linux-kernel - Re: [PATCH] sched/fair: Prefer idle CPU to cache affinity

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Message-ID: <20210301133720.GI2028034@linux.vnet.ibm.com>
Date:   Mon, 1 Mar 2021 19:07:20 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Rik van Riel <riel@...riel.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Parth Shah <parth@...ux.ibm.com>
Subject: Re: [PATCH] sched/fair: Prefer idle CPU to cache affinity

* Rik van Riel <riel@...riel.com> [2021-02-27 14:56:07]:

> > In the current situation where waker and previous CPUs are busy, but
> > only one of its LLC has an idle CPU, Scheduler may end up picking a
> > LLC
> > with no idle CPUs. To mitigate this, add a new step between 1 and 2
> > where Scheduler compares idle CPUs in waker and previous LLCs and
> > picks
> > the appropriate one.
> 
> I like that idea a lot. That could also solve some of the

Thanks.

> issues sometimes observed on multi-node x86 systems, and
> probably on the newer AMD chips with several LLCs on chip.
> 

Okay.

> > +	if (sched_feat(WA_WAKER) && tnr_busy < tllc_size)
> > +		return this_cpu;
> 
> I wonder if we need to use a slightly lower threshold on
> very large LLCs, both to account for the fact that the
> select_idle_cpu code may not find the single idle CPU
> among a dozen busy ones, or because on a system with
> hyperthreading we may often be better off picking another
> LLC for HT contention issues?

> 
> Maybe we could use "tnr_busy * 4 <
> tllc_size * 3" or
> something like that?
> 
> That way we will only try to find the last 5 idle
> CPUs
> in a 22 CPU LLC if the other LLC also has fewer than 6
> idle cores.
> 
> That might increase our chances of finding an idle CPU
> with SIS_PROP enabled, and might allow WA_WAKER to be
> true by default.

Agree we need to be conservative esp if we want to make WA_WAKER on by
default. I would still like to hear from other people if they think its ok
to enable it by default. I wonder if enabling it by default can cause some
load imbalances leading to more active load balance down the line.  I
haven't benchmarked with WA_WAKER enabled.

Thanks Rik for your inputs.

-- 
Thanks and Regards
Srikar Dronamraju