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Date:   Fri, 12 Jul 2019 09:53:18 +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
Message-ID: <20190712075318.GM3402@hirez.programming.kicks-ass.net>
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On Fri, Jul 12, 2019 at 11:10:08AM +0800, 王贇 wrote:
> On 2019/7/11 下午10:27, Peter Zijlstra wrote:

> >> 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 it will never return @prev if it is not in the same cache domain as
@target. See how everything is gated by:

  && cpus_share_cache(x, target)

> 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?

No, that's insane. You don't do select_idle_sibling() to then ignore the
result. You have to change @target before calling select_idle_sibling().