linux-kernel - Re: [PATCH] mm: fix draining remote pageset

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Message-ID: <1f21c7bd-4bba-e72a-c998-0e11fda8a5b1@os.amperecomputing.com>
Date:   Wed, 16 Aug 2023 13:23:00 -0700 (PDT)
From:   "Lameter, Christopher" <cl@...amperecomputing.com>
To:     "Huang, Ying" <ying.huang@...el.com>
cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm: fix draining remote pageset

On Wed, 16 Aug 2023, Huang, Ying wrote:

> Michal Hocko <mhocko@...e.com> writes:
>>>> Why is that a problem?
>>>
>>> The pages of the remote zone may be kept in the local per-CPU pageset
>>> for long time as long as there's no page allocation/freeing on the
>>> logical CPU.  In addition to the logical CPU goes idle, this is also
>>> possible if the logical CPU is busy in the user space.
>>
>> But why is this a problem? Is the scale of the problem sufficient to
>> trigger out of memory situations or be otherwise harmful?
>
> This may trigger premature page reclaiming.  The pages in the PCP of the
> remote zone would have been freed to satisfy the page allocation for the
> remote zone to avoid page reclaiming.  It's highly possible that the
> local CPU just allocate/free from/to the remote zone temporarily.  So,
> we should free PCP pages of the remote zone if there is no page
> allocation/freeing from/to the remote zone for 3 seconds.
>
> This will not trigger OOM, because all PCP will be drained if allocation
> failed after direct reclaiming.

I think this is a minor issue but its cleaner behavior. 
vmstat refresh's should continue as long as there are 
pages queued in remote pcps.