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[PATCH] sched: fix SMT scheduler bug 

The SMT scheduler incorrectly skips kernel threads even if they are
runnable (but they are preempted by a higher-prio user-space task which got
SMT-delayed by an even higher-priority task running on a sibling CPU).

Fix this for now by only doing the SMT-nice optimization if the
to-be-delayed task is the only runnable task.  (This should cover most of
the real-life cases anyway.)

This bug has been in the SMT scheduler since 2.6.17 or so, but has only
been noticed now by the active check in the dynticks code.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Ingo Molnar 2007-02-28 20:13:42 -08:00 committed by Linus Torvalds
parent 3f555c700b
commit 7355690ead
1 changed files with 1 additions and 1 deletions
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2
kernel/sched.c
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@ -3547,7 +3547,7 @@ asmlinkage void __sched schedule(void)
}
}
next->sleep_type = SLEEP_NORMAL;
if (dependent_sleeper(cpu, rq, next))
if (rq->nr_running == 1 && dependent_sleeper(cpu, rq, next))
next = rq->idle;
switch_tasks:
if (next == rq->idle)