On 16 Sep 2022 15:36:53 +0200 Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
>
> Hi Hillf,
>
> On Fri, 16 Sept 2022 at 14:03, Hillf Danton <hdanton@xxxxxxxx> wrote:
> >
> > Hello Vincent
> >
> > On 16 Sep 2022 10:03:02 +0200 Vincent Guittot <vincent.guittot@xxxxxxxxxx> 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
with latency_offset, since the gap becomes larger, preempt happens later
than ideal runtime thoughts IMO.
