linux-kernel - Re: [PATCH V3 3/3] mm: page_alloc: drain pcp lists before oom kill

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Message-ID: <ZVNQdQKQAMjgOK9y@tiehlicka>
Date:   Tue, 14 Nov 2023 11:48:21 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Charan Teja Kalla <quic_charante@...cinc.com>
Cc:     akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        david@...hat.com, vbabka@...e.cz, hannes@...xchg.org,
        quic_pkondeti@...cinc.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 3/3] mm: page_alloc: drain pcp lists before oom kill

On Fri 10-11-23 22:06:22, Charan Teja Kalla wrote:
> Thanks Michal!!
> 
> On 11/9/2023 4:03 PM, Michal Hocko wrote:
> >> VM system running with ~50MB of memory shown the below stats during OOM
> >> kill:
> >> Normal free:760kB boost:0kB min:768kB low:960kB high:1152kB
> >> reserved_highatomic:0KB managed:49152kB free_pcp:460kB
> >>
> >> Though in such system state OOM kill is imminent, but the current kill
> >> could have been delayed if the pcp is drained as pcp + free is even
> >> above the high watermark.
> > TBH I am not sure this is really worth it. Does it really reduce the
> > risk of the OOM in any practical situation?
> 
> At least in my particular stress test case it just delayed the OOM as i
> can see that at the time of OOM kill, there are no free pcp pages. My
> understanding of the OOM is that it should be the last resort and only
> after doing the enough reclaim retries. CMIW here.

Yes it is a last resort but it is a heuristic as well. So the real
questoin is whether this makes any practical difference outside of
artificial workloads. I do not see anything particularly worrying to
drain the pcp cache but it should be noted that this won't be 100%
either as racing freeing of memory will end up on pcp lists first.

-- 
Michal Hocko
SUSE Labs