Re: Scheduler bug related to rq->skip_clock_update?

[Yong Zhang <yong.zhang0@gmail.com>]

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?

Thanks,
Yong