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

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Date:   Sat, 27 Feb 2021 14:56:07 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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

On Fri, 2021-02-26 at 22:10 +0530, Srikar Dronamraju wrote:

> Current order of preference to pick a LLC while waking a wake-affine
> task:
> 1. Between the waker CPU and previous CPU, prefer the LLC of the CPU
>    that is idle.
> 
> 2. Between the waker CPU and previous CPU, prefer the LLC of the CPU
>    that is less lightly loaded.
> 
> 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
issues sometimes observed on multi-node x86 systems, and
probably on the newer AMD chips with several LLCs on chip.

> +	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.

> +	/* For better wakeup latency, prefer idler LLC to cache
> affinity */
> +	diff = tnr_busy * pllc_size - sync - pnr_busy * tllc_size;
> +	if (!diff)
> +		return nr_cpumask_bits;
> +	if (diff < 0)
> +		return this_cpu;
> +
> +	return prev_cpu;
> +}

-- 
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