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

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Date:   Mon, 23 Nov 2020 23:07:27 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Balbir Singh <bsingharora@...il.com>,
        "Joel Fernandes (Google)" <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>,
        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 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.

Thanks,
-Aubrey