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Date: Fri, 2 Feb 2024 17:20:00 +0530
From: Vishal Chourasia <vishalc@...ux.ibm.com>
To: Ze Gao <zegao2021@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ben Segall <bsegall@...gle.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
Mel Gorman <mgorman@...e.de>, Steven Rostedt <rostedt@...dmis.org>,
Valentin Schneider <vschneid@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
linux-kernel@...r.kernel.org, Ze Gao <zegao@...cent.com>
Subject: Re: [RFC PATCH] sched/eevdf: Use tunable knob
sysctl_sched_base_slice as explicit time quanta
On Wed, Jan 24, 2024 at 10:32:08AM +0800, Ze Gao wrote:
> > Hi, How are you setting custom request values for process A and B?
>
> I cherry-picked peter's commit[1], and adds a SCHED_QUANTA feature control
> for testing w/o my patch. You can check out [2] to see how it works.
>
Thank you sharing your setup.
Built the kernel according to [2] keeping v6.8.0-rc1 as base
// NO_SCHED_QUANTA
# perf script -i perf.data.old -s perf-latency.py
PID 355045: Average Delta = 87.72726154385964 ms, Max Delta = 110.015044 ms, Count = 57
PID 355044: Average Delta = 92.2655679245283 ms, Max Delta = 110.017182 ms, Count = 53
// SCHED_QUANTA
# perf script -i perf.data -s perf-latency.py
PID 355065: Average Delta = 10.00 ms, Max Delta = 10.012708 ms, Count = 500
PID 355064: Average Delta = 9.959 ms, Max Delta = 10.023588 ms, Count = 501
# cat /sys/kernel/debug/sched/base_slice_ns
3000000
base slice is not being enforced.
Next, Looking closing at the perf.data file
# perf script -i perf.data -C 1 | grep switch
..
stress-ng-cpu 355064 [001] 776706.003222: sched:sched_switch: stress-ng-cpu:355064 [120] R ==> stress-ng-cpu:355065 [120]
stress-ng-cpu 355065 [001] 776706.013218: sched:sched_switch: stress-ng-cpu:355065 [120] R ==> stress-ng-cpu:355064 [120]
stress-ng-cpu 355064 [001] 776706.023218: sched:sched_switch: stress-ng-cpu:355064 [120] R ==> stress-ng-cpu:355065 [120]
stress-ng-cpu 355065 [001] 776706.033218: sched:sched_switch: stress-ng-cpu:355065 [120] R ==> stress-ng-cpu:355064 [120]
..
Delta wait time is approx 0.01s or 10ms
So, switch is not happening at base_slice_ns boundary.
But why? is it possible base_slice_ns is not properly used in
arch != x86 ?
>
> echo NO_SCHED_QUANTA > /sys/kernel/debug/sched/features
> test
> sleep 2
> echo SCHED_QUANTA > /sys/kernel/debug/sched/features
> test
>
>
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/kernel/sched?h=sched/eevdf&id=98866150f92f268a2f08eb1d884de9677eb4ec8f
> [2]: https://github.com/zegao96/linux/tree/sched-eevdf
>
>
> Regards,
> -- Ze
>
> > >
> > > stress-ng-cpu:10705 stress-ng-cpu:10706
> > > ---------------------------------------------------------------------
> > > Slices(ms) 100 0.1
> > > Runtime(ms) 4934.206 5025.048
> > > Switches 58 67
> > > Average delay(ms) 87.074 73.863
> > > Maximum delay(ms) 101.998 101.010
> > >
> > > In contrast, using sysctl_sched_base_slice as the size of a 'quantum'
> > > in this patch gives us a better control of the allocation accuracy and
> > > the avg latency:
> > >
> > > stress-ng-cpu:10584 stress-ng-cpu:10583
> > > ---------------------------------------------------------------------
> > > Slices(ms) 100 0.1
> > > Runtime(ms) 4980.309 4981.356
> > > Switches 1253 1254
> > > Average delay(ms) 3.990 3.990
> > > Maximum delay(ms) 5.001 4.014
> > >
> > > Furthmore, with sysctl_sched_base_slice = 10ms, we might benefit from
> > > less switches at the cost of worse delay:
> > >
> > > stress-ng-cpu:11208 stress-ng-cpu:11207
> > > ---------------------------------------------------------------------
> > > Slices(ms) 100 0.1
> > > Runtime(ms) 4983.722 4977.035
> > > Switches 456 456
> > > Average delay(ms) 10.963 10.939
> > > Maximum delay(ms) 19.002 21.001
> > >
> > > By being able to tune sysctl_sched_base_slice knob, we can achieve
> > > the goal to strike a good balance between throughput and latency by
> > > adjusting the frequency of context switches, and the conclusions are
> > > much close to what's covered in [1] with the explicit definition of
> > > a time quantum. And it aslo gives more freedom to choose the eligible
> > > request length range(either through nice value or raw value)
> > > without worrying about overscheduling or underscheduling too much.
> > >
> > > Note this change should introduce no obvious regression because all
> > > processes have the same request length as sysctl_sched_base_slice as
> > > in the status quo. And the result of benchmarks proves this as well.
> > >
> > > schbench -m2 -F128 -n10 -r90 w/patch tip/6.7-rc7
> > > Wakeup (usec): 99.0th: 3028 95
> > > Request (usec): 99.0th: 14992 21984
> > > RPS (count): 50.0th: 5864 5848
> > >
> > > hackbench -s 512 -l 200 -f 25 -P w/patch tip/6.7-rc7
> > > -g 10 0.212 0.223
> > > -g 20 0.415 0.432
> > > -g 30 0.625 0.639
> > > -g 40 0.852 0.858
> > >
> > > [1]: https://dl.acm.org/doi/10.5555/890606
> > > [2]: https://lore.kernel.org/all/20230420150537.GC4253@hirez.programming.kicks-ass.net/T/#u
> > >
> > > Signed-off-by: Ze Gao <zegao@...cent.com>
> > > ---
> >
>
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