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authorPaul E. McKenney <paul.mckenney@linaro.org>2011-09-30 12:10:22 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2011-12-11 10:31:24 -0800
commit9b2e4f1880b789be1f24f9684f7a54b90310b5c0 (patch)
tree1fa922e0616e298837a7079cb49118188a58186c /Documentation/RCU
parentb804cb9e91c6c304959c69d4f9daeef4ffdba71c (diff)
rcu: Track idleness independent of idle tasks
Earlier versions of RCU used the scheduling-clock tick to detect idleness by checking for the idle task, but handled idleness differently for CONFIG_NO_HZ=y. But there are now a number of uses of RCU read-side critical sections in the idle task, for example, for tracing. A more fine-grained detection of idleness is therefore required. This commit presses the old dyntick-idle code into full-time service, so that rcu_idle_enter(), previously known as rcu_enter_nohz(), is always invoked at the beginning of an idle loop iteration. Similarly, rcu_idle_exit(), previously known as rcu_exit_nohz(), is always invoked at the end of an idle-loop iteration. This allows the idle task to use RCU everywhere except between consecutive rcu_idle_enter() and rcu_idle_exit() calls, in turn allowing architecture maintainers to specify exactly where in the idle loop that RCU may be used. Because some of the userspace upcall uses can result in what looks to RCU like half of an interrupt, it is not possible to expect that the irq_enter() and irq_exit() hooks will give exact counts. This patch therefore expands the ->dynticks_nesting counter to 64 bits and uses two separate bitfields to count process/idle transitions and interrupt entry/exit transitions. It is presumed that userspace upcalls do not happen in the idle loop or from usermode execution (though usermode might do a system call that results in an upcall). The counter is hard-reset on each process/idle transition, which avoids the interrupt entry/exit error from accumulating. Overflow is avoided by the 64-bitness of the ->dyntick_nesting counter. This commit also adds warnings if a non-idle task asks RCU to enter idle state (and these checks will need some adjustment before applying Frederic's OS-jitter patches (http://lkml.org/lkml/2011/10/7/246). In addition, validation of ->dynticks and ->dynticks_nesting is added. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Diffstat (limited to 'Documentation/RCU')
-rw-r--r--Documentation/RCU/trace.txt4
1 files changed, 0 insertions, 4 deletions
diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index aaf65f6c6cd7..49587abfc2f7 100644
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -105,14 +105,10 @@ o "dt" is the current value of the dyntick counter that is incremented
or one greater than the interrupt-nesting depth otherwise.
The number after the second "/" is the NMI nesting depth.
- This field is displayed only for CONFIG_NO_HZ kernels.
-
o "df" is the number of times that some other CPU has forced a
quiescent state on behalf of this CPU due to this CPU being in
dynticks-idle state.
- This field is displayed only for CONFIG_NO_HZ kernels.
-
o "of" is the number of times that some other CPU has forced a
quiescent state on behalf of this CPU due to this CPU being
offline. In a perfect world, this might never happen, but it

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