linux-kernel - Re: [RFC PATCH 11/13] sched: migration changes for core scheduling

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Date:   Fri, 12 Jun 2020 17:32:01 -0400
From:   Vineeth Remanan Pillai <vpillai@...italocean.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Aubrey Li <aubrey.li@...el.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.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 11/13] sched: migration changes for core scheduling

On Fri, Jun 12, 2020 at 9:21 AM Joel Fernandes <joel@...lfernandes.org> wrote:
>
> > +#ifdef CONFIG_SCHED_CORE
> > +             if (available_idle_cpu(cpu) &&
> > +                 sched_core_cookie_match(cpu_rq(cpu), p))
> > +                     break;
> > +#else
>
> select_idle_cpu() is called only if no idle core could be found in the LLC by
> select_idle_core().
>
> So, would it be better here to just do the cookie equality check directly
> instead of calling the sched_core_cookie_match() helper?  More so, because
> select_idle_sibling() is a fastpath.
>
Agree, this makes sense to me.

> AFAIR, that's what v4 did:
>
>                 if (available_idle_cpu(cpu))
> #ifdef CONFIG_SCHED_CORE
>                         if (sched_core_enabled(cpu_rq(cpu)) &&
>                             (p->core_cookie == cpu_rq(cpu)->core->core_cookie))
>                                 break;
> #else
>                         break;
> #endif
>
This patch was initially not in v4 and this is a merging of 4 patches
suggested post-v4. During the initial round, code was like above. But since
there looked like a code duplication in the different migration paths,
it was consolidated into sched_core_cookie_match() and it caused this
extra logic to this specific code path. As you mentioned, I also feel
we do not need to check for core idleness in this path.

Thanks,
Vineeth