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Date: Thu, 3 Mar 2022 14:27:45 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Nicolas Saenz Julienne <nsaenzju@...hat.com>,
akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
frederic@...nel.org, tglx@...utronix.de, mtosatti@...hat.com,
mgorman@...e.de, linux-rt-users@...r.kernel.org, cl@...ux.com,
paulmck@...nel.org, willy@...radead.org
Subject: Re: [PATCH 0/2] mm/page_alloc: Remote per-cpu lists drain support
On 2/8/22 11:07, 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.
>
> The new algorithm will atomically switch the pointer to the per-cpu page lists
> and use RCU to make sure it's not being concurrently used before draining the
> lists. And its main benefit of is that it fixes the issue for good, avoiding
> the need for configuration based heuristics or having to modify applications
> (i.e. using the isolation prctrl being worked by Marcello Tosatti ATM).
>
> All this with minimal performance implications: a page allocation
> microbenchmark was run on multiple systems and architectures generally showing
> no performance differences, only the more extreme cases showed a 1-3%
> degradation. See data below. Needless to say that I'd appreciate if someone
> could validate my values independently.
>
> The approach has been stress-tested: I forced 100 drains/s while running
> mmtests' pft in a loop for a full day on multiple machines and archs (arm64,
> x86_64, ppc64le).
>
> 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 completeness, what was the fate of the approach to have pcp->high = 0
for NOHZ cpus? [1] It would be nice to have documented why it wasn't
feasible. Too much overhead for when these CPUs eventually do allocate, or
some other unforeseen issue? Thanks.
[1]
https://lore.kernel.org/all/43462fe11258395f4e885c3d594a3ed1b604b858.camel@redhat.com/
> Previous RFC: https://lkml.org/lkml/2021/10/8/793
>
> Thanks!
>
> [1] https://lkml.org/lkml/2021/9/21/599
> [2] https://lkml.org/lkml/2021/11/3/644
>
> ---
>
> Changes since RFC:
> - Get rid of aesthetic changes that affected performance
> - Add more documentation
> - Add better commit messages
> - Pass sparse tests
> - Verify this_cpu_*() usage
> - Performance measurements
>
> Nicolas Saenz Julienne (2):
> mm/page_alloc: Access lists in 'struct per_cpu_pages' indirectly
> mm/page_alloc: Add remote draining support to per-cpu lists
>
> include/linux/mmzone.h | 28 +++++-
> mm/page_alloc.c | 212 ++++++++++++++++++++++++++---------------
> mm/vmstat.c | 6 +-
> 3 files changed, 162 insertions(+), 84 deletions(-)
>
>
> -------------------------Performance results-----------------------------
>
> I'm focusing on mmtests' Page Fault Test (pft), as it's page allocator
> intensive.
>
> - AMD Daytona Reference System, 2 sockets, AMD EPYC 7742, Zen 2, 64-Core,
> 4 NUMA nodes, x86_64
>
> pft timings:
> vanilla rcu
> Amean system-1 58.52 ( 0.00%) 58.92 * -0.68%*
> Amean system-4 61.00 ( 0.00%) 61.41 * -0.67%*
> Amean system-7 61.55 ( 0.00%) 61.74 * -0.30%*
> Amean system-12 64.91 ( 0.00%) 64.94 * -0.05%*
> Amean system-21 98.80 ( 0.00%) 99.92 * -1.13%*
> Amean system-30 147.68 ( 0.00%) 145.83 * 1.25%*
> Amean system-48 237.04 ( 0.00%) 241.29 * -1.79%*
> Amean system-79 286.61 ( 0.00%) 283.72 * 1.01%*
> Amean system-110 303.40 ( 0.00%) 299.91 * 1.15%*
> Amean system-128 345.07 ( 0.00%) 342.10 * 0.86%*
> Amean elapsed-1 61.21 ( 0.00%) 61.65 * -0.71%*
> Amean elapsed-4 15.94 ( 0.00%) 16.05 * -0.69%*
> Amean elapsed-7 9.24 ( 0.00%) 9.28 * -0.47%*
> Amean elapsed-12 5.70 ( 0.00%) 5.70 * -0.07%*
> Amean elapsed-21 5.11 ( 0.00%) 5.06 * 1.13%*
> Amean elapsed-30 5.28 ( 0.00%) 5.14 * 2.73%*
> Amean elapsed-48 5.28 ( 0.00%) 5.24 * 0.74%*
> Amean elapsed-79 4.41 ( 0.00%) 4.31 * 2.17%*
> Amean elapsed-110 3.45 ( 0.00%) 3.44 * 0.40%*
> Amean elapsed-128 2.75 ( 0.00%) 2.75 * -0.28%*
>
> - AMD Speedway Reference System, 2 sockets, AMD EPYC 7601, Zen 1, 64-core, 8
> NUMA nodes, x86_64. Lots of variance between tests on this platform. It'll
> easily swing -+5% on each result.
>
> pft timings:
> vanilla rcu
> Amean system-1 69.20 ( 0.00%) 66.21 * 4.32%*
> Amean system-4 70.79 ( 0.00%) 69.01 * 2.52%*
> Amean system-7 71.34 ( 0.00%) 69.16 * 3.05%*
> Amean system-12 74.00 ( 0.00%) 72.74 * 1.70%*
> Amean system-21 86.01 ( 0.00%) 85.70 * 0.36%*
> Amean system-30 89.21 ( 0.00%) 89.93 * -0.80%*
> Amean system-48 92.39 ( 0.00%) 92.43 * -0.04%*
> Amean system-79 120.19 ( 0.00%) 121.30 * -0.92%*
> Amean system-110 172.79 ( 0.00%) 179.37 * -3.81%*
> Amean system-128 201.70 ( 0.00%) 212.57 * -5.39%*
> Amean elapsed-1 72.23 ( 0.00%) 69.29 * 4.08%*
> Amean elapsed-4 18.69 ( 0.00%) 18.28 * 2.20%*
> Amean elapsed-7 10.80 ( 0.00%) 10.54 * 2.41%*
> Amean elapsed-12 6.62 ( 0.00%) 6.53 * 1.30%*
> Amean elapsed-21 4.68 ( 0.00%) 4.69 * -0.14%*
> Amean elapsed-30 3.44 ( 0.00%) 3.50 * -1.66%*
> Amean elapsed-48 2.40 ( 0.00%) 2.42 * -1.00%*
> Amean elapsed-79 2.05 ( 0.00%) 2.09 * -1.90%*
> Amean elapsed-110 1.83 ( 0.00%) 1.91 * -4.60%*
> Amean elapsed-128 1.75 ( 0.00%) 1.85 * -5.99%*
>
> - IBM 9006-22C system, 2 sockets, POWER9, 64-Core, 1 NUMA node per cpu,
> pppc64le.
>
> pft timings:
> vanilla rcu
> Amean system-1 1.82 ( 0.00%) 1.85 * -1.43%*
> Amean system-4 2.18 ( 0.00%) 2.22 * -2.02%*
> Amean system-7 3.27 ( 0.00%) 3.28 * -0.15%*
> Amean system-12 5.22 ( 0.00%) 5.20 * 0.26%*
> Amean system-21 10.10 ( 0.00%) 10.20 * -1.00%*
> Amean system-30 15.00 ( 0.00%) 14.52 * 3.20%*
> Amean system-48 26.41 ( 0.00%) 25.96 * 1.71%*
> Amean system-79 29.35 ( 0.00%) 29.70 * -1.21%*
> Amean system-110 24.01 ( 0.00%) 23.40 * 2.54%*
> Amean system-128 24.57 ( 0.00%) 25.32 * -3.06%*
> Amean elapsed-1 1.85 ( 0.00%) 1.87 * -1.28%*
> Amean elapsed-4 0.56 ( 0.00%) 0.57 * -1.72%*
> Amean elapsed-7 0.51 ( 0.00%) 0.50 * 0.07%*
> Amean elapsed-12 0.51 ( 0.00%) 0.51 * 0.06%*
> Amean elapsed-21 0.54 ( 0.00%) 0.54 * 0.06%*
> Amean elapsed-30 0.54 ( 0.00%) 0.53 * 2.22%*
> Amean elapsed-48 0.58 ( 0.00%) 0.57 * 1.73%*
> Amean elapsed-79 0.49 ( 0.00%) 0.48 * 0.89%*
> Amean elapsed-110 0.37 ( 0.00%) 0.37 * -1.08%*
> Amean elapsed-128 0.33 ( 0.00%) 0.33 * 0.00%*
>
> - Ampere MtSnow, 1 socket, Neoverse-N1, 80-Cores, 1 NUMA node, arm64.
>
> pft timings:
> vanilla rcu
> Amean system-1 11.92 ( 0.00%) 11.99 * -0.61%*
> Amean system-4 13.13 ( 0.00%) 13.09 * 0.31%*
> Amean system-7 13.91 ( 0.00%) 13.94 * -0.20%*
> Amean system-12 15.77 ( 0.00%) 15.69 * 0.48%*
> Amean system-21 21.32 ( 0.00%) 21.42 * -0.46%*
> Amean system-30 28.58 ( 0.00%) 29.12 * -1.90%*
> Amean system-48 47.41 ( 0.00%) 46.91 * 1.04%*
> Amean system-79 76.76 ( 0.00%) 77.16 * -0.52%*
> Amean system-80 77.98 ( 0.00%) 78.23 * -0.32%*
> Amean elapsed-1 12.46 ( 0.00%) 12.53 * -0.58%*
> Amean elapsed-4 3.47 ( 0.00%) 3.46 * 0.34%*
> Amean elapsed-7 2.18 ( 0.00%) 2.21 * -1.58%*
> Amean elapsed-12 1.41 ( 0.00%) 1.42 * -0.80%*
> Amean elapsed-21 1.09 ( 0.00%) 1.12 * -2.60%*
> Amean elapsed-30 0.98 ( 0.00%) 1.01 * -3.08%*
> Amean elapsed-48 1.08 ( 0.00%) 1.10 * -1.78%*
> Amean elapsed-79 1.32 ( 0.00%) 1.28 * 2.71%*
> Amean elapsed-80 1.32 ( 0.00%) 1.28 * 3.23%*
>
> - Dell R430, 2 sockets, Intel Xeon E5-2640 v3, Sandy Bridge, 16-Cores, 2 NUMA
> nodes, x86_64.
>
> pft timings:
> vanilla rcu
> Amean system-1 11.10 ( 0.00%) 11.07 * 0.24%*
> Amean system-3 11.14 ( 0.00%) 11.10 * 0.34%*
> Amean system-5 11.18 ( 0.00%) 11.13 * 0.47%*
> Amean system-7 11.21 ( 0.00%) 11.17 * 0.38%*
> Amean system-12 11.28 ( 0.00%) 11.28 ( -0.03%)
> Amean system-18 13.24 ( 0.00%) 13.25 * -0.11%*
> Amean system-24 17.12 ( 0.00%) 17.14 ( -0.13%)
> Amean system-30 21.10 ( 0.00%) 21.23 * -0.60%*
> Amean system-32 22.31 ( 0.00%) 22.47 * -0.71%*
> Amean elapsed-1 11.76 ( 0.00%) 11.73 * 0.29%*
> Amean elapsed-3 3.93 ( 0.00%) 3.93 * 0.17%*
> Amean elapsed-5 2.39 ( 0.00%) 2.37 * 0.74%*
> Amean elapsed-7 1.72 ( 0.00%) 1.71 * 0.81%*
> Amean elapsed-12 1.02 ( 0.00%) 1.03 ( -0.42%)
> Amean elapsed-18 1.13 ( 0.00%) 1.14 ( -0.18%)
> Amean elapsed-24 0.87 ( 0.00%) 0.88 * -0.65%*
> Amean elapsed-30 0.77 ( 0.00%) 0.78 * -0.86%*
> Amean elapsed-32 0.74 ( 0.00%) 0.74 ( 0.00%)
>
> - HPE Apollo 70, 2 sockets, Cavium ThunderX2, 128-Cores, 2 NUMA nodes, arm64.
> NOTE: The test here only goes up to 128 for some reason, although there are
> 256 CPUs. Maybe a mmtests issue? I didn't investigate.
>
> pft timings:
> vanilla rcu
> Amean system-1 4.42 ( 0.00%) 4.36 * 1.29%*
> Amean system-4 4.56 ( 0.00%) 4.51 * 1.05%*
> Amean system-7 4.63 ( 0.00%) 4.65 * -0.42%*
> Amean system-12 5.96 ( 0.00%) 6.02 * -1.00%*
> Amean system-21 10.97 ( 0.00%) 11.01 * -0.32%*
> Amean system-30 16.01 ( 0.00%) 16.04 * -0.19%*
> Amean system-48 26.81 ( 0.00%) 26.78 * 0.09%*
> Amean system-79 30.80 ( 0.00%) 30.85 * -0.16%*
> Amean system-110 31.87 ( 0.00%) 31.93 * -0.19%*
> Amean system-128 36.27 ( 0.00%) 36.31 * -0.10%*
> Amean elapsed-1 4.88 ( 0.00%) 4.85 * 0.60%*
> Amean elapsed-4 1.27 ( 0.00%) 1.26 * 1.00%*
> Amean elapsed-7 0.73 ( 0.00%) 0.74 * -0.46%*
> Amean elapsed-12 0.55 ( 0.00%) 0.55 * 1.09%*
> Amean elapsed-21 0.59 ( 0.00%) 0.60 * -0.96%*
> Amean elapsed-30 0.60 ( 0.00%) 0.60 * 0.28%*
> Amean elapsed-48 0.60 ( 0.00%) 0.60 * 0.44%*
> Amean elapsed-79 0.49 ( 0.00%) 0.49 * -0.07%*
> Amean elapsed-110 0.36 ( 0.00%) 0.36 * 0.28%*
> Amean elapsed-128 0.31 ( 0.00%) 0.31 * -0.43%*
>
> - Raspberry Pi 4, 1 socket, bcm2711, Cortex-A72, 4-Cores, 1 NUMA node, arm64.
>
> pft timings:
> vanilla rcu
> Amean system-1 0.67 ( 0.00%) 0.67 * -1.25%*
> Amean system-3 1.30 ( 0.00%) 1.29 * 0.62%*
> Amean system-4 1.61 ( 0.00%) 1.59 * 0.95%*
> Amean elapsed-1 0.71 ( 0.00%) 0.72 * -1.17%*
> Amean elapsed-3 0.45 ( 0.00%) 0.45 * 0.88%*
> Amean elapsed-4 0.42 ( 0.00%) 0.42 * 1.19%*
>
>
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