Re: [PATCH 00/10] sched/fair: wake_affine improvements

[Vincent Guittot <vincent.guittot@linaro.org>]

Hi Srikar,

On Thu, 22 Apr 2021 at 12:23, Srikar Dronamraju
<srikar@linux.vnet.ibm.com> wrote:
>
> Recently we found that some of the benchmark numbers on Power10 were lesser
> than expected. Some analysis showed that the problem lies in the fact that
> L2-Cache on Power10 is at core level i.e only 4 threads share the L2-cache.
>
> One probable solution to the problem was worked by Gautham where he posted
> http://lore.kernel.org/lkml/1617341874-1205-1-git-send-email-ego@linux.vnet.ibm.com/t/#u
> a patch that marks MC domain as LLC.
>
> Here the focus is on seeing if we can improve the current core scheduler's
> wakeup mechanism by looking at idle-cores and nr_busy_cpus that is already
> maintained per Last level cache(aka LLC) (first 8 patches) + explore the
> possibility to provide a fallback LLC domain, that can be preferred if the
> current LLC is busy (last 2 patches).
>
> Except the last 2 patches, the rest patches should work independently of the
> other proposed solution. i.e if the mc-llc patch is accepted, then the last
> two patches may not be needed for Power10. However this may be helpful for
> other archs/platforms.
>
> In the fallback approach, we look for a one-to-one mapping for each LLC.
> However this can be easily modified to look for all LLC's in the current
> LLC's parent. Also fallback is only used for sync wakeups. This is because
> that is where we expect the maximum benefit of moving the task closer to the
> task. For non-sync wakeups, its expected that CPU from previous LLC may be
> better off.
>
> Request you to please review and provide your feedback.
>
> Benchmarking numbers are from Power 10 but I have verified that we don't
> regress on Power 9 setup.
>
> # lscpu
> Architecture:        ppc64le
> Byte Order:          Little Endian
> CPU(s):              80
> On-line CPU(s) list: 0-79
> Thread(s) per core:  8
> Core(s) per socket:  10
> Socket(s):           1
> NUMA node(s):        1
> Model:               1.0 (pvr 0080 0100)
> Model name:          POWER10 (architected), altivec supported
> Hypervisor vendor:   pHyp
> Virtualization type: para
> L1d cache:           64K
> L1i cache:           32K
> L2 cache:            256K
> L3 cache:            8K
> NUMA node2 CPU(s):   0-79
>
> Hackbench: (latency, lower is better)
>
> v5.12-rc5
> instances = 1, min = 24.102529 usecs/op, median =  usecs/op, max = 24.102529 usecs/op
> instances = 2, min = 24.096112 usecs/op, median = 24.096112 usecs/op, max = 24.178903 usecs/op
> instances = 4, min = 24.080541 usecs/op, median = 24.082990 usecs/op, max = 24.166873 usecs/op
> instances = 8, min = 24.088969 usecs/op, median = 24.116081 usecs/op, max = 24.199853 usecs/op
> instances = 16, min = 24.267228 usecs/op, median = 26.204510 usecs/op, max = 29.218360 usecs/op
> instances = 32, min = 30.680071 usecs/op, median = 32.911664 usecs/op, max = 37.380470 usecs/op
> instances = 64, min = 43.908331 usecs/op, median = 44.454343 usecs/op, max = 46.210298 usecs/op
> instances = 80, min = 44.585754 usecs/op, median = 56.738546 usecs/op, max = 60.625826 usecs/op
>
> v5.12-rc5 + mc-llc
> instances = 1, min = 18.676505 usecs/op, median =  usecs/op, max = 18.676505 usecs/op
> instances = 2, min = 18.488627 usecs/op, median = 18.488627 usecs/op, max = 18.574946 usecs/op
> instances = 4, min = 18.428399 usecs/op, median = 18.589051 usecs/op, max = 18.872548 usecs/op
> instances = 8, min = 18.597389 usecs/op, median = 18.783815 usecs/op, max = 19.265532 usecs/op
> instances = 16, min = 21.922350 usecs/op, median = 22.737792 usecs/op, max = 24.832429 usecs/op
> instances = 32, min = 29.770446 usecs/op, median = 31.996687 usecs/op, max = 34.053042 usecs/op
> instances = 64, min = 53.067842 usecs/op, median = 53.295139 usecs/op, max = 53.473059 usecs/op
> instances = 80, min = 44.423288 usecs/op, median = 44.713767 usecs/op, max = 45.159761 usecs/op
>
> v5.12-rc5 + this patchset
> instances = 1, min = 19.368805 usecs/op, median =  usecs/op, max = 19.368805 usecs/op
> instances = 2, min = 19.423674 usecs/op, median = 19.423674 usecs/op, max = 19.506203 usecs/op
> instances = 4, min = 19.454523 usecs/op, median = 19.596947 usecs/op, max = 19.863620 usecs/op
> instances = 8, min = 20.005272 usecs/op, median = 20.239924 usecs/op, max = 20.878947 usecs/op
> instances = 16, min = 21.856779 usecs/op, median = 24.102147 usecs/op, max = 25.496110 usecs/op
> instances = 32, min = 31.460159 usecs/op, median = 32.809621 usecs/op, max = 33.939650 usecs/op
> instances = 64, min = 39.506553 usecs/op, median = 43.835221 usecs/op, max = 45.645505 usecs/op
> instances = 80, min = 43.805716 usecs/op, median = 44.314757 usecs/op, max = 48.910236 usecs/op
>
> Summary:
> mc-llc and this patchset seem to be performing much better than vanilla v5.12-rc5
>
> DayTrader (throughput, higher is better)
>                      v5.12-rc5   v5.12-rc5     v5.12-rc5
>                                  + mc-llc      + patchset
> 64CPUs/1JVM/ 60Users  6373.7      7520.5        7232.3
> 64CPUs/1JVM/ 80Users  6742.1      7940.9        7732.8
> 64CPUs/1JVM/100Users  6482.2      7730.3        7540
> 64CPUs/2JVM/ 60Users  6335        8081.6        7914.3
> 64CPUs/2JVM/ 80Users  6360.8      8259.6        8138.6
> 64CPUs/2JVM/100Users  6215.6      8046.5        8039.2
> 64CPUs/4JVM/ 60Users  5385.4      7685.3        7706.1
> 64CPUs/4JVM/ 80Users  5380.8      7753.3        7721.5
> 64CPUs/4JVM/100Users  5275.2      7549.2        7608.3
>
> Summary: Across all profiles, this patchset or mc-llc out perform
> vanilla v5.12-rc5
> Not: Only 64 cores were online during this test.
>
> schbench (latency: lesser is better)
> ======== Running schbench -m 3 -r 30 =================
> Latency percentiles (usec) runtime 10 (s) (2545 total samples)
> v5.12-rc5                  |  v5.12-rc5 + mc-llc                 | v5.12-rc5 + patchset
>
> 50.0th: 56 (1301 samples)  |     50.0th: 49 (1309 samples)       | 50.0th: 50 (1310 samples)
> 75.0th: 76 (623 samples)   |     75.0th: 66 (628 samples)        | 75.0th: 68 (632 samples)
> 90.0th: 93 (371 samples)   |     90.0th: 78 (371 samples)        | 90.0th: 80 (354 samples)
> 95.0th: 107 (123 samples)  |     95.0th: 87 (117 samples)        | 95.0th: 86 (126 samples)
> *99.0th: 12560 (102 samples)    *99.0th: 100 (97 samples)        | *99.0th: 103 (97 samples)
> 99.5th: 15312 (14 samples) |     99.5th: 104 (12 samples)        | 99.5th: 1202 (13 samples)
> 99.9th: 19936 (9 samples)  |     99.9th: 106 (8 samples)         | 99.9th: 14992 (10 samples)
> min=13, max=20684          |     min=15, max=113                 | min=15, max=18721
>
> Latency percentiles (usec) runtime 20 (s) (7649 total samples)
>
> 50.0th: 51 (3884 samples)  |     50.0th: 50 (3935 samples)       | 50.0th: 49 (3841 samples)
> 75.0th: 69 (1859 samples)  |     75.0th: 66 (1817 samples)       | 75.0th: 67 (1965 samples)
> 90.0th: 87 (1173 samples)  |     90.0th: 80 (1204 samples)       | 90.0th: 78 (1134 samples)
> 95.0th: 97 (368 samples)   |     95.0th: 87 (342 samples)        | 95.0th: 83 (359 samples)
> *99.0th: 8624 (290 samples)|     *99.0th: 98 (294 samples)       | *99.0th: 93 (296 samples)
> 99.5th: 11344 (37 samples) |     99.5th: 102 (37 samples)        | 99.5th: 98 (34 samples)
> 99.9th: 18592 (31 samples) |     99.9th: 106 (30 samples)        | 99.9th: 7544 (28 samples)
> min=13, max=20684          |     min=12, max=113                 | min=13, max=18721
>
> Latency percentiles (usec) runtime 30 (s) (12785 total samples)
>
> 50.0th: 50 (6614 samples)  |     50.0th: 49 (6544 samples)       | 50.0th: 48 (6527 samples)
> 75.0th: 67 (3059 samples)  |     75.0th: 65 (3100 samples)       | 75.0th: 64 (3143 samples)
> 90.0th: 84 (1894 samples)  |     90.0th: 79 (1912 samples)       | 90.0th: 76 (1985 samples)
> 95.0th: 94 (586 samples)   |     95.0th: 87 (646 samples)        | 95.0th: 81 (585 samples)
> *99.0th: 8304 (507 samples)|     *99.0th: 101 (496 samples)      | *99.0th: 90 (453 samples)
> 99.5th: 11696 (62 samples) |     99.5th: 104 (45 samples)        | 99.5th: 94 (66 samples)
> 99.9th: 18592 (51 samples) |     99.9th: 110 (51 samples)        | 99.9th: 1202 (49 samples)
> min=12, max=21421          |     min=1, max=126                  | min=3, max=18721
>
> Summary:
> mc-llc is the best option, but this patchset also helps compared to vanilla v5.12-rc5
>
>
> mongodb (threads=6) (throughput, higher is better)
>                                          Throughput         read        clean      update
>                                                             latency     latency    latency
> v5.12-rc5            JVM=YCSB_CLIENTS=14  68116.05 ops/sec   1109.82 us  944.19 us  1342.29 us
> v5.12-rc5            JVM=YCSB_CLIENTS=21  64802.69 ops/sec   1772.64 us  944.69 us  2099.57 us
> v5.12-rc5            JVM=YCSB_CLIENTS=28  61792.78 ops/sec   2490.48 us  930.09 us  2928.03 us
> v5.12-rc5            JVM=YCSB_CLIENTS=35  59604.44 ops/sec   3236.86 us  870.28 us  3787.48 us
>
> v5.12-rc5 + mc-llc   JVM=YCSB_CLIENTS=14  70948.51 ops/sec   1060.21 us  842.02 us  1289.44 us
> v5.12-rc5 + mc-llc   JVM=YCSB_CLIENTS=21  68732.48 ops/sec   1669.91 us  871.57 us  1975.19 us
> v5.12-rc5 + mc-llc   JVM=YCSB_CLIENTS=28  66674.81 ops/sec   2313.79 us  889.59 us  2702.36 us
> v5.12-rc5 + mc-llc   JVM=YCSB_CLIENTS=35  64397.51 ops/sec   3010.66 us  966.28 us  3484.19 us
>
> v5.12-rc5 + patchset JVM=YCSB_CLIENTS=14  67403.29 ops/sec   1121.80 us  797.81 us  1357.28 us
> v5.12-rc5 + patchset JVM=YCSB_CLIENTS=21  63952.79 ops/sec   1792.86 us  779.59 us  2130.54 us
> v5.12-rc5 + patchset JVM=YCSB_CLIENTS=28  62198.83 ops/sec   2469.60 us  780.00 us  2914.48 us
> v5.12-rc5 + patchset JVM=YCSB_CLIENTS=35  60333.81 ops/sec   3192.41 us  822.09 us  3748.24 us
>
> Summary:
> mc-llc outperforms, this patchset and upstream almost give similar performance.

So mc-llc patch seems to be the best approach IMHO. Although the
hemisphere don't share cache, they share enough resources so
cache-snooping is as efficient as sharing cache

>
>
> Cc: LKML <linux-kernel@vger.kernel.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>
> Cc: Parth Shah <parth@linux.ibm.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Rik van Riel <riel@surriel.com>
>
> Srikar Dronamraju (10):
>   sched/fair: Update affine statistics when needed
>   sched/fair: Maintain the identity of idle-core
>   sched/fair: Update idle-core more often
>   sched/fair: Prefer idle CPU to cache affinity
>   sched/fair: Call wake_affine only if necessary
>   sched/idle: Move busy_cpu accounting to idle callback
>   sched/fair: Remove ifdefs in waker_affine_idler_llc
>   sched/fair: Dont iterate if no idle CPUs
>   sched/topology: Introduce fallback LLC
>   powerpc/smp: Add fallback flag to powerpc MC domain
>
>  arch/powerpc/kernel/smp.c      |   7 +-
>  include/linux/sched/sd_flags.h |   7 +
>  include/linux/sched/topology.h |   3 +-
>  kernel/sched/fair.c            | 229 +++++++++++++++++++++++++++------
>  kernel/sched/features.h        |   1 +
>  kernel/sched/idle.c            |  33 ++++-
>  kernel/sched/sched.h           |   6 +
>  kernel/sched/topology.c        |  54 +++++++-
>  8 files changed, 296 insertions(+), 44 deletions(-)
>
> --
> 2.18.2
>