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Message-ID: <CAKfTPtBpYWDz=ZeKKa1BGOVZ+PqM=kbbgSBQpn7msxMV_5v5uA@mail.gmail.com>
Date: Wed, 13 May 2020 15:25:29 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Phil Auld <pauld@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
linux-kernel <linux-kernel@...r.kernel.org>,
Tao Zhou <ouwen210@...mail.com>,
Pavan Kondeti <pkondeti@...eaurora.org>
Subject: Re: [PATCH v2] sched/fair: enqueue_task_fair optimization
On Wed, 13 May 2020 at 15:18, Phil Auld <pauld@...hat.com> wrote:
>
> On Wed, May 13, 2020 at 03:15:53PM +0200 Vincent Guittot wrote:
> > On Wed, 13 May 2020 at 15:13, Phil Auld <pauld@...hat.com> wrote:
> > >
> > > On Wed, May 13, 2020 at 03:10:28PM +0200 Vincent Guittot wrote:
> > > > On Wed, 13 May 2020 at 14:45, Phil Auld <pauld@...hat.com> wrote:
> > > > >
> > > > > Hi Vincent,
> > > > >
> > > > > On Wed, May 13, 2020 at 02:33:35PM +0200 Vincent Guittot wrote:
> > > > > > enqueue_task_fair jumps to enqueue_throttle label when cfs_rq_of(se) is
> > > > > > throttled which means that se can't be NULL and we can skip the test.
> > > > > >
> > > > >
> > > > > s/be NULL/be non-NULL/
> > > > >
> > > > > I think.
> > > >
> > > > This sentence refers to the move of enqueue_throttle and the fact that
> > > > se can't be null when goto enqueue_throttle and we can jump directly
> > > > after the if statement, which is now removed in v2 because se is
> > > > always NULL if we don't use goto enqueue_throttle.
> > > >
> > > > I haven't change the commit message for the remove of if statement
> > > >
> > >
> > > Fair enough, it just seems backwards from the intent of the patch now.
> > >
> > > There is also an extra } after the update_overutilized_status.
> >
> > don't know what I did but it's crap. sorry about that
> >
>
> No worries. I didn't see it when I read it either. The compiler told me :)
Yeah, but i thought that i compiled it which is obviously not true
>
>
> > Let me prepare a v3
> >
> > >
> > >
> > > Cheers,
> > > Phil
> > >
> > >
> > >
> > > > >
> > > > > It's more like if it doesn't jump to the label then se must be NULL for
> > > > > the loop to terminate. The final loop is a NOP if se is NULL. The check
> > > > > wasn't protecting that.
> > > > >
> > > > > Otherwise still
> > > > >
> > > > > > Reviewed-by: Phil Auld <pauld@...hat.com>
> > > > >
> > > > > Cheers,
> > > > > Phil
> > > > >
> > > > >
> > > > > > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > > > > > ---
> > > > > >
> > > > > > v2 changes:
> > > > > > - Remove useless if statement
> > > > > >
> > > > > > kernel/sched/fair.c | 39 ++++++++++++++++++++-------------------
> > > > > > 1 file changed, 20 insertions(+), 19 deletions(-)
> > > > > >
> > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > > > index a0c690d57430..b51b12d63c39 100644
> > > > > > --- a/kernel/sched/fair.c
> > > > > > +++ b/kernel/sched/fair.c
> > > > > > @@ -5513,28 +5513,29 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > > > > > list_add_leaf_cfs_rq(cfs_rq);
> > > > > > }
> > > > > >
> > > > > > -enqueue_throttle:
> > > > > > - if (!se) {
> > > > > > - add_nr_running(rq, 1);
> > > > > > - /*
> > > > > > - * Since new tasks are assigned an initial util_avg equal to
> > > > > > - * half of the spare capacity of their CPU, tiny tasks have the
> > > > > > - * ability to cross the overutilized threshold, which will
> > > > > > - * result in the load balancer ruining all the task placement
> > > > > > - * done by EAS. As a way to mitigate that effect, do not account
> > > > > > - * for the first enqueue operation of new tasks during the
> > > > > > - * overutilized flag detection.
> > > > > > - *
> > > > > > - * A better way of solving this problem would be to wait for
> > > > > > - * the PELT signals of tasks to converge before taking them
> > > > > > - * into account, but that is not straightforward to implement,
> > > > > > - * and the following generally works well enough in practice.
> > > > > > - */
> > > > > > - if (flags & ENQUEUE_WAKEUP)
> > > > > > - update_overutilized_status(rq);
> > > > > > + /* At this point se is NULL and we are at root level*/
> > > > > > + add_nr_running(rq, 1);
> > > > > > +
> > > > > > + /*
> > > > > > + * Since new tasks are assigned an initial util_avg equal to
> > > > > > + * half of the spare capacity of their CPU, tiny tasks have the
> > > > > > + * ability to cross the overutilized threshold, which will
> > > > > > + * result in the load balancer ruining all the task placement
> > > > > > + * done by EAS. As a way to mitigate that effect, do not account
> > > > > > + * for the first enqueue operation of new tasks during the
> > > > > > + * overutilized flag detection.
> > > > > > + *
> > > > > > + * A better way of solving this problem would be to wait for
> > > > > > + * the PELT signals of tasks to converge before taking them
> > > > > > + * into account, but that is not straightforward to implement,
> > > > > > + * and the following generally works well enough in practice.
> > > > > > + */
> > > > > > + if (flags & ENQUEUE_WAKEUP)
> > > > > > + update_overutilized_status(rq);
> > > > > >
> > > > > > }
> > > > > >
> > > > > > +enqueue_throttle:
> > > > > > if (cfs_bandwidth_used()) {
> > > > > > /*
> > > > > > * When bandwidth control is enabled; the cfs_rq_throttled()
> > > > > > --
> > > > > > 2.17.1
> > > > > >
> > > > >
> > > > > --
> > > > >
> > > >
> > >
> > > --
> > >
> >
>
> --
>
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