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authorDario Faggioli <raistlin@linux.it>2011-03-24 14:00:18 +0100
committerIngo Molnar <mingo@elte.hu>2011-03-31 13:00:34 +0200
commita51e91981870d013fcfcc08b0117997edbcbc7a7 (patch)
tree6e673d3b27f6150dd2723d773a8704657e5bb261 /kernel
parent6aba74f2791287ec407e0f92487a725a25908067 (diff)
sched: Leave sched_setscheduler() earlier if possible, do not disturb SCHED_FIFO tasks
sched_setscheduler() (in sched.c) is called in order of changing the scheduling policy and/or the real-time priority of a task. Thus, if we find out that neither of those are actually being modified, it is possible to return earlier and save the overhead of a full deactivate+activate cycle of the task in question. Beside that, if we have more than one SCHED_FIFO task with the same priority on the same rq (which means they share the same priority queue) having one of them changing its position in the priority queue because of a sched_setscheduler (as it happens by means of the deactivate+activate) that does not actually change the priority violates POSIX which states, for SCHED_FIFO: "If a thread whose policy or priority has been modified by pthread_setschedprio() is a running thread or is runnable, the effect on its position in the thread list depends on the direction of the modification, as follows: a. <...> b. If the priority is unchanged, the thread does not change position in the thread list. c. <...>" http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_08.html (ed: And the POSIX specification here does, briefly and somewhat unexpectedly, match what common sense tells us as well. ) Signed-off-by: Dario Faggioli <raistlin@linux.it> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> LKML-Reference: <1300971618.3960.82.camel@Palantir> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/sched.c11
1 files changed, 11 insertions, 0 deletions
diff --git a/kernel/sched.c b/kernel/sched.c
index f592ce6f8616..a8845516ace6 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5011,6 +5011,17 @@ recheck:
return -EINVAL;
}
+ /*
+ * If not changing anything there's no need to proceed further:
+ */
+ if (unlikely(policy == p->policy && (!rt_policy(policy) ||
+ param->sched_priority == p->rt_priority))) {
+
+ __task_rq_unlock(rq);
+ raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ return 0;
+ }
+
#ifdef CONFIG_RT_GROUP_SCHED
if (user) {
/*

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