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    SubjectRe: [PATCH v4] sched/core: Preempt current task in favour of bound kthread
    On Thu, 12 Dec 2019 at 11:23, Peter Zijlstra <peterz@infradead.org> wrote:
    >
    > On Thu, Dec 12, 2019 at 11:14:24AM +0100, Peter Zijlstra wrote:
    > > On Thu, Dec 12, 2019 at 11:10:31AM +0100, Peter Zijlstra wrote:
    > > > @@ -4156,13 +4159,13 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
    > > > if (delta_exec < sysctl_sched_min_granularity)
    > > > return;
    > > >
    > > > - se = __pick_first_entity(cfs_rq);
    > > > + se = __pick_next_entity(cfs_rq, NULL);
    > > > delta = curr->vruntime - se->vruntime;
    > > >
    > > > if (delta < 0)
    > > > return;
    > >
    > > What I mean with the below comment is, when this isn't enough, try
    > > something like:
    > >
    > > if (se == cfs_rq->next)
    > > ideal_runtime /= 2;
    > >
    > > to make it yield sooner to 'next' buddies. Sadly, due to the whole
    > > cgroup mess, we can't say what actual task is on the end of it (without
    > > doing a full hierarchy pick, which is more expensive still).
    >
    > Just for giggles, that'd look something like:
    >
    > while (!entity_is_task(se) {
    > cfs_rq = group_cfs_rq(se);
    > se = pick_next_entity(cfs_rq, cfs_rq->curr);
    > }
    > p = task_of(se);
    >
    > if (is_per_cpu_kthread(p))
    > ideal_runtime /= 2;
    >
    > the core-scheduling patch set includes the right primitive for this I
    > think, pick_task_fair().

    why not only updating wan_gran() which is the only function which uses
    sysctl_sched_wakeup_granularity ?

    For per cpu kthread, we could set the gran to sched_min_granularity
    instead of scaling it with thread's priority so per cpu kthread can
    still preempt current ask even if sysctl_sched_wakeup_granularity is
    large

    >
    > > > - if (delta > ideal_runtime)
    > > > + if (delta > ideal_runtime) // maybe frob this too ?
    > > > resched_curr(rq_of(cfs_rq));
    > > > }

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