linux-kernel - Re: [PATCH 1/2] sched/fair: Remove redundant lookup of rq in check_preempt

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Message-ID: <YUmV/7puqVrYPfyL@hirez.programming.kicks-ass.net>
Date:   Tue, 21 Sep 2021 10:21:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/fair: Remove redundant lookup of rq in
 check_preempt_wakeup

On Tue, Sep 21, 2021 at 08:53:09AM +0100, Mel Gorman wrote:
> On Tue, Sep 21, 2021 at 09:21:16AM +0200, Vincent Guittot wrote:
> > On Mon, 20 Sept 2021 at 16:26, Mel Gorman <mgorman@...hsingularity.net> wrote:
> > >
> > > The rq for curr is read during the function preamble, remove the
> > > redundant lookup.
> > >
> > > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> > > ---
> > >  kernel/sched/fair.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index ff69f245b939..038edfaaae9e 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -7190,7 +7190,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
> > >         if (cse_is_idle != pse_is_idle)
> > >                 return;
> > >
> > > -       update_curr(cfs_rq_of(se));
> > > +       update_curr(cfs_rq);
> > 
> > se can have been modified by find_matching_se(&se, &pse)
> > 
> 
> I still expected the cfs_rq to be the same, particularly given that the
> context is about preempting the current task on a runqueue. Is that
> wrong?

Yes. There's a cfs_rq for every se. What we do in find_matching_se() is
walk up the hiarachy until both are in the same cfs_rq, otherwse we
cannot compare them.

Fundamentally this means the effective cfs_rq also changes.