Re: [PATCH v4 5/8] sched/fair: Take into account latency priority at wakeup

[Hillf Danton <hdanton@sina.com>]

On 18 Sep 2022 12:46:00 +0200 Vincent Guittot <vincent.guittot@linaro.org> wrote:
> On Sun, 18 Sept 2022 at 00:58, Hillf Danton <hdanton@sina.com> wrote:
> >
> > On 16 Sep 2022 15:36:53 +0200 Vincent Guittot <vincent.guittot@linaro.org> wrote:
> > >
> > > Hi Hillf,
> > >
> > > On Fri, 16 Sept 2022 at 14:03, Hillf Danton <hdanton@sina.com> wrote:
> > > >
> > > > Hello Vincent
> > > >
> > > > On 16 Sep 2022 10:03:02 +0200 Vincent Guittot <vincent.guittot@linaro.org> wrote:
> > > > >
> > > > > @@ -4606,6 +4608,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> > > > >
> > > > >       se = __pick_first_entity(cfs_rq);
> > > > >       delta = curr->vruntime - se->vruntime;
> > > > > +     delta -= wakeup_latency_gran(curr, se);
> > > > >
> > > > >       if (delta < 0)
> > > > >               return;
> > > >
> > > > What is derived from the latency nice you added is the runtime granulaity
> > > > which has a role in preempting the current task.
> > > >
> > > > Given the same defination of latency nice as the nice, the runtime granularity
> > > > can be computed without introducing the latency nice.
> > > >
> > > > Only for thoughts now.
> > > >
> > > > Hillf
> > > >
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -4569,7 +4569,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
> > > >  static void
> > > >  check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> > > >  {
> > > > -       unsigned long ideal_runtime, delta_exec;
> > > > +       unsigned long ideal_runtime, delta_exec, granu;
> > > >         struct sched_entity *se;
> > > >         s64 delta;
> > > >
> > > > @@ -4594,6 +4594,14 @@ check_preempt_tick(struct cfs_rq *cfs_rq
> > > >                 return;
> > > >
> > > >         se = __pick_first_entity(cfs_rq);
> > > > +
> > > > +       granu = sysctl_sched_min_granularity +
> > > > +               (ideal_runtime - sysctl_sched_min_granularity) *
> > > > +               (se->latency_nice + 20) / LATENCY_NICE_WIDTH;
> > >
> > > There is no latency_nice field in se but a latency_offset instead
> > >
> > > Also at this step, we are sure that curr has run at least
> > > sysctl_sched_min_granularity and we want now to compare curr vruntime
> > > with first se's one. We take the latency offset into account to make
> > > sure we will not preempt curr too early
> > >
> > > > +
> > > > +       if (delta_exec < granu)
> > > > +               return;
> > > > +
> > > >         delta = curr->vruntime - se->vruntime;
> > > >
> > > >         if (delta < 0)
> >                 return;
> >
> >             if (delta > ideal_runtime)
> >                 resched_curr(rq_of(cfs_rq));
> >
> > After another look, curr is not preempted without the gap in vruntime
> > between curr and the first entity growing more than ideal runtime, while
> 
> Curr can be preempted as it has run more than the ideal time (1st
> test). This one is to make sure that the diff does not become too
> large. Here we reuse the same comparison as wakeup to make sure that a
> newly curr will get a chance to run its ideal time after  having
> preempted at wakeup the previous current

IIUC it would take two checks to preempt correctly - diff in vruntime
is checked first to avoid preempting too early, then it is checked again
with latency_offset taken into account to avoid preempting too late.

+++ b/kernel/sched/fair.c
@@ -4571,7 +4571,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq
 {
 	unsigned long ideal_runtime, delta_exec;
 	struct sched_entity *se;
-	s64 delta;
+	s64 delta, d2;
 
 	ideal_runtime = sched_slice(cfs_rq, curr);
 	delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
@@ -4595,11 +4595,9 @@ check_preempt_tick(struct cfs_rq *cfs_rq
 
 	se = __pick_first_entity(cfs_rq);
 	delta = curr->vruntime - se->vruntime;
+	d2 = delta - wakeup_latency_gran(curr, se);	
 
-	if (delta < 0)
-		return;
-
-	if (delta > ideal_runtime)
+	if (delta > ideal_runtime || d2 > ideal_runtime)
 		resched_curr(rq_of(cfs_rq));
 }