linux-kernel - Re: [patch] mm, oom: stop reclaiming if GFP

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Message-ID: <94f9b716-b251-79d8-2c8c-70d63a255496@sony.com>
Date:   Wed, 29 Apr 2020 10:31:41 +0200
From:   peter enderborg <peter.enderborg@...y.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [patch] mm, oom: stop reclaiming if GFP_ATOMIC will start failing
 soon

On 4/28/20 9:43 AM, Michal Hocko wrote:
> On Mon 27-04-20 16:35:58, Andrew Morton wrote:
> [...]
>> No consumer of GFP_ATOMIC memory should consume an unbounded amount of
>> it.
>> Subsystems such as networking will consume a certain amount and
>> will then start recycling it.  The total amount in-flight will vary
>> over the longer term as workloads change.  A dynamically tuning
>> threshold system will need to adapt rapidly enough to sudden load
>> shifts, which might require unreasonable amounts of headroom.
> I do agree. __GFP_HIGH/__GFP_ATOMIC are bound by the size of the
> reserves under memory pressure. Then allocatios start failing very
> quickly and users have to cope with that, usually by deferring to a
> sleepable context. Tuning reserves dynamically for heavy reserves
> consumers would be possible but I am worried that this is far from
> trivial.
>
> We definitely need to understand what is going on here.  Why doesn't
> kswapd + N*direct reclaimers do not provide enough memory to satisfy
> both N threads + reserves consumers? How many times those direct
> reclaimers have to retry?

Was this not supposed to be avoided with PSI, user-space should
a fair change to take actions before it goes bad in user-space?


> We used to have the allocation stall warning as David mentioned in the
> patch description and I have seen it triggering without heavy reserves
> consumers (aka reported free pages corresponded to the min watermark).
> The underlying problem was usually kswapd being stuck on some FS locks,
> direct reclaimers stuck in shrinkers or way too overloaded system with
> dozens if not hundreds of processes stuck in the page allocator each
> racing with the reclaim and betting on luck. The last problem was the
> most annoying because it is really hard to tune for.