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SubjectRe: [PATCH 6/6] mm/page_alloc: Remotely drain per-cpu lists
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 ;)

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