Re: [PATCH 0/2] mm/page_alloc: Remote per-cpu lists drain support

[Mel Gorman]

On Tue, Feb 08, 2022 at 11:07:48AM +0100, Nicolas Saenz Julienne wrote:
> This series replaces mm/page_alloc's per-cpu page lists drain mechanism with
> one that allows accessing the lists remotely. Currently, only a local CPU is
> permitted to change its per-cpu lists, and it's expected to do so, on-demand,
> whenever a process demands it by means of queueing a drain task on the local
> CPU. This causes problems for NOHZ_FULL CPUs and real-time systems that can't
> take any sort of interruption and to some lesser extent inconveniences idle and
> virtualised systems.
> 

I know this has been sitting here for a long while. Last few weeks have
not been fun.

> Note that this is not the first attempt at fixing this per-cpu page lists:
>  - The first attempt[1] tried to conditionally change the pagesets locking
>    scheme based the NOHZ_FULL config. It was deemed hard to maintain as the
>    NOHZ_FULL code path would be rarely tested. Also, this only solves the issue
>    for NOHZ_FULL setups, which isn't ideal.
>  - The second[2] unanimously switched the local_locks to per-cpu spinlocks. The
>    performance degradation was too big.
> 

For unrelated reasons I looked at using llist to avoid locks entirely. It
turns out it's not possible and needs a lock. We know "local_locks to
per-cpu spinlocks" took a large penalty so I considered alternatives on
how a lock could be used.  I found it's possible to both remote drain
the lists and avoid the disable/enable of IRQs entirely as long as a
preempting IRQ is willing to take the zone lock instead (should be very
rare). The IRQ part is a bit hairy though as softirqs are also a problem
and preempt-rt needs different rules and the llist has to sort PCP
refills which might be a loss in total. However, the remote draining may
still be interesting. The full series is at
https://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git/ mm-pcpllist-v1r2

It's still waiting on tests to complete and not all the changelogs are
complete which is why it's not posted.

This is a comparison of vanilla vs "local_locks to per-cpu spinlocks"
versus the git series up to "mm/page_alloc: Remotely drain per-cpu lists"
for the page faulting microbench I originally complained about.  The test
machine is a 2-socket CascadeLake machine.

pft timings
                                 5.17.0-rc5             5.17.0-rc5             5.17.0-rc5
                                    vanilla    mm-remotedrain-v2r1       mm-pcpdrain-v1r1
Amean     elapsed-1        32.54 (   0.00%)       33.08 *  -1.66%*       32.82 *  -0.86%*
Amean     elapsed-4         8.66 (   0.00%)        9.24 *  -6.72%*        8.69 *  -0.38%*
Amean     elapsed-7         5.02 (   0.00%)        5.43 *  -8.16%*        5.05 *  -0.55%*
Amean     elapsed-12        3.07 (   0.00%)        3.38 * -10.00%*        3.09 *  -0.72%*
Amean     elapsed-21        2.36 (   0.00%)        2.38 *  -0.89%*        2.19 *   7.39%*
Amean     elapsed-30        1.75 (   0.00%)        1.87 *  -6.50%*        1.62 *   7.59%*
Amean     elapsed-48        1.71 (   0.00%)        2.00 * -17.32%*        1.71 (  -0.08%)
Amean     elapsed-79        1.56 (   0.00%)        1.62 *  -3.84%*        1.56 (  -0.02%)
Amean     elapsed-80        1.57 (   0.00%)        1.65 *  -5.31%*        1.57 (  -0.04%)

Note the local_lock conversion took 1 1-17% penalty while the git tree
takes a negligile penalty while still allowing remote drains. It might
have some potential while being less complex than the RCU approach.

-- 
Mel Gorman
SUSE Labs