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Date:   Thu, 12 May 2022 12:37:43 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Nicolas Saenz Julienne <nsaenzju@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 6/6] mm/page_alloc: Remotely drain per-cpu lists
Message-Id: <20220512123743.5be26b3ad4413f20d5f46564@linux-foundation.org>
In-Reply-To: <20220512085043.5234-7-mgorman@techsingularity.net>
References: <20220512085043.5234-1-mgorman@techsingularity.net>
        <20220512085043.5234-7-mgorman@techsingularity.net>
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On Thu, 12 May 2022 09:50:43 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:

> From: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> 
> Some setups, notably NOHZ_FULL CPUs, are too busy to handle the per-cpu
> drain work queued by __drain_all_pages(). So introduce a new mechanism to
> remotely drain the per-cpu lists. It is made possible by remotely locking
> 'struct per_cpu_pages' new per-cpu spinlocks. A benefit of this new scheme
> is that drain operations are now migration safe.
> 
> There was no observed performance degradation vs. the previous scheme.
> Both netperf and hackbench were run in parallel to triggering the
> __drain_all_pages(NULL, true) code path around ~100 times per second.
> The new scheme performs a bit better (~5%), although the important point
> here is there are no performance regressions vs. the previous mechanism.
> Per-cpu lists draining happens only in slow paths.
> 
> Minchan Kim tested this independently and reported;
> 
> 	My workload is not NOHZ CPUs but run apps under heavy memory
> 	pressure so they goes to direct reclaim and be stuck on
> 	drain_all_pages until work on workqueue run.
> 
> 	unit: nanosecond
> 	max(dur)        avg(dur)                count(dur)
> 	166713013       487511.77786438033      1283
> 
> 	From traces, system encountered the drain_all_pages 1283 times and
> 	worst case was 166ms and avg was 487us.
> 
> 	The other problem was alloc_contig_range in CMA. The PCP draining
> 	takes several hundred millisecond sometimes though there is no
> 	memory pressure or a few of pages to be migrated out but CPU were
> 	fully booked.
> 
> 	Your patch perfectly removed those wasted time.

I'm not getting a sense here of the overall effect upon userspace
performance.  As Thomas said last year in
https://lkml.kernel.org/r/87v92sgt3n.ffs@tglx

: The changelogs and the cover letter have a distinct void vs. that which
: means this is just another example of 'scratch my itch' changes w/o
: proper justification.

Is there more to all of this than itchiness and if so, well, you know
the rest ;)