Re: [RFC PATCH] sched/eevdf: Use tunable knob sysctl_sched_base_slice as explicit time quanta

[Vishal Chourasia <vishalc@linux.ibm.com>]

On Sun, Feb 04, 2024 at 11:05:22AM +0800, Ze Gao wrote:
> On Fri, Feb 2, 2024 at 7:50 PM Vishal Chourasia <vishalc@linux.ibm.com> wrote:
> >
> > 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
> 
> You can check out your HZ, which should be 100 in your settings
> in my best guess.That explains your results.
Yes. How much is it in your case? If I may ask.
> 
> > 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 ?
> 
> The thing is  in my RFC the effective quanta is actually
> 
>    max_t(u64, TICK_NSEC, sysctl_sched_base_slice)
> 
> where sysctl_sched_base_slice is precisely a handy tunable knob
> for users ( maybe i should make it loud and clear more ).
> 
> See what I do in update_entity_lag(), you will understand.
Thanks. I will look into it.
> 
> Note we have 3 time related concepts here:
> 1. TIME TICK: (schedule) accounting time unit.
> 2. TIME QUANTA （not necessarily the effective one): scheduling time unit
> 3. USER SLICE: time slice per request
To double check,
User slice is the request size submitted by a competing task for the time-shared resource (here,
processor) against other competing tasks.

Scheduler allocates time-shared resource (here, processor) in `q` quantum
which is our TIME QUANTA

TIME TICK is time period between two scheduler ticks.

Thanks, 
 -- vishal.c
> 
> To implement latency-nice while being as fair as possible, We must
> carefully consider the size relationship between them, and especially
> the value range of USER SLICE due to the cold fact that the lag(
> unfairness) is literally subject to both time quanta and user requested
> slices.
> 
> 
> Regards,
>         -- Ze
> 
> > >
> > > 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@tencent.com>
> > > > > ---
> > > >
> > >