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Date:   Thu, 11 Jul 2019 16:27:28 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     =?utf-8?B?546L6LSH?= <yun.wang@linux.alibaba.com>
Cc:     hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, mcgrof@kernel.org, keescook@chromium.org,
        linux-fsdevel@vger.kernel.org, cgroups@vger.kernel.org,
        Mel Gorman <mgorman@suse.de>, riel@surriel.com
Subject: Re: [PATCH 4/4] numa: introduce numa cling feature
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On Wed, Jul 03, 2019 at 11:34:16AM +0800, 王贇 wrote:
> Although we paid so many effort to settle down task on a particular
> node, there are still chances for a task to leave it's preferred
> node, that is by wakeup, numa swap migrations or load balance.
> 
> When we are using cpu cgroup in share way, since all the workloads
> see all the cpus, it could be really bad especially when there
> are too many fast wakeup, although now we can numa group the tasks,
> they won't really stay on the same node, for example we have numa
> group ng_A, ng_B, ng_C, ng_D, it's very likely result as:
> 
> 	CPU Usage:
> 		Node 0		Node 1
> 		ng_A(600%)	ng_A(400%)
> 		ng_B(400%)	ng_B(600%)
> 		ng_C(400%)	ng_C(600%)
> 		ng_D(600%)	ng_D(400%)
> 
> 	Memory Ratio:
> 		Node 0		Node 1
> 		ng_A(60%)	ng_A(40%)
> 		ng_B(40%)	ng_B(60%)
> 		ng_C(40%)	ng_C(60%)
> 		ng_D(60%)	ng_D(40%)
> 
> Locality won't be too bad but far from the best situation, we want
> a numa group to settle down thoroughly on a particular node, with
> every thing balanced.
> 
> Thus we introduce the numa cling, which try to prevent tasks leaving
> the preferred node on wakeup fast path.


> @@ -6195,6 +6447,13 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  	if ((unsigned)i < nr_cpumask_bits)
>  		return i;
> 
> +	/*
> +	 * Failed to find an idle cpu, wake affine may want to pull but
> +	 * try stay on prev-cpu when the task cling to it.
> +	 */
> +	if (task_numa_cling(p, cpu_to_node(prev), cpu_to_node(target)))
> +		return prev;
> +
>  	return target;
>  }

Select idle sibling should never cross node boundaries and is thus the
entirely wrong place to fix anything.