linux-kernel - Re: [PATCH] mm: terminate shrink

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Date:   Thu, 7 Dec 2017 23:34:53 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        hannes@...xchg.org, hillf.zj@...baba-inc.com, minchan@...nel.org,
        mgorman@...hsingularity.net, ying.huang@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        timmurray@...gle.com, tkjos@...gle.com
Subject: Re: [PATCH] mm: terminate shrink_slab loop if signal is pending

On 2017/12/07 17:34, Michal Hocko wrote:
> On Wed 06-12-17 11:20:26, Suren Baghdasaryan wrote:
>> Slab shrinkers can be quite time consuming and when signal
>> is pending they can delay handling of the signal. If fatal
>> signal is pending there is no point in shrinking that process
>> since it will be killed anyway. This change checks for pending
>> fatal signals inside shrink_slab loop and if one is detected
>> terminates this loop early.
> 
> This is not enough. You would have to make sure the direct reclaim will
> bail out immeditally which is not at all that simple. We do check fatal
> signals in throttle_direct_reclaim and conditionally in shrink_inactive_list
> so even if you bail out from shrinkers we could still finish the full
> reclaim cycle.
> 
> Besides that shrinkers shouldn't really take very long so this looks
> like it papers over a real bug somewhere else. I am not saying the patch
> is wrong but it would deserve much more details to judge wether this is
> the right way to go for your particular problem.
> 

I wish that normal threads do not invoke direct reclaim operation.
Only dedicated kernel threads (such as filesystem's writeback) invoke
direct reclaim operation. Then, we can implement __GFP_KILLABLE for
normal threads, and hopefully get rid of distinction between GFP_NOIO/
GFP_NOFS/GFP_KERNEL because reclaim (and locking) dependency becomes
simpler.