linux-kernel - Re: [PATCH -tip 13/32] sched: Trivial forced-newidle balancer

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Message-ID: <20201125212828.GA163610@balbir-desktop>
Date:   Thu, 26 Nov 2020 08:28:28 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 13/32] sched: Trivial forced-newidle balancer

On Tue, Nov 24, 2020 at 08:32:01AM +0800, Li, Aubrey wrote:
> On 2020/11/24 7:35, Balbir Singh wrote:
> > On Mon, Nov 23, 2020 at 11:07:27PM +0800, Li, Aubrey wrote:
> >> On 2020/11/23 12:38, Balbir Singh wrote:
> >>> On Tue, Nov 17, 2020 at 06:19:43PM -0500, Joel Fernandes (Google) wrote:
> >>>> From: Peter Zijlstra <peterz@...radead.org>
> >>>>
> >>>> When a sibling is forced-idle to match the core-cookie; search for
> >>>> matching tasks to fill the core.
> >>>>
> >>>> rcu_read_unlock() can incur an infrequent deadlock in
> >>>> sched_core_balance(). Fix this by using the RCU-sched flavor instead.
> >>>>
> >>> ...
> >>>> +
> >>>> +		if (p->core_occupation > dst->idle->core_occupation)
> >>>> +			goto next;
> >>>> +
> >>>
> >>> I am unable to understand this check, a comment or clarification in the
> >>> changelog will help. I presume we are looking at either one or two cpus
> >>> to define the core_occupation and we expect to match it against the
> >>> destination CPU.
> >>
> >> IIUC, this check prevents a task from keeping jumping among the cores forever.
> >>
> >> For example, on a SMT2 platform:
> >> - core0 runs taskA and taskB, core_occupation is 2
> >> - core1 runs taskC, core_occupation is 1
> >>
> >> Without this check, taskB could ping-pong between core0 and core1 by core load
> >> balance.
> > 
> > But the comparison is p->core_occuption (as in tasks core occuptation,
> > not sure what that means, can a task have a core_occupation of > 1?)
> >
> 
> p->core_occupation is assigned to the core occupation in the last pick_next_task.
> (so yes, it can have a > 1 core_occupation).
>

Hmm.. I find that hard to interpret that. But I am happy to re-read the
code again.

Balbir Singh.