linux-kernel - Re: [PATCH v4] mm/vmscan: more restrictive condition for retry in do_try_to_free

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Message-ID: <20170317145020.GA8106@cmpxchg.org>
Date:   Fri, 17 Mar 2017 10:50:20 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yisheng Xie <xieyisheng1@...wei.com>
Cc:     akpm@...ux-foundation.org, mgorman@...e.de, vbabka@...e.cz,
        mhocko@...e.com, riel@...hat.com, shakeelb@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        guohanjun@...wei.com, qiuxishi@...wei.com
Subject: Re: [PATCH v4] mm/vmscan: more restrictive condition for retry in
 do_try_to_free_pages

On Wed, Mar 15, 2017 at 07:36:48PM +0800, Yisheng Xie wrote:
> By reviewing code, I find that when enter do_try_to_free_pages, the
> may_thrash is always clear, and it will retry shrink zones to tap
> cgroup's reserves memory by setting may_thrash when the former
> shrink_zones reclaim nothing.
> 
> However, when memcg is disabled or on legacy hierarchy, or there do not
> have any memcg protected by low limit, it should not do this useless retry
> at all, for we do not have any cgroup's reserves memory to tap, and we
> have already done hard work but made no progress.
> 
> To avoid this unneeded retrying, add a new field in scan_control named
> memcg_low_protection, set it if there is any memcg protected by low limit
> and only do the retry when memcg_low_protection is set while may_thrash
> is clear.
> 
> Signed-off-by: Yisheng Xie <xieyisheng1@...wei.com>
> Suggested-by: Michal Hocko <mhocko@...nel.org>
> Suggested-by: Shakeel Butt <shakeelb@...gle.com>
> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>

I don't see the point of this patch. It adds more code just to
marginally optimize a near-OOM cold path.