Re: [RFC PATCH v4 0/2] sched/fair: Choose the CPU where short task is running during wake up
From: Yicong Yang
Date: Mon Jan 16 2023 - 21:57:35 EST
Hi Chen Yu,
On 2022/12/16 14:08, Chen Yu wrote:
> The main purpose of this change is to avoid too many cross CPU
> wake up when it is unnecessary. The frequent cross CPU wake up
> brings significant damage to some workloads, especially on high
> core count systems.
>
> This patch set inhibits the cross CPU wake-up by placing the wakee
> on waking CPU or previous CPU, if both the waker and wakee are
> short-duration tasks.
>
> The first patch is to introduce the definition of a short-duration
> task. The second patch leverages the first patch to choose a local
> or previous CPU for wakee.
>
Tested with tbench and netperf locally and MySQL remotely. Unluckily
cannot reproduce the improvement and see some regression. Detailed
information below. The tests are done based on 6.2-rc1 on a 2x64 CPUs
Kunpeng 920 server with 4 nodes and no SMT, CONFIG_SCHED_CLUSTER
enabled.
For tbench and netperf tested on single node and two nodes on one socket.
tbench(node 0)
vanilla sis_short
1: 326.2950 312.3700 ( -4.27%)
4: 1299.7700 1241.3400 ( -4.50%)
8: 2613.2900 2476.4700 ( -5.24%)
16: 5091.9400 5096.3100 ( 0.09%)
32: 10104.1000 9905.8500 ( -1.96%)
64: 7558.8500 7826.2000 ( 3.54%)
128: 6641.4000 6608.6600 ( -0.49%)
tbench(node 0-1)
vanilla sis_short
1: 328.0880 313.5830 ( -4.42%)
4: 1318.4700 1237.2100 ( -6.16%)
8: 2546.1700 2372.5900 ( -6.82%)
16: 5147.1700 4651.3400 ( -9.63%)
32: 9275.1100 8772.0100 ( -5.42%)
64: 17526.6000 17918.8000 ( 2.23%)
128: 13108.1000 13317.0000 ( 1.59%)
netperf TCP_RR(node 0)
vanilla sis_short
1: 76444.3100 72681.4200 ( -4.92%)
4: 77615.7475 73436.4575 ( -5.38%)
8: 78066.5887 69473.9312 ( -11.01%)
16: 77134.8562 76473.4144 ( -0.86%)
32: 76626.3322 63785.1650 ( -16.76%)
64: 29072.7872 24552.9916 ( -15.55%)
128: 11875.6830 10532.4508 ( -11.31%)
netperf TCP_RR(node 0-1)
vanilla sis_short
1: 77645.3700 72879.3300 ( -6.14%)
4: 78180.0150 72926.2225 ( -6.72%)
8: 77878.7675 66724.6137 ( -14.32%)
16: 77427.6944 65682.6012 ( -15.17%)
32: 71774.5309 71887.1747 ( 0.16%)
64: 75023.3972 62730.7395 ( -16.39%)
128: 28468.2309 25048.1859 ( -12.01%)
netperf UDP_RR(node 0)
vanilla sis_short
1: 91614.6400 89437.6600 ( -2.38%)
4: 91926.3425 90511.9750 ( -1.54%)
8: 92596.4850 87336.7162 ( -5.68%)
16: 91803.2525 95212.5069 ( 3.71%)
32: 98482.9212 79793.1803 ( -18.98%)
64: 36373.5367 30577.2208 ( -15.94%)
128: 14440.2914 12727.7759 ( -11.86%)
netperf UDP_RR(node 0-1)
vanilla sis_short
1: 92609.3400 90915.8600 ( -1.83%)
4: 93034.8425 91742.4825 ( -1.39%)
8: 91963.3950 85801.8250 ( -6.70%)
16: 91589.4900 83339.7944 ( -9.01%)
32: 83161.0234 90298.3934 ( 8.58%)
64: 88860.0155 74933.8544 ( -15.67%)
128: 24715.9842 28022.6998 ( 13.38%)
MySQL tests are done among two servers with same hardware setup and kernel
updated on both kernel. Network device locates on node 0 and network
interrupts is balanced evenly by irqbalance. No NUMA binding applied.
oltp_read_only
8-TPS 4315.11 4236.45 -1.82%
8-QPS 69041.74 67783.19 -1.82%
8-avg-lat 1.85 1.89 -1.98%
8-95th-lat 2.14 2.24 -4.35%
16-TPS 8315.80 7993.78 -3.87%
16-QPS 133052.87 127900.49 -3.87%
16-avg-lat 1.92 2.00 -3.99%
16-95th-lat 2.36 2.60 -10.17%
24-TPS 11601.29 11387.88 -1.84%
24-QPS 185620.63 182206.08 -1.84%
24-avg-lat 2.07 2.11 -1.94%
24-95th-lat 2.61 2.74 -4.97%
32-TPS 13882.59 13277.71 -4.36%
32-QPS 222121.46 212443.37 -4.36%
32-avg-lat 2.31 2.41 -4.48%
32-95th-lat 2.93 3.38 -15.47%
64-TPS 17558.38 16200.13 -7.74%
64-QPS 280934.13 259202.00 -7.74%
64-avg-lat 3.64 3.95 -8.42%
64-95th-lat 5.12 5.40 -5.53%
96-TPS 21858.44 21104.80 -3.45%
96-QPS 349735.03 337676.78 -3.45%
96-avg-lat 4.39 4.55 -3.57%
96-95th-lat 5.92 6.09 -2.93%
128-TPS 24633.09 24278.48 -1.44%
128-QPS 394129.44 388455.76 -1.44%
128-avg-lat 5.19 5.27 -1.54%
128-95th-lat 6.51 6.79 -4.30%
256-TPS 40793.59 41878.29 2.66%
256-QPS 652697.45 670052.64 2.66%
256-avg-lat 6.27 6.11 2.55%
256-95th-lat 9.67 9.79 -1.21%
512-TPS 50461.71 50593.95 0.26%
512-QPS 807387.36 809503.29 0.26%
512-avg-lat 10.13 10.10 0.30%
512-95th-lat 14.64 14.73 -0.61%
700-TPS 50848.04 50899.43 0.10%
700-QPS 813568.68 814390.94 0.10%
700-avg-lat 13.74 13.73 0.10%
700-95th-lat 18.95 19.06 -0.60%
Further looked into 64 threads since it regress most and mainly looked on the
server(mysqld) side. In this case most loads will be on node 0 and partly
on node 1. With SIS_SHORT the utilization on node 0 keeps almost the same
but utilization on node 1 remains nearly half comparing to baseline. That
may partly answers why the performance decreases.
mpstat 60s, non-idle%
vanilla sis_short
node 0 94.25 91.67 -2.73%
node 1 46.41 30.03 -35.29%
Previously there's argument that sync wakeup from interrupts maybe incorrectly
and SIS_SHORT seems to consolidate that and tie more tasks on the interrupt node.
SIS_SHORT will likely to make short tasks wakeup locally but seems to increase the
wait time in this case. schedstat for CPUs on node 0 shows:
vanilla sis_short
time elapsed 60.201085s 60.200395s
[cpu Average, counts/sec]
#0 cpu zeros 0.0 0.0
#1 sched_yied() 0.03 0.12
#2 zeros 0.0 0.0
#3 schedule() 409415.36 340304.86
#4 sched->idle() 125616.92 91078.52
#5 try_to_wake_up() 317404.25 280896.88
#6 try_to_wakeup(local) 115451.87 140567.68
#7 run time 26071494674.87 26164834556.84
#8 wait time 3861592891.75 7361228637.54 (+ 90.63%)
#9 timeslices, 283747.1 249182.34
Since it's a 128 CPU machine the min_granularity is 3000000ns which means a short
task running within 375000ns. Most mysql worker threads' se.dur_avg is around
110000ns and will be regarded as short tasks.
To exclude the influence of Patch 1 (which we'll always record the dur_avg and related
informations), tested 64 threads case with SIS_SHORT enabled or not through debugfs
control and seems Patch 2 mainly dominate the results.
NO_SIS_SHORT SIS_SHORT
TPS 17446.54 16464.83 -5.63%
QPS 279144.61 263437.21 -5.63%
avg lat 3.67 3.89 -5.99%
95th lat 5.09 5.37 -5.50%
Please let me know if you need more information.
Thanks.
> Changes since v3:
> 1. Honglei and Josh have concern that the threshold of short
> task duration could be too long. Decreased the threshold from
> sysctl_sched_min_granularity to (sysctl_sched_min_granularity / 8),
> and the '8' comes from get_update_sysctl_factor().
> 2. Export p->se.dur_avg to /proc/{pid}/sched per Yicong's suggestion.
> 3. Move the calculation of average duration from put_prev_task_fair()
> to dequeue_task_fair(). Because there is an issue in v3 that,
> put_prev_task_fair() will not be invoked by pick_next_task_fair()
> in fast path, thus the dur_avg could not be updated timely.
> 4. Fix the comment in PATCH 2/2, that "WRITE_ONCE(CPU1->ttwu_pending, 1);"
> on CPU0 is earlier than CPU1 getting "ttwu_list->p0", per Tianchen.
> 5. Move the scan for CPU with short duration task from select_idle_cpu()
> to select_idle_siblings(), because there is no CPU scan involved, per
> Yicong.
>
> Changes since v2:
>
> 1. Peter suggested comparing the duration of waker and the cost to
> scan for an idle CPU: If the cost is higher than the task duration,
> do not waste time finding an idle CPU, choose the local or previous
> CPU directly. A prototype was created based on this suggestion.
> However, according to the test result, this prototype does not inhibit
> the cross CPU wakeup and did not bring improvement. Because the cost
> to find an idle CPU is small in the problematic scenario. The root
> cause of the problem is a race condition between scanning for an idle
> CPU and task enqueue(please refer to the commit log in PATCH 2/2).
> So v3 does not change the core logic of v2, with some refinement based
> on Peter's suggestion.
>
> 2. Simplify the logic to record the task duration per Peter and Abel's suggestion.
>
> This change brings overall improvement on some microbenchmarks, both on
> Intel and AMD platforms.
>
> v3: https://lore.kernel.org/lkml/cover.1669862147.git.yu.c.chen@xxxxxxxxx/
> v2: https://lore.kernel.org/all/cover.1666531576.git.yu.c.chen@xxxxxxxxx/
> v1: https://lore.kernel.org/lkml/20220915165407.1776363-1-yu.c.chen@xxxxxxxxx/
>
> Chen Yu (2):
> sched/fair: Introduce short duration task check
> sched/fair: Choose the CPU where short task is running during wake up
>
> include/linux/sched.h | 3 +++
> kernel/sched/core.c | 2 ++
> kernel/sched/debug.c | 1 +
> kernel/sched/fair.c | 32 ++++++++++++++++++++++++++++++++
> kernel/sched/features.h | 1 +
> 5 files changed, 39 insertions(+)
>