Re: [RFC PATCH v7] sched/fair: select idle cpu from idle cpumask for task wakeup

Li, Aubrey
2020-12-10T08:25:39Z

Hi Mel,

On 2020/12/9 22:36, Mel Gorman wrote:
> On Wed, Dec 09, 2020 at 02:24:04PM +0800, Aubrey Li wrote:
>> Add idle cpumask to track idle cpus in sched domain. Every time
>> a CPU enters idle, the CPU is set in idle cpumask to be a wakeup
>> target. And if the CPU is not in idle, the CPU is cleared in idle
>> cpumask during scheduler tick to ratelimit idle cpumask update.
>>
>> When a task wakes up to select an idle cpu, scanning idle cpumask
>> has lower cost than scanning all the cpus in last level cache domain,
>> especially when the system is heavily loaded.
>>
>> Benchmarks including hackbench, schbench, uperf, sysbench mysql
>> and kbuild were tested on a x86 4 socket system with 24 cores per
>> socket and 2 hyperthreads per core, total 192 CPUs, no regression
>> found.
>>
>
> I ran this patch with tbench on top of of the schedstat patches that
> track SIS efficiency. The tracking adds overhead so it's not a perfect
> performance comparison but the expectation would be that the patch reduces
> the number of runqueues that are scanned

Thanks for the measurement! I don't play with tbench so may need a while
to digest the data.

>
> tbench4
> 5.10.0-rc6 5.10.0-rc6
> schedstat-v1r1 idlemask-v7r1
> Hmean 1 504.76 ( 0.00%) 500.14 * -0.91%*
> Hmean 2 1001.22 ( 0.00%) 970.37 * -3.08%*
> Hmean 4 1930.56 ( 0.00%) 1880.96 * -2.57%*
> Hmean 8 3688.05 ( 0.00%) 3537.72 * -4.08%*
> Hmean 16 6352.71 ( 0.00%) 6439.53 * 1.37%*
> Hmean 32 10066.37 ( 0.00%) 10124.65 * 0.58%*
> Hmean 64 12846.32 ( 0.00%) 11627.27 * -9.49%*
> Hmean 128 22278.41 ( 0.00%) 22304.33 * 0.12%*
> Hmean 256 21455.52 ( 0.00%) 20900.13 * -2.59%*
> Hmean 320 21802.38 ( 0.00%) 21928.81 * 0.58%*
>
> Not very optimistic result. The schedstats indicate;

How many client threads was the following schedstats collected?

>
> 5.10.0-rc6 5.10.0-rc6
> schedstat-v1r1 idlemask-v7r1
> Ops TTWU Count 5599714302.00 5589495123.00
> Ops TTWU Local 2687713250.00 2563662550.00
> Ops SIS Search 5596677950.00 5586381168.00
> Ops SIS Domain Search 3268344934.00 3229088045.00
> Ops SIS Scanned 15909069113.00 16568899405.00
> Ops SIS Domain Scanned 13580736097.00 14211606282.00
> Ops SIS Failures 2944874939.00 2843113421.00
> Ops SIS Core Search 262853975.00 311781774.00
> Ops SIS Core Hit 185189656.00 216097102.00
> Ops SIS Core Miss 77664319.00 95684672.00
> Ops SIS Recent Used Hit 124265515.00 146021086.00
> Ops SIS Recent Used Miss 338142547.00 403547579.00
> Ops SIS Recent Attempts 462408062.00 549568665.00
> Ops SIS Search Efficiency 35.18 33.72
> Ops SIS Domain Search Eff 24.07 22.72
> Ops SIS Fast Success Rate 41.60 42.20
> Ops SIS Success Rate 47.38 49.11
> Ops SIS Recent Success Rate 26.87 26.57
>
> The field I would expect to decrease is SIS Domain Scanned -- the number
> of runqueues that were examined but it's actually worse and graphing over
> time shows it's worse for the client thread counts. select_idle_cpu()
> is definitely being called because "Domain Search" is 10 times higher than
> "Core Search" and there "Core Miss" is non-zero.

Why SIS Domain Scanned can be decreased?

I thought SIS Scanned was supposed to be decreased but it seems not on your side.

I printed some trace log on my side by uperf workload, and it looks properly.
To make the log easy to read, I started a 4 VCPU VM to run 2-second uperf 8 threads.

stage 1: system idle, update_idle_cpumask is called from idle thread, set cpumask to 0-3
========================================================================================
<idle>-0 [002] d..1 137.408681: update_idle_cpumask: set_idle-1, cpumask: 2
<idle>-0 [000] d..1 137.408713: update_idle_cpumask: set_idle-1, cpumask: 0,2
<idle>-0 [003] d..1 137.408924: update_idle_cpumask: set_idle-1, cpumask: 0,2-3
<idle>-0 [001] d..1 137.409035: update_idle_cpumask: set_idle-1, cpumask: 0-3

stage 2: uperf ramp up, cpumask changes back and forth
========================================================
uperf-561 [003] d..3 137.410620: select_task_rq_fair: scanning: 0-3
uperf-560 [000] d..5 137.411384: select_task_rq_fair: scanning: 0-3
kworker/u8:3-110 [000] d..4 137.411436: select_task_rq_fair: scanning: 0-3
uperf-560 [000] d.h1 137.412562: update_idle_cpumask: set_idle-0, cpumask: 1-3
uperf-570 [002] d.h2 137.412580: update_idle_cpumask: set_idle-0, cpumask: 1,3
<idle>-0 [002] d..1 137.412917: update_idle_cpumask: set_idle-1, cpumask: 1-3
<idle>-0 [000] d..1 137.413004: update_idle_cpumask: set_idle-1, cpumask: 0-3
uperf-560 [000] d..5 137.415856: select_task_rq_fair: scanning: 0-3
kworker/u8:3-110 [001] d..4 137.415956: select_task_rq_fair: scanning: 0-3
sshd-504 [003] d.h1 137.416562: update_idle_cpumask: set_idle-0, cpumask: 0-2
uperf-560 [000] d.h1 137.416598: update_idle_cpumask: set_idle-0, cpumask: 1-2
<idle>-0 [003] d..1 137.416638: update_idle_cpumask: set_idle-1, cpumask: 1-3
<idle>-0 [000] d..1 137.417076: update_idle_cpumask: set_idle-1, cpumask: 0-3
tmux: server-528 [001] d.h. 137.448566: update_idle_cpumask: set_idle-0, cpumask: 0,2-3
<idle>-0 [001] d..1 137.448980: update_idle_cpumask: set_idle-1, cpumask: 0-3

stage 3: uperf running, select_idle_cpu scan all the CPUs in the scheduler domain at the beginning
===================================================================================================
uperf-560 [000] d..3 138.418494: select_task_rq_fair: scanning: 0-3
uperf-560 [000] d..3 138.418506: select_task_rq_fair: scanning: 0-3
uperf-560 [000] d..3 138.418514: select_task_rq_fair: scanning: 0-3
uperf-560 [000] dN.3 138.418534: select_task_rq_fair: scanning: 0-3
uperf-560 [000] dN.3 138.418543: select_task_rq_fair: scanning: 0-3
uperf-560 [000] dN.3 138.418551: select_task_rq_fair: scanning: 0-3
uperf-561 [003] d..3 138.418577: select_task_rq_fair: scanning: 0-3
uperf-561 [003] d..3 138.418600: select_task_rq_fair: scanning: 0-3
uperf-561 [003] d..3 138.418617: select_task_rq_fair: scanning: 0-3
uperf-561 [003] d..3 138.418640: select_task_rq_fair: scanning: 0-3
uperf-561 [003] d..3 138.418652: select_task_rq_fair: scanning: 0-3
uperf-561 [003] d..3 138.418662: select_task_rq_fair: scanning: 0-3
uperf-561 [003] d..3 138.418672: select_task_rq_fair: scanning: 0-3
uperf-560 [000] d..5 138.418676: select_task_rq_fair: scanning: 0-3
uperf-561 [003] d..3 138.418693: select_task_rq_fair: scanning: 0-3
kworker/u8:3-110 [002] d..4 138.418746: select_task_rq_fair: scanning: 0-3

stage 4: scheduler tick comes, update idle cpumask to EMPTY
============================================================
uperf-572 [002] d.h. 139.420568: update_idle_cpumask: set_idle-0, cpumask: 1,3
uperf-574 [000] d.H2 139.420568: update_idle_cpumask: set_idle-0, cpumask: 1,3
uperf-565 [003] d.H6 139.420569: update_idle_cpumask: set_idle-0, cpumask: 1
tmux: server-528 [001] d.h2 139.420572: update_idle_cpumask: set_idle-0, cpumask:

stage 5: uperf continue running, select_idle_cpu does not scan idle cpu
========================================================================
<I only run 2 seconds uperf, during this two seconds, no idle cpu in cpumask to scan>

uperf-565 [003] d.sa 139.420587: select_task_rq_fair: scanning:
uperf-572 [002] d.sa 139.420670: select_task_rq_fair: scanning:
............
uperf-561 [003] d..5 141.421620: select_task_rq_fair: scanning:
uperf-571 [001] d.sa 141.421630: select_task_rq_fair: scanning:

stage 6: uperf benchmark finished, idle thread switch on
=========================================================

<idle>-0 [002] d..1 141.421631: update_idle_cpumask: set_idle-1, cpumask: 2
<idle>-0 [000] d..1 141.421654: update_idle_cpumask: set_idle-1, cpumask: 0,2
<idle>-0 [001] d..1 141.421665: update_idle_cpumask: set_idle-1, cpumask: 0-2
uperf-561 [003] d..5 141.421712: select_task_rq_fair: scanning: 0-2
<idle>-0 [003] d..1 141.421807: update_idle_cpumask: set_idle-1, cpumask: 0-3

stage 7: uperf ramp down
==========================
uperf-560 [000] d..5 141.423075: select_task_rq_fair: scanning: 0-3
uperf-560 [000] d..5 141.423107: select_task_rq_fair: scanning: 0-3
uperf-560 [000] d..5 141.423259: select_task_rq_fair: scanning: 0-3
tmux: server-528 [002] d..5 141.730489: select_task_rq_fair: scanning: 1-3
kworker/u8:1-96 [003] d..4 141.731924: select_task_rq_fair: scanning: 1-3
<idle>-0 [000] d..1 141.734560: update_idle_cpumask: set_idle-1, cpumask: 0-3
tmux: server-528 [002] d.h1 141.736568: update_idle_cpumask: set_idle-0, cpumask: 0-1
uperf.sh-558 [003] d.h. 141.736570: update_idle_cpumask: set_idle-0, cpumask: 0-1
<idle>-0 [002] d..1 141.736718: update_idle_cpumask: set_idle-1, cpumask: 0-2
<idle>-0 [003] d..1 141.738179: update_idle_cpumask: set_idle-1, cpumask: 0-3
pkill-578 [001] d.h1 141.740569: update_idle_cpumask: set_idle-0, cpumask: 0,2-3
<idle>-0 [001] d..1 141.741875: update_idle_cpumask: set_idle-1, cpumask: 0-3
pkill-578 [001] d.h. 141.744570: update_idle_cpumask: set_idle-0, cpumask: 0,2-3
pkill-578 [001] d..6 141.770012: select_task_rq_fair: scanning: 0,2-3
<idle>-0 [001] d..1 141.770938: update_idle_cpumask: set_idle-1, cpumask: 0-3

In this case, SIS Scanned should be decreased. I'll apply your schedstat patch to see if
the data matches.

>
> I suspect the issue is that the mask is only marked busy from the tick
> context which is a very wide window. If select_idle_cpu() picks an idle
> CPU from the mask, it's still marked as idle in the mask.
>
That should be fine because we still check available_idle_cpu() and sched_idle_cpu for the selected
CPU. And even if that CPU is marked as idle in the mask, that mask should not be worse(bigger) than
the default sched_domain_span(sd).

Thanks,
-Aubrey