linux-kernel - Re: [PATCH 3/6] mm/page_alloc: Adjust pcp->high after CPU hotplug events

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Date:   Mon, 24 May 2021 17:01:01 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/6] mm/page_alloc: Adjust pcp->high after CPU hotplug
 events

On Mon, May 24, 2021 at 08:52:02AM -0700, Dave Hansen wrote:
> > To address your point requires much deeper surgery.
> ...
> > There is value to doing something like this but it's beyond what this
> > series is trying to do and doing the work without introducing regressions
> > would be very difficult.
> 
> Agreed, such a solution is outside of the scope of what this set is
> trying to do.
> 
> It would be nice to touch on this counter-intuitive property in the
> changelog, and *maybe* add a WARN_ON_ONCE() if we hit an edge case.
> Maybe WARN_ON_ONCE() if pcp->high gets below pcp->batch*SOMETHING.
> 

I think it's reasonable to ensure pcp->batch is never above pcp->high so
I have this in zone_highsize now

+       /*
+        * Ensure high is at least batch*4. The multiple is based on the
+        * historical relationship between high and batch.
+        */
+       high = max(high, batch << 2);

Performance tests are running and I'll post a v2 assuming they pass.

-- 
Mel Gorman
SUSE Labs