On Mon, 2010-12-20 at 11:49 -0800, Greg KH wrote: > This patch doesn't apply to the .36 stable tree > If someone wants it applied there, please email the backport > to [email protected]
Subject: Sched: fix skip_clock_update optimization From: Mike Galbraith <[email protected]> Date: Wed Dec 8 11:05:42 2010 +0100 commit f26f9aff6aaf67e9a430d16c266f91b13a5bff64 upstream idle_balance() drops/retakes rq->lock, leaving the previous task vulnerable to set_tsk_need_resched(). Clear it after we return from balancing instead, and in setup_thread_stack() as well, so no successfully descheduled or never scheduled task has it set. Need resched confused the skip_clock_update logic, which assumes that the next call to update_rq_clock() will come nearly immediately after being set. Make the optimization robust against the waking a sleeper before it sucessfully deschedules case by checking that the current task has not been dequeued before setting the flag, since it is that useless clock update we're trying to save, and clear unconditionally in schedule() proper instead of conditionally in put_prev_task(). Signed-off-by: Mike Galbraith <[email protected]> Reported-by: Bjoern B. Brandenburg <[email protected]> Tested-by: Yong Zhang <[email protected]> Signed-off-by: Peter Zijlstra <[email protected]> Cc: [email protected] LKML-Reference: <[email protected]> Signed-off-by: Ingo Molnar <[email protected]> --- kernel/fork.c | 1 + kernel/sched.c | 6 +++--- 2 files changed, 4 insertions(+), 3 deletions(-) Index: linux-2.6.36/kernel/sched.c =================================================================== --- linux-2.6.36.orig/kernel/sched.c +++ linux-2.6.36/kernel/sched.c @@ -571,7 +571,7 @@ void check_preempt_curr(struct rq *rq, s * A queue event has occurred, and we're going to schedule. In * this case, we can save a useless back to back clock update. */ - if (test_tsk_need_resched(p)) + if (rq->curr->se.on_rq && test_tsk_need_resched(rq->curr)) rq->skip_clock_update = 1; } @@ -3702,7 +3702,6 @@ static void put_prev_task(struct rq *rq, { if (prev->se.on_rq) update_rq_clock(rq); - rq->skip_clock_update = 0; prev->sched_class->put_prev_task(rq, prev); } @@ -3764,7 +3763,6 @@ need_resched_nonpreemptible: hrtick_clear(rq); raw_spin_lock_irq(&rq->lock); - clear_tsk_need_resched(prev); switch_count = &prev->nivcsw; if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) { @@ -3796,6 +3794,8 @@ need_resched_nonpreemptible: put_prev_task(rq, prev); next = pick_next_task(rq); + clear_tsk_need_resched(prev); + rq->skip_clock_update = 0; if (likely(prev != next)) { sched_info_switch(prev, next); Index: linux-2.6.36/kernel/fork.c =================================================================== --- linux-2.6.36.orig/kernel/fork.c +++ linux-2.6.36/kernel/fork.c @@ -274,6 +274,7 @@ static struct task_struct *dup_task_stru setup_thread_stack(tsk, orig); clear_user_return_notifier(tsk); + clear_tsk_need_resched(tsk); stackend = end_of_stack(tsk); *stackend = STACK_END_MAGIC; /* for overflow detection */ _______________________________________________ stable mailing list [email protected] http://linux.kernel.org/mailman/listinfo/stable
