linux-kernel - Re: [PATCH v4] sched/fair: Consider cpu affinity when allowing NUMA imbalance in find_idlest

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Date:   Thu, 17 Feb 2022 16:53:51 +0530
From:   K Prateek Nayak <kprateek.nayak@....com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     peterz@...radead.org, aubrey.li@...ux.intel.com, efault@....de,
        gautham.shenoy@....com, linux-kernel@...r.kernel.org,
        mingo@...nel.org, song.bao.hua@...ilicon.com,
        srikar@...ux.vnet.ibm.com, valentin.schneider@....com,
        vincent.guittot@...aro.org
Subject: Re: [PATCH v4] sched/fair: Consider cpu affinity when allowing NUMA
 imbalance in find_idlest_group

Hello Mel,


Thank you for looking into the patch.

On 2/17/2022 3:35 PM, Mel Gorman wrote:
> Thanks Prateek,
>
> On Thu, Feb 17, 2022 at 11:24:08AM +0530, K Prateek Nayak wrote:
>> [..snip..]
>>
>> Eg: numactl -C 0,16,32,48,64,80,96,112 ./stream8
>>
> In this case the stream threads can use any CPU of the subset, presumably
> this is parallelised with OpenMP without specifying spread or bind
> directives.
Yes it is parallelized using OpenMP without specifying any directive.
> [..snip..]
> One concern I have is that we incur a cpumask setup and cpumask_weight
> cost on every clone whether a restricted CPU mask is used or not.  Peter,
> is it acceptable to avoid the cpumask check if there is no restrictions
> on allowed cpus like this?
>
> 	imb = sd->imb_numa_nr;
> 	if (p->nr_cpus_allowed != num_online_cpus())
> 		struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
>
> 		cpumask_and(cpus, sched_group_span(local), p->cpus_ptr);
> 		imb = min(cpumask_weight(cpus), imb);
> 	}
Can we optimize this further as:

	imb = sd->imb_numa_nr;
	if (unlikely(p->nr_cpus_allowed != num_online_cpus()))
		struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);

		cpumask_and(cpus, sched_group_span(local), p->cpus_ptr);
		imb = min(cpumask_weight(cpus), imb);
	}

As for most part, p->nr_cpus_allowed will be equal to num_online_cpus()
unless user has specifically pinned the task.

--
Thanks and Regards,
Prateek