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authorJ. Bruce Fields <bfields@citi.umich.edu>2008-02-07 08:13:37 (GMT)
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2008-02-07 16:42:17 (GMT)
commit9b8eae7248dad42091204f83ed3448e661456af1 (patch)
tree1e300d41f8aaa9c258c179024ba63799a79f5a6f /Documentation/scheduler
parentd3cf91d0e201962a6367191e5926f5b0920b0339 (diff)
Documentation: create new scheduler/ subdirectory
The top-level Documentation/ directory is unmanageably large, so we should take any obvious opportunities to move stuff into subdirectories. These sched-*.txt files seem an obvious easy case. Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu> Cc: Ingo Molnar <mingo@elte.hu> Acked-by: Randy Dunlap <randy.dunlap@oracle.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/scheduler')
-rw-r--r--Documentation/scheduler/00-INDEX16
-rw-r--r--Documentation/scheduler/sched-arch.txt89
-rw-r--r--Documentation/scheduler/sched-coding.txt126
-rw-r--r--Documentation/scheduler/sched-design-CFS.txt186
-rw-r--r--Documentation/scheduler/sched-design.txt165
-rw-r--r--Documentation/scheduler/sched-domains.txt70
-rw-r--r--Documentation/scheduler/sched-nice-design.txt108
-rw-r--r--Documentation/scheduler/sched-stats.txt156
8 files changed, 916 insertions, 0 deletions
diff --git a/Documentation/scheduler/00-INDEX b/Documentation/scheduler/00-INDEX
new file mode 100644
index 0000000..b5f5ca0
--- /dev/null
+++ b/Documentation/scheduler/00-INDEX
@@ -0,0 +1,16 @@
+00-INDEX
+ - this file.
+sched-arch.txt
+ - CPU Scheduler implementation hints for architecture specific code.
+sched-coding.txt
+ - reference for various scheduler-related methods in the O(1) scheduler.
+sched-design.txt
+ - goals, design and implementation of the Linux O(1) scheduler.
+sched-design-CFS.txt
+ - goals, design and implementation of the Complete Fair Scheduler.
+sched-domains.txt
+ - information on scheduling domains.
+sched-nice-design.txt
+ - How and why the scheduler's nice levels are implemented.
+sched-stats.txt
+ - information on schedstats (Linux Scheduler Statistics).
diff --git a/Documentation/scheduler/sched-arch.txt b/Documentation/scheduler/sched-arch.txt
new file mode 100644
index 0000000..941615a
--- /dev/null
+++ b/Documentation/scheduler/sched-arch.txt
@@ -0,0 +1,89 @@
+ CPU Scheduler implementation hints for architecture specific code
+
+ Nick Piggin, 2005
+
+Context switch
+==============
+1. Runqueue locking
+By default, the switch_to arch function is called with the runqueue
+locked. This is usually not a problem unless switch_to may need to
+take the runqueue lock. This is usually due to a wake up operation in
+the context switch. See include/asm-ia64/system.h for an example.
+
+To request the scheduler call switch_to with the runqueue unlocked,
+you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file
+(typically the one where switch_to is defined).
+
+Unlocked context switches introduce only a very minor performance
+penalty to the core scheduler implementation in the CONFIG_SMP case.
+
+2. Interrupt status
+By default, the switch_to arch function is called with interrupts
+disabled. Interrupts may be enabled over the call if it is likely to
+introduce a significant interrupt latency by adding the line
+`#define __ARCH_WANT_INTERRUPTS_ON_CTXSW` in the same place as for
+unlocked context switches. This define also implies
+`__ARCH_WANT_UNLOCKED_CTXSW`. See include/asm-arm/system.h for an
+example.
+
+
+CPU idle
+========
+Your cpu_idle routines need to obey the following rules:
+
+1. Preempt should now disabled over idle routines. Should only
+ be enabled to call schedule() then disabled again.
+
+2. need_resched/TIF_NEED_RESCHED is only ever set, and will never
+ be cleared until the running task has called schedule(). Idle
+ threads need only ever query need_resched, and may never set or
+ clear it.
+
+3. When cpu_idle finds (need_resched() == 'true'), it should call
+ schedule(). It should not call schedule() otherwise.
+
+4. The only time interrupts need to be disabled when checking
+ need_resched is if we are about to sleep the processor until
+ the next interrupt (this doesn't provide any protection of
+ need_resched, it prevents losing an interrupt).
+
+ 4a. Common problem with this type of sleep appears to be:
+ local_irq_disable();
+ if (!need_resched()) {
+ local_irq_enable();
+ *** resched interrupt arrives here ***
+ __asm__("sleep until next interrupt");
+ }
+
+5. TIF_POLLING_NRFLAG can be set by idle routines that do not
+ need an interrupt to wake them up when need_resched goes high.
+ In other words, they must be periodically polling need_resched,
+ although it may be reasonable to do some background work or enter
+ a low CPU priority.
+
+ 5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter
+ an interrupt sleep, it needs to be cleared then a memory
+ barrier issued (followed by a test of need_resched with
+ interrupts disabled, as explained in 3).
+
+arch/i386/kernel/process.c has examples of both polling and
+sleeping idle functions.
+
+
+Possible arch/ problems
+=======================
+
+Possible arch problems I found (and either tried to fix or didn't):
+
+h8300 - Is such sleeping racy vs interrupts? (See #4a).
+ The H8/300 manual I found indicates yes, however disabling IRQs
+ over the sleep mean only NMIs can wake it up, so can't fix easily
+ without doing spin waiting.
+
+ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)
+
+sh64 - Is sleeping racy vs interrupts? (See #4a)
+
+sparc - IRQs on at this point(?), change local_irq_save to _disable.
+ - TODO: needs secondary CPUs to disable preempt (See #1)
+
diff --git a/Documentation/scheduler/sched-coding.txt b/Documentation/scheduler/sched-coding.txt
new file mode 100644
index 0000000..cbd8db7
--- /dev/null
+++ b/Documentation/scheduler/sched-coding.txt
@@ -0,0 +1,126 @@
+ Reference for various scheduler-related methods in the O(1) scheduler
+ Robert Love <rml@tech9.net>, MontaVista Software
+
+
+Note most of these methods are local to kernel/sched.c - this is by design.
+The scheduler is meant to be self-contained and abstracted away. This document
+is primarily for understanding the scheduler, not interfacing to it. Some of
+the discussed interfaces, however, are general process/scheduling methods.
+They are typically defined in include/linux/sched.h.
+
+
+Main Scheduling Methods
+-----------------------
+
+void load_balance(runqueue_t *this_rq, int idle)
+ Attempts to pull tasks from one cpu to another to balance cpu usage,
+ if needed. This method is called explicitly if the runqueues are
+ imbalanced or periodically by the timer tick. Prior to calling,
+ the current runqueue must be locked and interrupts disabled.
+
+void schedule()
+ The main scheduling function. Upon return, the highest priority
+ process will be active.
+
+
+Locking
+-------
+
+Each runqueue has its own lock, rq->lock. When multiple runqueues need
+to be locked, lock acquires must be ordered by ascending &runqueue value.
+
+A specific runqueue is locked via
+
+ task_rq_lock(task_t pid, unsigned long *flags)
+
+which disables preemption, disables interrupts, and locks the runqueue pid is
+running on. Likewise,
+
+ task_rq_unlock(task_t pid, unsigned long *flags)
+
+unlocks the runqueue pid is running on, restores interrupts to their previous
+state, and reenables preemption.
+
+The routines
+
+ double_rq_lock(runqueue_t *rq1, runqueue_t *rq2)
+
+and
+
+ double_rq_unlock(runqueue_t *rq1, runqueue_t *rq2)
+
+safely lock and unlock, respectively, the two specified runqueues. They do
+not, however, disable and restore interrupts. Users are required to do so
+manually before and after calls.
+
+
+Values
+------
+
+MAX_PRIO
+ The maximum priority of the system, stored in the task as task->prio.
+ Lower priorities are higher. Normal (non-RT) priorities range from
+ MAX_RT_PRIO to (MAX_PRIO - 1).
+MAX_RT_PRIO
+ The maximum real-time priority of the system. Valid RT priorities
+ range from 0 to (MAX_RT_PRIO - 1).
+MAX_USER_RT_PRIO
+ The maximum real-time priority that is exported to user-space. Should
+ always be equal to or less than MAX_RT_PRIO. Setting it less allows
+ kernel threads to have higher priorities than any user-space task.
+MIN_TIMESLICE
+MAX_TIMESLICE
+ Respectively, the minimum and maximum timeslices (quanta) of a process.
+
+Data
+----
+
+struct runqueue
+ The main per-CPU runqueue data structure.
+struct task_struct
+ The main per-process data structure.
+
+
+General Methods
+---------------
+
+cpu_rq(cpu)
+ Returns the runqueue of the specified cpu.
+this_rq()
+ Returns the runqueue of the current cpu.
+task_rq(pid)
+ Returns the runqueue which holds the specified pid.
+cpu_curr(cpu)
+ Returns the task currently running on the given cpu.
+rt_task(pid)
+ Returns true if pid is real-time, false if not.
+
+
+Process Control Methods
+-----------------------
+
+void set_user_nice(task_t *p, long nice)
+ Sets the "nice" value of task p to the given value.
+int setscheduler(pid_t pid, int policy, struct sched_param *param)
+ Sets the scheduling policy and parameters for the given pid.
+int set_cpus_allowed(task_t *p, unsigned long new_mask)
+ Sets a given task's CPU affinity and migrates it to a proper cpu.
+ Callers must have a valid reference to the task and assure the
+ task not exit prematurely. No locks can be held during the call.
+set_task_state(tsk, state_value)
+ Sets the given task's state to the given value.
+set_current_state(state_value)
+ Sets the current task's state to the given value.
+void set_tsk_need_resched(struct task_struct *tsk)
+ Sets need_resched in the given task.
+void clear_tsk_need_resched(struct task_struct *tsk)
+ Clears need_resched in the given task.
+void set_need_resched()
+ Sets need_resched in the current task.
+void clear_need_resched()
+ Clears need_resched in the current task.
+int need_resched()
+ Returns true if need_resched is set in the current task, false
+ otherwise.
+yield()
+ Place the current process at the end of the runqueue and call schedule.
diff --git a/Documentation/scheduler/sched-design-CFS.txt b/Documentation/scheduler/sched-design-CFS.txt
new file mode 100644
index 0000000..88bcb87
--- /dev/null
+++ b/Documentation/scheduler/sched-design-CFS.txt
@@ -0,0 +1,186 @@
+
+This is the CFS scheduler.
+
+80% of CFS's design can be summed up in a single sentence: CFS basically
+models an "ideal, precise multi-tasking CPU" on real hardware.
+
+"Ideal multi-tasking CPU" is a (non-existent :-)) CPU that has 100%
+physical power and which can run each task at precise equal speed, in
+parallel, each at 1/nr_running speed. For example: if there are 2 tasks
+running then it runs each at 50% physical power - totally in parallel.
+
+On real hardware, we can run only a single task at once, so while that
+one task runs, the other tasks that are waiting for the CPU are at a
+disadvantage - the current task gets an unfair amount of CPU time. In
+CFS this fairness imbalance is expressed and tracked via the per-task
+p->wait_runtime (nanosec-unit) value. "wait_runtime" is the amount of
+time the task should now run on the CPU for it to become completely fair
+and balanced.
+
+( small detail: on 'ideal' hardware, the p->wait_runtime value would
+ always be zero - no task would ever get 'out of balance' from the
+ 'ideal' share of CPU time. )
+
+CFS's task picking logic is based on this p->wait_runtime value and it
+is thus very simple: it always tries to run the task with the largest
+p->wait_runtime value. In other words, CFS tries to run the task with
+the 'gravest need' for more CPU time. So CFS always tries to split up
+CPU time between runnable tasks as close to 'ideal multitasking
+hardware' as possible.
+
+Most of the rest of CFS's design just falls out of this really simple
+concept, with a few add-on embellishments like nice levels,
+multiprocessing and various algorithm variants to recognize sleepers.
+
+In practice it works like this: the system runs a task a bit, and when
+the task schedules (or a scheduler tick happens) the task's CPU usage is
+'accounted for': the (small) time it just spent using the physical CPU
+is deducted from p->wait_runtime. [minus the 'fair share' it would have
+gotten anyway]. Once p->wait_runtime gets low enough so that another
+task becomes the 'leftmost task' of the time-ordered rbtree it maintains
+(plus a small amount of 'granularity' distance relative to the leftmost
+task so that we do not over-schedule tasks and trash the cache) then the
+new leftmost task is picked and the current task is preempted.
+
+The rq->fair_clock value tracks the 'CPU time a runnable task would have
+fairly gotten, had it been runnable during that time'. So by using
+rq->fair_clock values we can accurately timestamp and measure the
+'expected CPU time' a task should have gotten. All runnable tasks are
+sorted in the rbtree by the "rq->fair_clock - p->wait_runtime" key, and
+CFS picks the 'leftmost' task and sticks to it. As the system progresses
+forwards, newly woken tasks are put into the tree more and more to the
+right - slowly but surely giving a chance for every task to become the
+'leftmost task' and thus get on the CPU within a deterministic amount of
+time.
+
+Some implementation details:
+
+ - the introduction of Scheduling Classes: an extensible hierarchy of
+ scheduler modules. These modules encapsulate scheduling policy
+ details and are handled by the scheduler core without the core
+ code assuming about them too much.
+
+ - sched_fair.c implements the 'CFS desktop scheduler': it is a
+ replacement for the vanilla scheduler's SCHED_OTHER interactivity
+ code.
+
+ I'd like to give credit to Con Kolivas for the general approach here:
+ he has proven via RSDL/SD that 'fair scheduling' is possible and that
+ it results in better desktop scheduling. Kudos Con!
+
+ The CFS patch uses a completely different approach and implementation
+ from RSDL/SD. My goal was to make CFS's interactivity quality exceed
+ that of RSDL/SD, which is a high standard to meet :-) Testing
+ feedback is welcome to decide this one way or another. [ and, in any
+ case, all of SD's logic could be added via a kernel/sched_sd.c module
+ as well, if Con is interested in such an approach. ]
+
+ CFS's design is quite radical: it does not use runqueues, it uses a
+ time-ordered rbtree to build a 'timeline' of future task execution,
+ and thus has no 'array switch' artifacts (by which both the vanilla
+ scheduler and RSDL/SD are affected).
+
+ CFS uses nanosecond granularity accounting and does not rely on any
+ jiffies or other HZ detail. Thus the CFS scheduler has no notion of
+ 'timeslices' and has no heuristics whatsoever. There is only one
+ central tunable (you have to switch on CONFIG_SCHED_DEBUG):
+
+ /proc/sys/kernel/sched_granularity_ns
+
+ which can be used to tune the scheduler from 'desktop' (low
+ latencies) to 'server' (good batching) workloads. It defaults to a
+ setting suitable for desktop workloads. SCHED_BATCH is handled by the
+ CFS scheduler module too.
+
+ Due to its design, the CFS scheduler is not prone to any of the
+ 'attacks' that exist today against the heuristics of the stock
+ scheduler: fiftyp.c, thud.c, chew.c, ring-test.c, massive_intr.c all
+ work fine and do not impact interactivity and produce the expected
+ behavior.
+
+ the CFS scheduler has a much stronger handling of nice levels and
+ SCHED_BATCH: both types of workloads should be isolated much more
+ agressively than under the vanilla scheduler.
+
+ ( another detail: due to nanosec accounting and timeline sorting,
+ sched_yield() support is very simple under CFS, and in fact under
+ CFS sched_yield() behaves much better than under any other
+ scheduler i have tested so far. )
+
+ - sched_rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler
+ way than the vanilla scheduler does. It uses 100 runqueues (for all
+ 100 RT priority levels, instead of 140 in the vanilla scheduler)
+ and it needs no expired array.
+
+ - reworked/sanitized SMP load-balancing: the runqueue-walking
+ assumptions are gone from the load-balancing code now, and
+ iterators of the scheduling modules are used. The balancing code got
+ quite a bit simpler as a result.
+
+
+Group scheduler extension to CFS
+================================
+
+Normally the scheduler operates on individual tasks and strives to provide
+fair CPU time to each task. Sometimes, it may be desirable to group tasks
+and provide fair CPU time to each such task group. For example, it may
+be desirable to first provide fair CPU time to each user on the system
+and then to each task belonging to a user.
+
+CONFIG_FAIR_GROUP_SCHED strives to achieve exactly that. It lets
+SCHED_NORMAL/BATCH tasks be be grouped and divides CPU time fairly among such
+groups. At present, there are two (mutually exclusive) mechanisms to group
+tasks for CPU bandwidth control purpose:
+
+ - Based on user id (CONFIG_FAIR_USER_SCHED)
+ In this option, tasks are grouped according to their user id.
+ - Based on "cgroup" pseudo filesystem (CONFIG_FAIR_CGROUP_SCHED)
+ This options lets the administrator create arbitrary groups
+ of tasks, using the "cgroup" pseudo filesystem. See
+ Documentation/cgroups.txt for more information about this
+ filesystem.
+
+Only one of these options to group tasks can be chosen and not both.
+
+Group scheduler tunables:
+
+When CONFIG_FAIR_USER_SCHED is defined, a directory is created in sysfs for
+each new user and a "cpu_share" file is added in that directory.
+
+ # cd /sys/kernel/uids
+ # cat 512/cpu_share # Display user 512's CPU share
+ 1024
+ # echo 2048 > 512/cpu_share # Modify user 512's CPU share
+ # cat 512/cpu_share # Display user 512's CPU share
+ 2048
+ #
+
+CPU bandwidth between two users are divided in the ratio of their CPU shares.
+For ex: if you would like user "root" to get twice the bandwidth of user
+"guest", then set the cpu_share for both the users such that "root"'s
+cpu_share is twice "guest"'s cpu_share
+
+
+When CONFIG_FAIR_CGROUP_SCHED is defined, a "cpu.shares" file is created
+for each group created using the pseudo filesystem. See example steps
+below to create task groups and modify their CPU share using the "cgroups"
+pseudo filesystem
+
+ # mkdir /dev/cpuctl
+ # mount -t cgroup -ocpu none /dev/cpuctl
+ # cd /dev/cpuctl
+
+ # mkdir multimedia # create "multimedia" group of tasks
+ # mkdir browser # create "browser" group of tasks
+
+ # #Configure the multimedia group to receive twice the CPU bandwidth
+ # #that of browser group
+
+ # echo 2048 > multimedia/cpu.shares
+ # echo 1024 > browser/cpu.shares
+
+ # firefox & # Launch firefox and move it to "browser" group
+ # echo <firefox_pid> > browser/tasks
+
+ # #Launch gmplayer (or your favourite movie player)
+ # echo <movie_player_pid> > multimedia/tasks
diff --git a/Documentation/scheduler/sched-design.txt b/Documentation/scheduler/sched-design.txt
new file mode 100644
index 0000000..1605bf0
--- /dev/null
+++ b/Documentation/scheduler/sched-design.txt
@@ -0,0 +1,165 @@
+ Goals, Design and Implementation of the
+ new ultra-scalable O(1) scheduler
+
+
+ This is an edited version of an email Ingo Molnar sent to
+ lkml on 4 Jan 2002. It describes the goals, design, and
+ implementation of Ingo's new ultra-scalable O(1) scheduler.
+ Last Updated: 18 April 2002.
+
+
+Goal
+====
+
+The main goal of the new scheduler is to keep all the good things we know
+and love about the current Linux scheduler:
+
+ - good interactive performance even during high load: if the user
+ types or clicks then the system must react instantly and must execute
+ the user tasks smoothly, even during considerable background load.
+
+ - good scheduling/wakeup performance with 1-2 runnable processes.
+
+ - fairness: no process should stay without any timeslice for any
+ unreasonable amount of time. No process should get an unjustly high
+ amount of CPU time.
+
+ - priorities: less important tasks can be started with lower priority,
+ more important tasks with higher priority.
+
+ - SMP efficiency: no CPU should stay idle if there is work to do.
+
+ - SMP affinity: processes which run on one CPU should stay affine to
+ that CPU. Processes should not bounce between CPUs too frequently.
+
+ - plus additional scheduler features: RT scheduling, CPU binding.
+
+and the goal is also to add a few new things:
+
+ - fully O(1) scheduling. Are you tired of the recalculation loop
+ blowing the L1 cache away every now and then? Do you think the goodness
+ loop is taking a bit too long to finish if there are lots of runnable
+ processes? This new scheduler takes no prisoners: wakeup(), schedule(),
+ the timer interrupt are all O(1) algorithms. There is no recalculation
+ loop. There is no goodness loop either.
+
+ - 'perfect' SMP scalability. With the new scheduler there is no 'big'
+ runqueue_lock anymore - it's all per-CPU runqueues and locks - two
+ tasks on two separate CPUs can wake up, schedule and context-switch
+ completely in parallel, without any interlocking. All
+ scheduling-relevant data is structured for maximum scalability.
+
+ - better SMP affinity. The old scheduler has a particular weakness that
+ causes the random bouncing of tasks between CPUs if/when higher
+ priority/interactive tasks, this was observed and reported by many
+ people. The reason is that the timeslice recalculation loop first needs
+ every currently running task to consume its timeslice. But when this
+ happens on eg. an 8-way system, then this property starves an
+ increasing number of CPUs from executing any process. Once the last
+ task that has a timeslice left has finished using up that timeslice,
+ the recalculation loop is triggered and other CPUs can start executing
+ tasks again - after having idled around for a number of timer ticks.
+ The more CPUs, the worse this effect.
+
+ Furthermore, this same effect causes the bouncing effect as well:
+ whenever there is such a 'timeslice squeeze' of the global runqueue,
+ idle processors start executing tasks which are not affine to that CPU.
+ (because the affine tasks have finished off their timeslices already.)
+
+ The new scheduler solves this problem by distributing timeslices on a
+ per-CPU basis, without having any global synchronization or
+ recalculation.
+
+ - batch scheduling. A significant proportion of computing-intensive tasks
+ benefit from batch-scheduling, where timeslices are long and processes
+ are roundrobin scheduled. The new scheduler does such batch-scheduling
+ of the lowest priority tasks - so nice +19 jobs will get
+ 'batch-scheduled' automatically. With this scheduler, nice +19 jobs are
+ in essence SCHED_IDLE, from an interactiveness point of view.
+
+ - handle extreme loads more smoothly, without breakdown and scheduling
+ storms.
+
+ - O(1) RT scheduling. For those RT folks who are paranoid about the
+ O(nr_running) property of the goodness loop and the recalculation loop.
+
+ - run fork()ed children before the parent. Andrea has pointed out the
+ advantages of this a few months ago, but patches for this feature
+ do not work with the old scheduler as well as they should,
+ because idle processes often steal the new child before the fork()ing
+ CPU gets to execute it.
+
+
+Design
+======
+
+The core of the new scheduler contains the following mechanisms:
+
+ - *two* priority-ordered 'priority arrays' per CPU. There is an 'active'
+ array and an 'expired' array. The active array contains all tasks that
+ are affine to this CPU and have timeslices left. The expired array
+ contains all tasks which have used up their timeslices - but this array
+ is kept sorted as well. The active and expired array is not accessed
+ directly, it's accessed through two pointers in the per-CPU runqueue
+ structure. If all active tasks are used up then we 'switch' the two
+ pointers and from now on the ready-to-go (former-) expired array is the
+ active array - and the empty active array serves as the new collector
+ for expired tasks.
+
+ - there is a 64-bit bitmap cache for array indices. Finding the highest
+ priority task is thus a matter of two x86 BSFL bit-search instructions.
+
+the split-array solution enables us to have an arbitrary number of active
+and expired tasks, and the recalculation of timeslices can be done
+immediately when the timeslice expires. Because the arrays are always
+access through the pointers in the runqueue, switching the two arrays can
+be done very quickly.
+
+this is a hybride priority-list approach coupled with roundrobin
+scheduling and the array-switch method of distributing timeslices.
+
+ - there is a per-task 'load estimator'.
+
+one of the toughest things to get right is good interactive feel during
+heavy system load. While playing with various scheduler variants i found
+that the best interactive feel is achieved not by 'boosting' interactive
+tasks, but by 'punishing' tasks that want to use more CPU time than there
+is available. This method is also much easier to do in an O(1) fashion.
+
+to establish the actual 'load' the task contributes to the system, a
+complex-looking but pretty accurate method is used: there is a 4-entry
+'history' ringbuffer of the task's activities during the last 4 seconds.
+This ringbuffer is operated without much overhead. The entries tell the
+scheduler a pretty accurate load-history of the task: has it used up more
+CPU time or less during the past N seconds. [the size '4' and the interval
+of 4x 1 seconds was found by lots of experimentation - this part is
+flexible and can be changed in both directions.]
+
+the penalty a task gets for generating more load than the CPU can handle
+is a priority decrease - there is a maximum amount to this penalty
+relative to their static priority, so even fully CPU-bound tasks will
+observe each other's priorities, and will share the CPU accordingly.
+
+the SMP load-balancer can be extended/switched with additional parallel
+computing and cache hierarchy concepts: NUMA scheduling, multi-core CPUs
+can be supported easily by changing the load-balancer. Right now it's
+tuned for my SMP systems.
+
+i skipped the prev->mm == next->mm advantage - no workload i know of shows
+any sensitivity to this. It can be added back by sacrificing O(1)
+schedule() [the current and one-lower priority list can be searched for a
+that->mm == current->mm condition], but costs a fair number of cycles
+during a number of important workloads, so i wanted to avoid this as much
+as possible.
+
+- the SMP idle-task startup code was still racy and the new scheduler
+triggered this. So i streamlined the idle-setup code a bit. We do not call
+into schedule() before all processors have started up fully and all idle
+threads are in place.
+
+- the patch also cleans up a number of aspects of sched.c - moves code
+into other areas of the kernel where it's appropriate, and simplifies
+certain code paths and data constructs. As a result, the new scheduler's
+code is smaller than the old one.
+
+ Ingo
diff --git a/Documentation/scheduler/sched-domains.txt b/Documentation/scheduler/sched-domains.txt
new file mode 100644
index 0000000..a9e990a
--- /dev/null
+++ b/Documentation/scheduler/sched-domains.txt
@@ -0,0 +1,70 @@
+Each CPU has a "base" scheduling domain (struct sched_domain). These are
+accessed via cpu_sched_domain(i) and this_sched_domain() macros. The domain
+hierarchy is built from these base domains via the ->parent pointer. ->parent
+MUST be NULL terminated, and domain structures should be per-CPU as they
+are locklessly updated.
+
+Each scheduling domain spans a number of CPUs (stored in the ->span field).
+A domain's span MUST be a superset of it child's span (this restriction could
+be relaxed if the need arises), and a base domain for CPU i MUST span at least
+i. The top domain for each CPU will generally span all CPUs in the system
+although strictly it doesn't have to, but this could lead to a case where some
+CPUs will never be given tasks to run unless the CPUs allowed mask is
+explicitly set. A sched domain's span means "balance process load among these
+CPUs".
+
+Each scheduling domain must have one or more CPU groups (struct sched_group)
+which are organised as a circular one way linked list from the ->groups
+pointer. The union of cpumasks of these groups MUST be the same as the
+domain's span. The intersection of cpumasks from any two of these groups
+MUST be the empty set. The group pointed to by the ->groups pointer MUST
+contain the CPU to which the domain belongs. Groups may be shared among
+CPUs as they contain read only data after they have been set up.
+
+Balancing within a sched domain occurs between groups. That is, each group
+is treated as one entity. The load of a group is defined as the sum of the
+load of each of its member CPUs, and only when the load of a group becomes
+out of balance are tasks moved between groups.
+
+In kernel/sched.c, rebalance_tick is run periodically on each CPU. This
+function takes its CPU's base sched domain and checks to see if has reached
+its rebalance interval. If so, then it will run load_balance on that domain.
+rebalance_tick then checks the parent sched_domain (if it exists), and the
+parent of the parent and so forth.
+
+*** Implementing sched domains ***
+The "base" domain will "span" the first level of the hierarchy. In the case
+of SMT, you'll span all siblings of the physical CPU, with each group being
+a single virtual CPU.
+
+In SMP, the parent of the base domain will span all physical CPUs in the
+node. Each group being a single physical CPU. Then with NUMA, the parent
+of the SMP domain will span the entire machine, with each group having the
+cpumask of a node. Or, you could do multi-level NUMA or Opteron, for example,
+might have just one domain covering its one NUMA level.
+
+The implementor should read comments in include/linux/sched.h:
+struct sched_domain fields, SD_FLAG_*, SD_*_INIT to get an idea of
+the specifics and what to tune.
+
+For SMT, the architecture must define CONFIG_SCHED_SMT and provide a
+cpumask_t cpu_sibling_map[NR_CPUS], where cpu_sibling_map[i] is the mask of
+all "i"'s siblings as well as "i" itself.
+
+Architectures may retain the regular override the default SD_*_INIT flags
+while using the generic domain builder in kernel/sched.c if they wish to
+retain the traditional SMT->SMP->NUMA topology (or some subset of that). This
+can be done by #define'ing ARCH_HASH_SCHED_TUNE.
+
+Alternatively, the architecture may completely override the generic domain
+builder by #define'ing ARCH_HASH_SCHED_DOMAIN, and exporting your
+arch_init_sched_domains function. This function will attach domains to all
+CPUs using cpu_attach_domain.
+
+Implementors should change the line
+#undef SCHED_DOMAIN_DEBUG
+to
+#define SCHED_DOMAIN_DEBUG
+in kernel/sched.c as this enables an error checking parse of the sched domains
+which should catch most possible errors (described above). It also prints out
+the domain structure in a visual format.
diff --git a/Documentation/scheduler/sched-nice-design.txt b/Documentation/scheduler/sched-nice-design.txt
new file mode 100644
index 0000000..e2bae5a
--- /dev/null
+++ b/Documentation/scheduler/sched-nice-design.txt
@@ -0,0 +1,108 @@
+This document explains the thinking about the revamped and streamlined
+nice-levels implementation in the new Linux scheduler.
+
+Nice levels were always pretty weak under Linux and people continuously
+pestered us to make nice +19 tasks use up much less CPU time.
+
+Unfortunately that was not that easy to implement under the old
+scheduler, (otherwise we'd have done it long ago) because nice level
+support was historically coupled to timeslice length, and timeslice
+units were driven by the HZ tick, so the smallest timeslice was 1/HZ.
+
+In the O(1) scheduler (in 2003) we changed negative nice levels to be
+much stronger than they were before in 2.4 (and people were happy about
+that change), and we also intentionally calibrated the linear timeslice
+rule so that nice +19 level would be _exactly_ 1 jiffy. To better
+understand it, the timeslice graph went like this (cheesy ASCII art
+alert!):
+
+
+ A
+ \ | [timeslice length]
+ \ |
+ \ |
+ \ |
+ \ |
+ \|___100msecs
+ |^ . _
+ | ^ . _
+ | ^ . _
+ -*----------------------------------*-----> [nice level]
+ -20 | +19
+ |
+ |
+
+So that if someone wanted to really renice tasks, +19 would give a much
+bigger hit than the normal linear rule would do. (The solution of
+changing the ABI to extend priorities was discarded early on.)
+
+This approach worked to some degree for some time, but later on with
+HZ=1000 it caused 1 jiffy to be 1 msec, which meant 0.1% CPU usage which
+we felt to be a bit excessive. Excessive _not_ because it's too small of
+a CPU utilization, but because it causes too frequent (once per
+millisec) rescheduling. (and would thus trash the cache, etc. Remember,
+this was long ago when hardware was weaker and caches were smaller, and
+people were running number crunching apps at nice +19.)
+
+So for HZ=1000 we changed nice +19 to 5msecs, because that felt like the
+right minimal granularity - and this translates to 5% CPU utilization.
+But the fundamental HZ-sensitive property for nice+19 still remained,
+and we never got a single complaint about nice +19 being too _weak_ in
+terms of CPU utilization, we only got complaints about it (still) being
+too _strong_ :-)
+
+To sum it up: we always wanted to make nice levels more consistent, but
+within the constraints of HZ and jiffies and their nasty design level
+coupling to timeslices and granularity it was not really viable.
+
+The second (less frequent but still periodically occuring) complaint
+about Linux's nice level support was its assymetry around the origo
+(which you can see demonstrated in the picture above), or more
+accurately: the fact that nice level behavior depended on the _absolute_
+nice level as well, while the nice API itself is fundamentally
+"relative":
+
+ int nice(int inc);
+
+ asmlinkage long sys_nice(int increment)
+
+(the first one is the glibc API, the second one is the syscall API.)
+Note that the 'inc' is relative to the current nice level. Tools like
+bash's "nice" command mirror this relative API.
+
+With the old scheduler, if you for example started a niced task with +1
+and another task with +2, the CPU split between the two tasks would
+depend on the nice level of the parent shell - if it was at nice -10 the
+CPU split was different than if it was at +5 or +10.
+
+A third complaint against Linux's nice level support was that negative
+nice levels were not 'punchy enough', so lots of people had to resort to
+run audio (and other multimedia) apps under RT priorities such as
+SCHED_FIFO. But this caused other problems: SCHED_FIFO is not starvation
+proof, and a buggy SCHED_FIFO app can also lock up the system for good.
+
+The new scheduler in v2.6.23 addresses all three types of complaints:
+
+To address the first complaint (of nice levels being not "punchy"
+enough), the scheduler was decoupled from 'time slice' and HZ concepts
+(and granularity was made a separate concept from nice levels) and thus
+it was possible to implement better and more consistent nice +19
+support: with the new scheduler nice +19 tasks get a HZ-independent
+1.5%, instead of the variable 3%-5%-9% range they got in the old
+scheduler.
+
+To address the second complaint (of nice levels not being consistent),
+the new scheduler makes nice(1) have the same CPU utilization effect on
+tasks, regardless of their absolute nice levels. So on the new
+scheduler, running a nice +10 and a nice 11 task has the same CPU
+utilization "split" between them as running a nice -5 and a nice -4
+task. (one will get 55% of the CPU, the other 45%.) That is why nice
+levels were changed to be "multiplicative" (or exponential) - that way
+it does not matter which nice level you start out from, the 'relative
+result' will always be the same.
+
+The third complaint (of negative nice levels not being "punchy" enough
+and forcing audio apps to run under the more dangerous SCHED_FIFO
+scheduling policy) is addressed by the new scheduler almost
+automatically: stronger negative nice levels are an automatic
+side-effect of the recalibrated dynamic range of nice levels.
diff --git a/Documentation/scheduler/sched-stats.txt b/Documentation/scheduler/sched-stats.txt
new file mode 100644
index 0000000..442e14d
--- /dev/null
+++ b/Documentation/scheduler/sched-stats.txt
@@ -0,0 +1,156 @@
+Version 14 of schedstats includes support for sched_domains, which hit the
+mainline kernel in 2.6.20 although it is identical to the stats from version
+12 which was in the kernel from 2.6.13-2.6.19 (version 13 never saw a kernel
+release). Some counters make more sense to be per-runqueue; other to be
+per-domain. Note that domains (and their associated information) will only
+be pertinent and available on machines utilizing CONFIG_SMP.
+
+In version 14 of schedstat, there is at least one level of domain
+statistics for each cpu listed, and there may well be more than one
+domain. Domains have no particular names in this implementation, but
+the highest numbered one typically arbitrates balancing across all the
+cpus on the machine, while domain0 is the most tightly focused domain,
+sometimes balancing only between pairs of cpus. At this time, there
+are no architectures which need more than three domain levels. The first
+field in the domain stats is a bit map indicating which cpus are affected
+by that domain.
+
+These fields are counters, and only increment. Programs which make use
+of these will need to start with a baseline observation and then calculate
+the change in the counters at each subsequent observation. A perl script
+which does this for many of the fields is available at
+
+ http://eaglet.rain.com/rick/linux/schedstat/
+
+Note that any such script will necessarily be version-specific, as the main
+reason to change versions is changes in the output format. For those wishing
+to write their own scripts, the fields are described here.
+
+CPU statistics
+--------------
+cpu<N> 1 2 3 4 5 6 7 8 9 10 11 12
+
+NOTE: In the sched_yield() statistics, the active queue is considered empty
+ if it has only one process in it, since obviously the process calling
+ sched_yield() is that process.
+
+First four fields are sched_yield() statistics:
+ 1) # of times both the active and the expired queue were empty
+ 2) # of times just the active queue was empty
+ 3) # of times just the expired queue was empty
+ 4) # of times sched_yield() was called
+
+Next three are schedule() statistics:
+ 5) # of times we switched to the expired queue and reused it
+ 6) # of times schedule() was called
+ 7) # of times schedule() left the processor idle
+
+Next two are try_to_wake_up() statistics:
+ 8) # of times try_to_wake_up() was called
+ 9) # of times try_to_wake_up() was called to wake up the local cpu
+
+Next three are statistics describing scheduling latency:
+ 10) sum of all time spent running by tasks on this processor (in jiffies)
+ 11) sum of all time spent waiting to run by tasks on this processor (in
+ jiffies)
+ 12) # of timeslices run on this cpu
+
+
+Domain statistics
+-----------------
+One of these is produced per domain for each cpu described. (Note that if
+CONFIG_SMP is not defined, *no* domains are utilized and these lines
+will not appear in the output.)
+
+domain<N> <cpumask> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
+
+The first field is a bit mask indicating what cpus this domain operates over.
+
+The next 24 are a variety of load_balance() statistics in grouped into types
+of idleness (idle, busy, and newly idle):
+
+ 1) # of times in this domain load_balance() was called when the
+ cpu was idle
+ 2) # of times in this domain load_balance() checked but found
+ the load did not require balancing when the cpu was idle
+ 3) # of times in this domain load_balance() tried to move one or
+ more tasks and failed, when the cpu was idle
+ 4) sum of imbalances discovered (if any) with each call to
+ load_balance() in this domain when the cpu was idle
+ 5) # of times in this domain pull_task() was called when the cpu
+ was idle
+ 6) # of times in this domain pull_task() was called even though
+ the target task was cache-hot when idle
+ 7) # of times in this domain load_balance() was called but did
+ not find a busier queue while the cpu was idle
+ 8) # of times in this domain a busier queue was found while the
+ cpu was idle but no busier group was found
+
+ 9) # of times in this domain load_balance() was called when the
+ cpu was busy
+ 10) # of times in this domain load_balance() checked but found the
+ load did not require balancing when busy
+ 11) # of times in this domain load_balance() tried to move one or
+ more tasks and failed, when the cpu was busy
+ 12) sum of imbalances discovered (if any) with each call to
+ load_balance() in this domain when the cpu was busy
+ 13) # of times in this domain pull_task() was called when busy
+ 14) # of times in this domain pull_task() was called even though the
+ target task was cache-hot when busy
+ 15) # of times in this domain load_balance() was called but did not
+ find a busier queue while the cpu was busy
+ 16) # of times in this domain a busier queue was found while the cpu
+ was busy but no busier group was found
+
+ 17) # of times in this domain load_balance() was called when the
+ cpu was just becoming idle
+ 18) # of times in this domain load_balance() checked but found the
+ load did not require balancing when the cpu was just becoming idle
+ 19) # of times in this domain load_balance() tried to move one or more
+ tasks and failed, when the cpu was just becoming idle
+ 20) sum of imbalances discovered (if any) with each call to
+ load_balance() in this domain when the cpu was just becoming idle
+ 21) # of times in this domain pull_task() was called when newly idle
+ 22) # of times in this domain pull_task() was called even though the
+ target task was cache-hot when just becoming idle
+ 23) # of times in this domain load_balance() was called but did not
+ find a busier queue while the cpu was just becoming idle
+ 24) # of times in this domain a busier queue was found while the cpu
+ was just becoming idle but no busier group was found
+
+ Next three are active_load_balance() statistics:
+ 25) # of times active_load_balance() was called
+ 26) # of times active_load_balance() tried to move a task and failed
+ 27) # of times active_load_balance() successfully moved a task
+
+ Next three are sched_balance_exec() statistics:
+ 28) sbe_cnt is not used
+ 29) sbe_balanced is not used
+ 30) sbe_pushed is not used
+
+ Next three are sched_balance_fork() statistics:
+ 31) sbf_cnt is not used
+ 32) sbf_balanced is not used
+ 33) sbf_pushed is not used
+
+ Next three are try_to_wake_up() statistics:
+ 34) # of times in this domain try_to_wake_up() awoke a task that
+ last ran on a different cpu in this domain
+ 35) # of times in this domain try_to_wake_up() moved a task to the
+ waking cpu because it was cache-cold on its own cpu anyway
+ 36) # of times in this domain try_to_wake_up() started passive balancing
+
+/proc/<pid>/schedstat
+----------------
+schedstats also adds a new /proc/<pid/schedstat file to include some of
+the same information on a per-process level. There are three fields in
+this file correlating for that process to:
+ 1) time spent on the cpu
+ 2) time spent waiting on a runqueue
+ 3) # of timeslices run on this cpu
+
+A program could be easily written to make use of these extra fields to
+report on how well a particular process or set of processes is faring
+under the scheduler's policies. A simple version of such a program is
+available at
+ http://eaglet.rain.com/rick/linux/schedstat/v12/latency.c

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