linux-kernel - Re: [PATCH v4] sched/core: Preempt current task in favour of bound kthread

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Date:   Thu, 12 Dec 2019 12:20:01 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Chinner <david@...morbit.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Phil Auld <pauld@...hat.com>, Ming Lei <ming.lei@...hat.com>,
        linux-block <linux-block@...r.kernel.org>,
        linux-fs <linux-fsdevel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        Eric Sandeen <sandeen@...hat.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v4] sched/core: Preempt current task in favour of bound kthread

On Thu, 12 Dec 2019 at 11:23, Peter Zijlstra <peterz@...radead.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));
> > >  }