Re: [PATCH v7 1/9] sched/fair: fix unfairness at wakeup

Vincent Guittot
2022-11-07T08:00:31Z

On Sat, 5 Nov 2022 at 15:33, Chen Yu <yu.c.chen@intel.com> wrote:
>
> On 2022-10-28 at 11:33:55 +0200, Vincent Guittot wrote:
> [snip]
> > +static inline unsigned long get_latency_max(void)
> > +{
> > + unsigned long thresh = get_sched_latency(false);
> > +
> > + thresh -= sysctl_sched_min_granularity;
> > +
> May I know why we substract sysctl_sched_min_granularity above?
> I thought thresh -= 1 would be enough to let the waking task preempt
> the current one, because:
> When a long sleeping task is enqueued on this rq, its vruntime is set
> to cfs_rq->min_vtime - latency, so
> diff = curr->vtime - cfs_rq->min_vtime + latency,
> since (curr->vtime - cfs_rq->min_vtime) >= 0, if we set the thresh to
> (latency - 1), the diff is guaranteed to be bigger than thresh and
> the waking task can preempt current task.

If the waking task failed to preempt current it could to wait up to
sysctl_sched_min_granularity before preempting it during next tick.

Vincent

>
> thanks,
> Chenyu
> > + return thresh;
> > +}