On 18 Sep 2022 12:46:00 +0200 Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> On Sun, 18 Sept 2022 at 00:58, Hillf Danton <hdanton@xxxxxxxx> wrote:
> >
> > 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
>
> 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));
}
