linux-kernel - Re: [PATCH -tip 14/32] sched: migration changes for core scheduling

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Date:   Thu, 26 Nov 2020 09:57:31 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...el.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 14/32] sched: migration changes for core scheduling

On Wed, Nov 25, 2020 at 11:12:53AM +0800, Li, Aubrey wrote:
> On 2020/11/24 23:42, Peter Zijlstra wrote:
> > On Mon, Nov 23, 2020 at 12:36:10PM +0800, Li, Aubrey wrote:
> >>>> +#ifdef CONFIG_SCHED_CORE
> >>>> +		/*
> >>>> +		 * Skip this cpu if source task's cookie does not match
> >>>> +		 * with CPU's core cookie.
> >>>> +		 */
> >>>> +		if (!sched_core_cookie_match(cpu_rq(cpu), env->p))
> >>>> +			continue;
> >>>> +#endif
> >>>> +
> >>>
> >>> Any reason this is under an #ifdef? In sched_core_cookie_match() won't
> >>> the check for sched_core_enabled() do the right thing even when
> >>> CONFIG_SCHED_CORE is not enabed?> 
> >> Yes, sched_core_enabled works properly when CONFIG_SCHED_CORE is not
> >> enabled. But when CONFIG_SCHED_CORE is not enabled, it does not make
> >> sense to leave a core scheduler specific function here even at compile
> >> time. Also, for the cases in hot path, this saves CPU cycles to avoid
> >> a judgment.
> > 
> > No, that's nonsense. If it works, remove the #ifdef. Less (#ifdef) is
> > more.
> > 
> 
> Okay, I pasted the refined patch here.
> @Joel, please let me know if you want me to send it in a separated thread.
>

You still have a bunch of #ifdefs, can't we just do

#ifndef CONFIG_SCHED_CORE
static inline bool sched_core_enabled(struct rq *rq)
{
        return false;
}
#endif

and frankly I think even that is not needed because there is a jump
label __sched_core_enabled that tells us if sched_core is enabled or
not.

Balbir