Messages in this thread Patch in this message | | | Subject | Re: Scheduler bug related to rq->skip_clock_update? | From | Mike Galbraith <> | Date | Mon, 22 Nov 2010 09:19:41 -0700 |
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On Sun, 2010-11-21 at 23:29 -0500, Bjoern B. Brandenburg wrote: > On Sun, 21 Nov 2010, Mike Galbraith wrote: > > > On Sat, 2010-11-20 at 23:22 -0500, Bjoern B. Brandenburg wrote: > > > > > I was under the impression that, as an invariant, tasks should not have > > > TIF_NEED_RESCHED set after they've blocked. In this case, the idle load > > > balancer should not mark the task that's on its way out with > > > set_tsk_need_resched(). > > > > Nice find. > > > > > In any case, check_preempt_curr() seems to assume that a resuming task cannot > > > have TIF_NEED_RESCHED already set. Setting skip_clock_update on a remote CPU > > > that hasn't even been notified via IPI seems wrong. > > > > Yes. Does the below fix it up for you? > > The patch definitely changes the behavior, but it doesn't seem to solve (all > of) the root cause(s). The failsafe kicks in and clears the flag the next > time that update_rq_clock() is called, but there can still be a significant > delay between setting and clearing the flag. Right after boot, I'm now seeing > values that go up to ~21ms.
A pull isn't the only vulnerability. Since idle_balance() drops rq->lock, so another cpu can wake to this rq.
> Please let me know if there is something else that I should test.
Sched: clear_tsk_need_resched() after NEWIDLE balancing
idle_balance() drops/retakes rq->lock, leaving the previous task vulnerable to set_tsk_need_resched() from another CPU. Clear it after NEWIDLE balancing to maintain the invariant that descheduled tasks are NOT marked for resched.
This also confuses the skip_clock_update logic, which assumes that the next call to update_rq_clock() will come nearly ĩmmediately after being set. Make the optimization more robust by clearing before we balance and in update_rq_clock().
Signed-off-by: Mike Galbraith <efault@gmx.de> Cc: Ingo Molnar <mingo@elte.hu> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Bjoern B. Brandenburg <bbb.lst@gmail.com> Reported-by: Bjoern B. Brandenburg <bbb.lst@gmail.com>
--- kernel/sched.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
Index: linux-2.6/kernel/sched.c =================================================================== --- linux-2.6.orig/kernel/sched.c +++ linux-2.6/kernel/sched.c @@ -657,6 +657,7 @@ inline void update_rq_clock(struct rq *r sched_irq_time_avg_update(rq, irq_time); } + rq->skip_clock_update = 0; } /* @@ -3714,7 +3715,6 @@ 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); } @@ -3799,8 +3799,16 @@ need_resched_nonpreemptible: pre_schedule(rq, prev); - if (unlikely(!rq->nr_running)) + if (unlikely(!rq->nr_running)) { idle_balance(cpu, rq); + /* + * idle_balance() releases/retakes rq->lock, leaving prev + * vulnerable to set_tsk_need_resched() from another cpu. + * Clock updates should not be skipped while we're away. + */ + clear_tsk_need_resched(prev); + rq->skip_clock_update = 0; + } put_prev_task(rq, prev); next = pick_next_task(rq);
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