Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to nr_running

[Phil Auld <pauld@redhat.com>]

On Thu, Sep 23, 2021 at 10:40:48AM +0200 Vincent Guittot wrote:
> On Thu, 23 Sept 2021 at 03:47, Mike Galbraith <efault@gmx.de> wrote:
> >
> > On Wed, 2021-09-22 at 20:22 +0200, Vincent Guittot wrote:
> > > On Wed, 22 Sept 2021 at 19:38, Mel Gorman <mgorman@techsingularity.net> wrote:
> > > >
> > > >
> > > > I'm not seeing an alternative suggestion that could be turned into
> > > > an implementation. The current value for sched_wakeup_granularity
> > > > was set 12 years ago was exposed for tuning which is no longer
> > > > the case. The intent was to allow some dynamic adjustment between
> > > > sysctl_sched_wakeup_granularity and sysctl_sched_latency to reduce
> > > > over-scheduling in the worst case without disabling preemption entirely
> > > > (which the first version did).
> >
> > I don't think those knobs were ever _intended_ for general purpose
> > tuning, but they did get used that way by some folks.
> >
> > > >
> > > > Should we just ignore this problem and hope it goes away or just let
> > > > people keep poking silly values into debugfs via tuned?
> > >
> > > We should certainly not add a bandaid because people will continue to
> > > poke silly value at the end. And increasing
> > > sysctl_sched_wakeup_granularity based on the number of running threads
> > > is not the right solution.
> >
> > Watching my desktop box stack up large piles of very short running
> > threads, I agree, instantaneous load looks like a non-starter.
> >
> > >  According to the description of your
> > > problem that the current task doesn't get enough time to move forward,
> > > sysctl_sched_min_granularity should be part of the solution. Something
> > > like below will ensure that current got a chance to move forward
> >
> > Nah, progress is guaranteed, the issue is a zillion very similar short
> > running threads preempting each other with no win to be had, thus
> > spending cycles in the scheduler that are utterly wasted.  It's a valid
> > issue, trouble is teaching the scheduler to recognize that situation
> > without mucking up other situations where there IS a win for even very
> > short running threads say, doing a synchronous handoff; preemption is
> > cheaper than scheduling off if the waker is going be awakened again in
> > very short order.
> >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 9bf540f04c2d..39d4e4827d3d 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -7102,6 +7102,7 @@ static void check_preempt_wakeup(struct rq *rq,
> > > struct task_struct *p, int wake_
> > >         int scale = cfs_rq->nr_running >= sched_nr_latency;
> > >         int next_buddy_marked = 0;
> > >         int cse_is_idle, pse_is_idle;
> > > +       unsigned long delta_exec;
> > >
> > >         if (unlikely(se == pse))
> > >                 return;
> > > @@ -7161,6 +7162,13 @@ static void check_preempt_wakeup(struct rq *rq,
> > > struct task_struct *p, int wake_
> > >                 return;
> > >
> > >         update_curr(cfs_rq_of(se));
> > > +       delta_exec = se->sum_exec_runtime - se->prev_sum_exec_runtime;
> > > +       /*
> > > +        * Ensure that current got a chance to move forward
> > > +        */
> > > +       if (delta_exec < sysctl_sched_min_granularity)
> > > +               return;
> > > +
> > >         if (wakeup_preempt_entity(se, pse) == 1) {
> > >                 /*
> > >                  * Bias pick_next to pick the sched entity that is
> >
> > Yikes!  If you do that, you may as well go the extra nanometer and rip
> > wakeup preemption out entirely, same result, impressive diffstat.
> 
> This patch is mainly there to show that there are other ways to ensure
> progress without using some load heuristic.
> sysctl_sched_min_granularity has the problem of scaling with the
> number of cpus and this can generate large values. At least we should
> use the normalized_sysctl_sched_min_granularity or even a smaller
> value but wakeup preemption still happens with this change. It only
> ensures that we don't waste time preempting each other without any
> chance to do actual stuff.
>

It's capped at 8 cpus, which is pretty easy to reach these days, so the
values don't get too large.  That scaling is almost a no-op these days.


Cheers,
Phil


> a 100us value should even be enough to fix Mel's problem without
> impacting common wakeup preemption cases.
> 
> 
> >
> >         -Mike
> 

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