linux-kernel - Re: [RFC PATCH v2 16/17] sched: Wake up sibling if it has something to run

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Date:   Mon, 29 Apr 2019 08:36:42 -0400
From:   Julien Desfossez <jdesfossez@...italocean.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, subhra.mazumdar@...cle.com,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>, Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v2 16/17] sched: Wake up sibling if it has something
 to run

On 26-Apr-2019 05:03:37 PM, Peter Zijlstra wrote:
> On Tue, Apr 23, 2019 at 04:18:21PM +0000, Vineeth Remanan Pillai wrote:
> 
> (you lost From: Julien)
> 
> > During core scheduling, it can happen that the current rq selects a
> > non-tagged process while the sibling might be idling even though it
> > had something to run (because the sibling selected idle to match the
> > tagged process in previous tag matching iteration). We need to wake up
> > the sibling if such a situation arise.
> > 
> > Signed-off-by: Vineeth Remanan Pillai <vpillai@...italocean.com>
> > Signed-off-by: Julien Desfossez <jdesfossez@...italocean.com>
> > ---
> >  kernel/sched/core.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index e8f5ec641d0a..0e3c51a1b54a 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3775,6 +3775,21 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >  			 */
> >  			if (i == cpu && !rq->core->core_cookie && !p->core_cookie) {
> >  				next = p;
> > +				rq->core_pick = NULL;
> > + 
> > +				/*
> > +				 * If the sibling is idling, we might want to wake it
> > +				 * so that it can check for any runnable tasks that did
> > +				 * not get a chance to run due to previous task matching.
> > +				 */
> > +				for_each_cpu(j, smt_mask) {
> > +					struct rq *rq_j = cpu_rq(j);
> > +					rq_j->core_pick = NULL;
> > +					if (j != cpu &&
> > +					    is_idle_task(rq_j->curr) && rq_j->nr_running) {
> > +						resched_curr(rq_j);
> > +					}
> > +				}
> >  				goto done;
> >  			}
> 
> Anyway, as written here:
> 
>   https://lkml.kernel.org/r/20190410150116.GI2490@worktop.programming.kicks-ass.net
> 
> I think this isn't quite right. Does the below patch (which actually
> removes lines) also work?
> 
> As written before; the intent was to not allow that optimization if the
> last pick had a cookie; thereby doing a (last) core wide selection when
> we go to a 0-cookie, and this then includes kicking forced-idle cores.

It works and the performance is similar to our previous solution :-)

Thanks,

Julien