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SubjectRe: Scheduler bug related to rq->skip_clock_update?
On Sat, Dec 04, 2010 at 03:42:36PM +0800, Yong Zhang wrote:
> On Mon, Nov 22, 2010 at 01:14:47PM -0500, Bjoern B. Brandenburg wrote:
> > On Mon, 22 Nov 2010, Mike Galbraith wrote:
> >
> > > 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().
> >
> > Unfortunately that doesn't seem to do it yet.
> >
> > After running five 'find /' instances to completion on the ARM platform,
> > I'm still seeing delays close to 10ms.
> >
> > bbb@district10:~$ egrep 'cpu#|skip' /proc/sched_debug
> > cpu#0
> > .skip_clock_count : 89606
> > .skip_clock_recent_max : 9817250
> > .skip_clock_max : 21992375
> > cpu#1
> > .skip_clock_count : 81978
> > .skip_clock_recent_max : 9582500
> > .skip_clock_max : 17201750
> > cpu#2
> > .skip_clock_count : 74565
> > .skip_clock_recent_max : 9678000
> > .skip_clock_max : 9879250
> > cpu#3
> > .skip_clock_count : 81685
> > .skip_clock_recent_max : 9300125
> > .skip_clock_max : 14115750
> >
> > On the x86_64 host, I've changed to HZ=100 and am now also seeing delays
> > close to 10ms after 'make clean && make -j8 bzImage'.
> >
> > bbb@koruna:~$ egrep 'cpu#|skip' /proc/sched_debug
> > cpu#0, 2493.476 MHz
> > .skip_clock_count : 29703
> > .skip_clock_recent_max : 9999858
> > .skip_clock_max : 40645942
> > cpu#1, 2493.476 MHz
> > .skip_clock_count : 32696
> > .skip_clock_recent_max : 9959118
> > .skip_clock_max : 35074771
> > cpu#2, 2493.476 MHz
> > .skip_clock_count : 31742
> > .skip_clock_recent_max : 9788654
> > .skip_clock_max : 24821765
> > cpu#3, 2493.476 MHz
> > .skip_clock_count : 31123
> > .skip_clock_recent_max : 9858546
> > .skip_clock_max : 44276033
> > cpu#4, 2493.476 MHz
> > .skip_clock_count : 28346
> > .skip_clock_recent_max : 10000775
> > .skip_clock_max : 18681753
> > cpu#5, 2493.476 MHz
> > .skip_clock_count : 29421
> > .skip_clock_recent_max : 9997656
> > .skip_clock_max : 138473407
> > cpu#6, 2493.476 MHz
> > .skip_clock_count : 27721
> > .skip_clock_recent_max : 9992074
> > .skip_clock_max : 53436918
> > cpu#7, 2493.476 MHz
> > .skip_clock_count : 29637
> > .skip_clock_recent_max : 9994516
> > .skip_clock_max : 566793528
> >
> > These numbers were recorded with the below patch.
> >
> > Please let me know if I can help by testing or tracing something else.
>
> Seems the checking for <if (prev->se.on_rq)> in put_prev_task()
> is the culprit.
>
> Because if we preempt a going sleep process on another CPU,
> we will fail to update the rq clock for that CPU in schedule.
> For example:
>
> CPUA CPUB
> process xxx == current
> check_preempt_curr() for CPUB ... skip_clock_update==1
> going to sleep
> ->schedule()
> ->deactivate_task() fail to update rq clock
> because skip_clock_update==1
> ->put_prev_task() fail to update rq clock
> because prev->se.on_rq==false
>
> Then rq clock on CPUB will updated until another schedule envent
> comes.
>
> So Bjoern, is deleting the checking for prev->se.on_rq in
> put_prev_task() helpful?

My test show there indeed is some improvement.
But I just notice skip_clock_recent_max/max is based on
_nanosecond_, so the 10ms delay mentioned by Bjoern
should be _10us_.

So I'm not sure if my suggestion is necessary.

>
> Thanks,
> Yong
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