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

[Mel Gorman]

On Thu, Dec 09, 2021 at 02:45:35PM -0300, Marcelo Tosatti wrote:
> On Fri, Dec 03, 2021 at 02:13:06PM +0000, Mel Gorman wrote:
> > 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,
> 
> Have you read Nicolas RCU patches?
> 

I agree with Vlastimil's review on overhead.

I think it would be more straight-forward to disable the pcp allocator for
NOHZ_FULL CPUs like what zone_pcp_disable except for individual CPUs with
care taken to not accidentally re-enable nohz CPus in zone_pcp_enable. The
downside is that there will be a performance penalty if an application
running on a NOHZ_FULL CPU is page allocator intensive for whatever
reason.  However, I guess this is unlikely because if there was a lot
of kernel activity for a NOHZ_FULL CPU, the vmstat shepherd would also
cause interference.

-- 
Mel Gorman
SUSE Labs