linux-kernel - Re: [PATCH v2 3/3] mm/page_alloc: Remotely drain per-cpu lists

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Date:   Fri, 3 Dec 2021 14:13:06 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, frederic@...nel.org, tglx@...utronix.de,
        peterz@...radead.org, mtosatti@...hat.com, nilal@...hat.com,
        linux-rt-users@...r.kernel.org, vbabka@...e.cz, cl@...ux.com,
        ppandit@...hat.com
Subject: Re: [PATCH v2 3/3] mm/page_alloc: Remotely drain per-cpu lists

On Wed, Nov 03, 2021 at 06:05:12PM +0100, Nicolas Saenz Julienne wrote:
> Some setups, notably NOHZ_FULL CPUs, are too busy to handle the per-cpu
> drain work queued by __drain_all_pages(). So introduce new a 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.
> 

netperf and hackbench are not great indicators of page allocator
performance as IIRC they are more slab-intensive than page allocator
intensive. I ran the series through a few benchmarks and can confirm
that there was negligible difference to netperf and hackbench.

However, on Page Fault Test (pft in mmtests), it is noticable. On a
2-socket cascadelake machine I get

pft timings
                                 5.16.0-rc1             5.16.0-rc1
                                    vanilla    mm-remotedrain-v2r1
Amean     system-1         27.48 (   0.00%)       27.85 *  -1.35%*
Amean     system-4         28.65 (   0.00%)       30.84 *  -7.65%*
Amean     system-7         28.70 (   0.00%)       32.43 * -13.00%*
Amean     system-12        30.33 (   0.00%)       34.21 * -12.80%*
Amean     system-21        37.14 (   0.00%)       41.51 * -11.76%*
Amean     system-30        36.79 (   0.00%)       46.15 * -25.43%*
Amean     system-48        58.95 (   0.00%)       65.28 * -10.73%*
Amean     system-79       111.61 (   0.00%)      114.78 *  -2.84%*
Amean     system-80       113.59 (   0.00%)      116.73 *  -2.77%*
Amean     elapsed-1        32.83 (   0.00%)       33.12 *  -0.88%*
Amean     elapsed-4         8.60 (   0.00%)        9.17 *  -6.66%*
Amean     elapsed-7         4.97 (   0.00%)        5.53 * -11.30%*
Amean     elapsed-12        3.08 (   0.00%)        3.43 * -11.41%*
Amean     elapsed-21        2.19 (   0.00%)        2.41 * -10.06%*
Amean     elapsed-30        1.73 (   0.00%)        2.04 * -17.87%*
Amean     elapsed-48        1.73 (   0.00%)        2.03 * -17.77%*
Amean     elapsed-79        1.61 (   0.00%)        1.64 *  -1.90%*
Amean     elapsed-80        1.60 (   0.00%)        1.64 *  -2.50%*

It's not specific to cascade lake, I see varying size regressions on
different Intel and AMD chips, some better and worse than this result.
The smallest regression was on a single CPU skylake machine with a 2-6%
hit. Worst was Zen1 with a 3-107% hit.

I didn't profile it to establish why but in all cases the system CPU
usage was much higher. It *might* be because the spinlock in
per_cpu_pages crosses a new cache line and it might be cold although the
penalty seems a bit high for that to be the only factor.

Code-wise, the patches look fine but the apparent penalty for PFT is
too severe.

-- 
Mel Gorman
SUSE Labs