Re: Scheduler bug related to rq->skip_clock_update?

[Yong Zhang]

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 Ämmediately 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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