linux-kernel - Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access

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Date:   Tue, 2 Apr 2019 09:42:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Subhra Mazumdar <subhra.mazumdar@...cle.com>
Cc:     Julien Desfossez <jdesfossez@...italocean.com>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, tim.c.chen@...ux.intel.com,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Vineeth Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>
Subject: Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access

On Fri, Mar 29, 2019 at 03:23:14PM -0700, Subhra Mazumdar wrote:
> 
> On 3/29/19 6:35 AM, Julien Desfossez wrote:
> > On Fri, Mar 22, 2019 at 8:09 PM Subhra Mazumdar <subhra.mazumdar@...cle.com>
> > wrote:
> > > Is the core wide lock primarily responsible for the regression? I ran
> > > upto patch
> > > 12 which also has the core wide lock for tagged cgroups and also calls
> > > newidle_balance() from pick_next_task(). I don't see any regression.  Of
> > > course
> > > the core sched version of pick_next_task() may be doing more but
> > > comparing with
> > > the __pick_next_task() it doesn't look too horrible.
> > On further testing and investigation, we also agree that spinlock contention
> > is not the major cause for the regression, but we feel that it should be one
> > of the major contributing factors to this performance loss.
> > 
> > 
> I finally did some code bisection and found the following lines are
> basically responsible for the regression. Commenting them out I don't see
> the regressions. Can you confirm? I am yet to figure if this is needed for
> the correctness of core scheduling and if so can we do this better?

It was meant to be an optimization; specifically, when no cookie was
set, don't bother to schedule the sibling(s).

> -------->8-------------
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index fe3918c..3b3388a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3741,8 +3741,8 @@ pick_next_task(struct rq *rq, struct task_struct
> *prev, struct rq_flags *rf)
>                                  * If there weren't no cookies; we don't
> need
>                                  * to bother with the other siblings.
> */
> -                               if (i == cpu && !rq->core->core_cookie)
> -                                       goto next_class;
> +                               //if (i == cpu && !rq->core->core_cookie)
> +                                       //goto next_class;
> 
> continue;
>                         }