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Subject: Re: [PATCH 4/4] numa: introduce numa cling feature
To:     Peter Zijlstra <peterz@infradead.org>
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
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From:   =?UTF-8?B?546L6LSH?= <yun.wang@linux.alibaba.com>
Message-ID: <82f42063-ce51-dd34-ba95-5b32ee733de7@linux.alibaba.com>
Date:   Fri, 12 Jul 2019 11:10:08 +0800
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On 2019/7/11 下午10:27, Peter Zijlstra wrote:
[snip]
>> 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.

Hmm.. in our early testing the printk show both select_task_rq_fair() and
task_numa_find_cpu() will call select_idle_sibling with prev and target on
different node, thus we pick this point to save few lines.

But if the semantics of select_idle_sibling() is to return cpu on the same
node of target, what about move the logical after select_idle_sibling() for
the two callers?

Regards,
Michael Wang

>