linux-kernel - Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during memory offline

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Date:   Fri, 4 Sep 2020 08:32:54 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Pavel Tatashin <pasha.tatashin@...een.com>,
        David Hildenbrand <david@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during
 memory offline

On 9/3/20 8:23 PM, Pavel Tatashin wrote:
>>
>> As expressed in reply to v2, I dislike this hack. There is strong
>> synchronization, just PCP is special. Allocating from MIGRATE_ISOLATE is
>> just plain ugly.
>>
>> Can't we temporarily disable PCP (while some pageblock in the zone is
>> isolated, which we know e.g., due to the counter), so no new pages get
>> put into PCP lists after draining, and re-enable after no pageblocks are
>> isolated again? We keep draining the PCP, so it doesn't seem to be of a
>> lot of use during that period, no? It's a performance hit already.
>>
>> Then, we would only need exactly one drain. And we would only have to
>> check on the free path whether PCP is temporarily disabled.
> 
> Hm, we could use a static branches to disable it, that would keep
> release code just as fast, but I am worried it will make code even
> uglier. Let's see what others in this thread think about this idea.

Maybe we could just set pcp->high = 0 or something, make sure the
pcplist user only reads this value while irqs are disabled. Then the the
IPI in drain_all_pages() should guarantee there's nobody racing freeing
to pcplist. But careful to not introduce bugs like the one fixed with
[1]. And not sure if this guarantee survives when RT comes and replaces
the disabled irq's with local_lock or something.

[1]
https://lore.kernel.org/linux-mm/1597150703-19003-1-git-send-email-charante@codeaurora.org/


> Thank you,
> Pasha
>