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

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Date:   Mon, 21 Aug 2023 09:55:27 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm: fix draining remote pageset

On Wed 16-08-23 15:08:23, Huang, Ying wrote:
> Michal Hocko <mhocko@...e.com> writes:
> 
> > On Mon 14-08-23 09:59:51, Huang, Ying wrote:
> >> Hi, Michal,
> >> 
> >> Michal Hocko <mhocko@...e.com> writes:
> >> 
> >> > On Fri 11-08-23 17:08:19, Huang Ying wrote:
> >> >> If there is no memory allocation/freeing in the remote pageset after
> >> >> some time (3 seconds for now), the remote pageset will be drained to
> >> >> avoid memory wastage.
> >> >> 
> >> >> But in the current implementation, vmstat updater worker may not be
> >> >> re-queued when we are waiting for the timeout (pcp->expire != 0) if
> >> >> there are no vmstat changes, for example, when CPU goes idle.
> >> >
> >> > 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.

I am slightly confused here but I suspect by zone you mean remote pcp.
But more importantly is this a concern seen in real workload? Can you
quantify it in some manner? E.g. with this patch we have X more kswapd
scanning or even hit direct reclaim much less often.

> 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.

Well, I would argue this depends a lot. There are workloads which really
like to have CPUs idle and yet they would like to benefit from the
allocator fast path after that CPU goes out of idle because idling is
their power saving opportunity while workloads want to act quickly after
there is something to run.

That being said, we really need some numbers (ideally from real world)
that proves this is not just a theoretical concern.
-- 
Michal Hocko
SUSE Labs