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Subject: Re: [PATCH] sched/fair: Prefer idle CPU to cache affinity
To:     Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Rik van Riel <riel@surriel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Michael Neuling <mikey@neuling.org>,
        Gautham R Shenoy <ego@linux.vnet.ibm.com>,
        Parth Shah <parth@linux.ibm.com>
References: <20210226164029.122432-1-srikar@linux.vnet.ibm.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <19bfd781-05f7-b220-d8a2-fde8ea2d9959@arm.com>
Date:   Tue, 2 Mar 2021 10:53:06 +0100
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On 26/02/2021 17:40, Srikar Dronamraju wrote:

[...]

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8a8bd7b13634..d49bfcdc4a19 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5869,6 +5869,36 @@ wake_affine_weight(struct sched_domain *sd, struct task_struct *p,
>  	return this_eff_load < prev_eff_load ? this_cpu : nr_cpumask_bits;
>  }
> 
> +static int prefer_idler_llc(int this_cpu, int prev_cpu, int sync)
> +{
> +	struct sched_domain_shared *tsds, *psds;
> +	int pnr_busy, pllc_size, tnr_busy, tllc_size, diff;
> +
> +	tsds = rcu_dereference(per_cpu(sd_llc_shared, this_cpu));
> +	tnr_busy = atomic_read(&tsds->nr_busy_cpus);
> +	tllc_size = per_cpu(sd_llc_size, this_cpu);
> +
> +	psds = rcu_dereference(per_cpu(sd_llc_shared, prev_cpu));
> +	pnr_busy = atomic_read(&psds->nr_busy_cpus);
> +	pllc_size = per_cpu(sd_llc_size, prev_cpu);
> +
> +	/* No need to compare, if both LLCs are fully loaded */
> +	if (pnr_busy == pllc_size && tnr_busy == pllc_size)

                                                     ^
                                           shouldn't this be tllc_size ?

> +		return nr_cpumask_bits;
> +
> +	if (sched_feat(WA_WAKER) && tnr_busy < tllc_size)
> +		return this_cpu;
> +
> +	/* 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;
> +}
> +