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Date:   Fri, 22 May 2020 16:38:57 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Jirka Hladky <jhladky@redhat.com>, Phil Auld <pauld@redhat.com>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
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        Dietmar Eggemann <dietmar.eggemann@arm.com>,
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        Valentin Schneider <valentin.schneider@arm.com>,
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        Waiman Long <longman@redhat.com>,
        Joe Mario <jmario@redhat.com>, Bill Gray <bgray@redhat.com>,
        riel@surriel.com
Subject: Re: [PATCH 00/13] Reconcile NUMA balancing decisions with the load
 balancer v6
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On Fri, May 22, 2020 at 02:28:54PM +0100, Mel Gorman wrote:

> Is something like this on top of your patch what you had in mind?

All under the assumption that is makes it go faster of course ;-)

> ---8<---

static inline bool ttwu_queue_cond()
{
	/*
	 * If the CPU does not share cache, then queue the task on the
	 * remote rqs wakelist to avoid accessing remote data.
	 */
	if (!cpus_share_cache(smp_processor_id(), cpu))
		return true;

	/*
	 * If the task is descheduling and the only running task on the
	 * CPU, ....
	 */
	if ((wake_flags & WF_ON_RQ) && cpu_rq(cpu)->nr_running <= 1)
		return true;

	return false;
}

> -static bool ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
> +static bool ttwu_queue_wakelist(struct task_struct *p, int cpu, int wake_flags)
>  {
> -	if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {
> -		sched_clock_cpu(cpu); /* Sync clocks across CPUs */
> -		__ttwu_queue_remote(p, cpu, wake_flags);
> -		return true;
> +	if (sched_feat(TTWU_QUEUE)) {
> +		/*
> +		 * If CPU does not share cache then queue the task on the remote
> +		 * rqs wakelist to avoid accessing remote data. Alternatively,
> +		 * if the task is descheduling and the only running task on the
> +		 * CPU then use the wakelist to offload the task activation to
> +		 * the CPU that will soon be idle so the waker can continue.
> +		 * nr_running is checked to avoid unnecessary task stacking.
> +		 */
> +		if (!cpus_share_cache(smp_processor_id(), cpu) ||
> +		    ((wake_flags & WF_ON_RQ) && cpu_rq(cpu)->nr_running <= 1)) {
> +			sched_clock_cpu(cpu); /* Sync clocks across CPUs */
> +			__ttwu_queue_wakelist(p, cpu, wake_flags);
> +			return true;
> +		}

	if (sched_feat(TTWU_QUEUE) && ttwu_queue_cond(cpu, wake_flags)) {
		sched_clock_cpu(cpu); /* Sync clocks across CPUs */
		__ttwu_queue_remote(p, cpu, wake_flags);
		return true;

>  	}
>  
>  	return false;


might be easier to read...