Re: [RFC PATCH 4/5] sched/fair: Use EAS also when overutilized

From: Vincent Guittot
Date: Tue Oct 15 2024 - 08:48:13 EST

On Fri, 11 Oct 2024 at 14:52, Pierre Gondois <pierre.gondois@xxxxxxx> wrote:
>
> Hello Vincent,
>
> On 10/9/24 10:53, Vincent Guittot wrote:
> > Hi Pierre,
> >
> > On Mon, 7 Oct 2024 at 09:03, Pierre Gondois <pierre.gondois@xxxxxxx> wrote:
> >>
> >> Hello Vincent,
> >>
> >> Sorry for the delay:
> >>
> >> On 9/25/24 15:28, Vincent Guittot wrote:
> >>> On Thu, 19 Sept 2024 at 10:26, Pierre Gondois <pierre.gondois@xxxxxxx> wrote:
> >>>>
> >>>> Hello Vincent,
> >>>> I tried this patch on a Pixel 6 (8 CPUs, 4 little, 2 mid, 2 big)
> >>>> with patches 1-4/5 using these workloads:
> >>>> ---
> >>>> A.
> >>>> a. 8 tasks at 2%/5%/10% during 1s
> >>>> b. 1 task:
> >>>> - sleeping during 0.3s
> >>>> - at 100% during 0.3s
> >>>> - sleeping during 0.3s
> >>>>
> >>>> b. is used to reach the overutilized state during a limited amount of time.
> >>>> EAS is then operating, then the load balancer does the task placement, then EAS
> >>>> is operating again.
> >>>>
> >>>> B.
> >>>> a. 8 tasks at 2%/5%/10% during 1s
> >>>> b. 1 task:
> >>>> - at 100% during 1s
> >>>>
> >>>> ---
> >>>> I'm seeing the energy consumption increase in some cases. This seems to be
> >>>> due to feec() migrating tasks more often than what the load balancer does
> >>>> for this workload. This leads to utilization 'spikes' and then frequency
> >>>> 'spikes', increasing the overall energy consumption.
> >>>> This is not entirely related to this patch though,, as the task placement seems
> >>>> correct. I.e. feec() effectively does an optimal placement given the EM and
> >>>> task utilization. The task placement is just a bit less stable than with
> >>>> the load balancer.
> >>>
> >>> Would patch 5 help to keep things better placed ? in particular if
> >>> task b is misplaced at some point because of load balance ?
> >>
> >> I assume so, it would require more testing on my side
> >>
> >>>
> >>> I agree that load balance might still contribute to migrate task in a
> >>> not energy efficient way
> >>>
> >>>>
> >>>> ---
> >>>> Regarding hackbench, I've reproduced the test you've run on the same Pixel6.
> >>>> I have CONFIG_SCHED_CLUSTER enabled, which allows having a sched domain for
> >>>> each little/mid/big CPUs (without the config, these group would no exist).
> >>>
> >>> Why did you do this ? All cpus are expected to be in same sched domain
> >>> as they share their LLC
> >>
> >> I did this to observe the loa balancer a bit more carefully while reviewing
> >> the first patch:
> >> sched/fair: Filter false overloaded_group case for EAS
> >> I've let this configuration, but effectively this should not bring anything more.
> >>
> >>
> >>>
> >>>>
> >>>> I see an important regression in the result.
> >>>> I replaced the condition to run feec() through select_task_rq_fair() by:
> >>>> if (get_rd_overloaded(cpu_rq(cpu)->rd) == 0)) {
> >>>
> >>> overloaded is enable when more than 1 task runs on a cpu whatever the
> >>> utilization
> >>
> >> Yes right, this idea has little sense.
> >>
> >>>
> >>>> new_cpu = find_energy_efficient_cpu(p, prev_cpu);
> >>>> ...
> >>>> }
> >>>> and obtained better results.
> >>>>
> >>>> Indeed, for such intensive workload:
> >>>> - EAS would not have any energy benefit, so better prioritize performance
> >>>> (as Christian mentioned)
> >>>> - EAS would not be able to find a fitting CPU, so running feec() should be
> >>>> avoided
> >>>> - as you mention in the commit message, shuffling tasks when one CPU becomes
> >>>> momentarily overutilized is inefficient energy-wise (even though I don't have
> >>>> the numbers, it should make sense).
> >>>> So detecting when the system is overloaded should be a better compromise I
> >>>> assume. The condition in sched_balance_find_src_group() to let the load balancer
> >>>> operate might also need to be updated.
> >>>>
> >>>> Note:
> >>>> - base: with patches 1-4/5
> >>>> - _ou: run feec() when not overutilized
> >>>> - _ol: run feec() when not overloaded
> >>>> - mean: hackbench execution time in s.
> >>>> - delta: negative is better. Value is in percentage.
> >>>
> >>> Could you share your command line ? As explained in the cover letter I
> >>> have seen some perf regressions but not in the range that you have
> >>> below
> >>>
> >>> What is your base ? tip/sched/core ?
> >>
> >> I am working on a Pixel6, with a branch based on v6.8 with some scheduler patches
> >> to be able to apply your patches cleanly.
> >
> > TBH, I'm always cautious with those kind of frankenstein kernel
> > especially with all changes that have happened on the scheduler since
> > v6.8 compared to tip/sched/core
>
> Yes I understand, I'll re-test it on a Juno with a newer kernel.
>
> >
> >>
> >> The mapping id -> command line is as:
> >
> > Thanks for the commands details, I'm going to have a look
> >
> >> (1) hackbench -l 5120 -g 1
> >> (2) hackbench -l 1280 -g 4
> >> (3) hackbench -l 640 -g 8
> >> (4) hackbench -l 320 -g 16
> >>
> >> (5) hackbench -p -l 5120 -g 1
> >> (6) hackbench -p -l 1280 -g 4
> >> (7) hackbench -p -l 640 -g 8
> >> (8) hackbench -p -l 320 -g 16
> >>
> >> (9) hackbench -T -l 5120 -g 1
> >> (10) hackbench -T -l 1280 -g 4
> >> (11) hackbench -T -l 640 -g 8
> >> (12) hackbench -T -l 320 -g 16
> >>
> >> (13) hackbench -T -p -l 5120 -g 1
> >> (14) hackbench -T -p -l 1280 -g 4
> >> (15) hackbench -T -p -l 640 -g 8
> >> (16) hackbench -T -p -l 320 -g 16
> >>
> >>
> >>>
> >>>> ┌─────┬───────────┬──────────┬─────────┬──────────┬─────────┬──────────┬──────────┬──────────┐
> >>>> │ id ┆ mean_base ┆ std_base ┆ mean_ou ┆ std_ou ┆ mean_ol ┆ std_ol ┆ delta_ou ┆ delta_ol │
> >>>> ╞═════╪═══════════╪══════════╪═════════╪══════════╪═════════╪══════════╪══════════╪══════════╡
> >>>> │ 1 ┆ 1.9786 ┆ 0.04719 ┆ 3.0856 ┆ 0.122209 ┆ 2.1734 ┆ 0.045203 ┆ 55.95 ┆ 9.85 │
> >
> > I might have misunderstood your results above last time.
> > mean_base results include patches 1 to 4 and mean_ou revert patch 4.
> > Does it mean that it is 55% better with patch 4 ? I originally thought
> > there was a regression with patch 4 but I'm not sure that I understood
> > correctly after re reading the table.
>
> The columns are:
> - the _base configuration disables EAS/feec() when in the overutilized state,
> i.e. patches 1-3 are applied.

your original description
"
- base: with patches 1-4/5
- _ou: run feec() when not overutilized
- _ol: run feec() when not overloaded
"
was quite confusing :-)

Thanks for the clarification

> - the _ou configuration keeps running EAS/feec() when in the overutilized state
> i.e. patches 1-4 are applied
> - the _ol configuration should be ignored as previously established
>
>
> >
> >>>> │ 2 ┆ 1.8991 ┆ 0.019768 ┆ 2.6672 ┆ 0.135266 ┆ 1.98875 ┆ 0.055132 ┆ 40.45 ┆ 4.72 │
> >>>> │ 3 ┆ 1.9053 ┆ 0.014795 ┆ 2.5761 ┆ 0.141693 ┆ 2.06425 ┆ 0.045901 ┆ 35.21 ┆ 8.34 │
> >>>> │ 4 ┆ 1.9586 ┆ 0.023439 ┆ 2.5823 ┆ 0.110399 ┆ 2.0955 ┆ 0.053818 ┆ 31.84 ┆ 6.99 │
> >>>> │ 5 ┆ 1.746 ┆ 0.055676 ┆ 3.3437 ┆ 0.279107 ┆ 1.88 ┆ 0.038184 ┆ 91.51 ┆ 7.67 │
> >>>> │ 6 ┆ 1.5476 ┆ 0.050131 ┆ 2.6835 ┆ 0.140497 ┆ 1.5645 ┆ 0.081644 ┆ 73.4 ┆ 1.09 │
> >>>> │ 7 ┆ 1.4562 ┆ 0.062457 ┆ 2.3568 ┆ 0.119213 ┆ 1.48425 ┆ 0.06212 ┆ 61.85 ┆ 1.93 │
> >>>> │ 8 ┆ 1.3554 ┆ 0.031757 ┆ 2.0609 ┆ 0.112869 ┆ 1.4085 ┆ 0.036601 ┆ 52.05 ┆ 3.92 │
> >>>> │ 9 ┆ 2.0391 ┆ 0.035732 ┆ 3.4045 ┆ 0.277307 ┆ 2.2155 ┆ 0.019053 ┆ 66.96 ┆ 8.65 │
> >>>> │ 10 ┆ 1.9247 ┆ 0.056472 ┆ 2.6605 ┆ 0.119417 ┆ 2.02775 ┆ 0.05795 ┆ 38.23 ┆ 5.35 │
> >>>> │ 11 ┆ 1.8923 ┆ 0.038222 ┆ 2.8113 ┆ 0.120623 ┆ 2.089 ┆ 0.025259 ┆ 48.57 ┆ 10.39 │
> >>>> │ 12 ┆ 1.9444 ┆ 0.034856 ┆ 2.6675 ┆ 0.219585 ┆ 2.1035 ┆ 0.076514 ┆ 37.19 ┆ 8.18 │
> >>>> │ 13 ┆ 1.7107 ┆ 0.04874 ┆ 3.4443 ┆ 0.154481 ┆ 1.8275 ┆ 0.036665 ┆ 101.34 ┆ 6.83 │
> >>>> │ 14 ┆ 1.5565 ┆ 0.056595 ┆ 2.8241 ┆ 0.158643 ┆ 1.5515 ┆ 0.040813 ┆ 81.44 ┆ -0.32 │
> >>>> │ 15 ┆ 1.4932 ┆ 0.085256 ┆ 2.6841 ┆ 0.135623 ┆ 1.50475 ┆ 0.028336 ┆ 79.75 ┆ 0.77 │
> >>>> │ 16 ┆ 1.4263 ┆ 0.067666 ┆ 2.3971 ┆ 0.145928 ┆ 1.414 ┆ 0.061422 ┆ 68.06 ┆ -0.86 │
> >>>> └─────┴───────────┴──────────┴─────────┴──────────┴─────────┴──────────┴──────────┴──────────┘
> >>>>
> >>>> On 9/17/24 22:24, Christian Loehle wrote:
> >>>>> On 8/30/24 14:03, Vincent Guittot wrote:
> >>>>>> Keep looking for an energy efficient CPU even when the system is
> >>>>>> overutilized and use the CPU returned by feec() if it has been able to find
> >>>>>> one. Otherwise fallback to the default performance and spread mode of the
> >>>>>> scheduler.
> >>>>>> A system can become overutilized for a short time when workers of a
> >>>>>> workqueue wake up for a short background work like vmstat update.
> >>>>>> Continuing to look for a energy efficient CPU will prevent to break the
> >>>>>> power packing of tasks.
> >>>>>>
> >>>>>> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> >>>>>> ---
> >>>>>> kernel/sched/fair.c | 2 +-
> >>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>>>
> >>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>>>>> index 2273eecf6086..e46af2416159 100644
> >>>>>> --- a/kernel/sched/fair.c
> >>>>>> +++ b/kernel/sched/fair.c
> >>>>>> @@ -8505,7 +8505,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >>>>>> cpumask_test_cpu(cpu, p->cpus_ptr))
> >>>>>> return cpu;
> >>>>>>
> >>>>>> - if (!is_rd_overutilized(this_rq()->rd)) {
> >>>>>> + if (sched_energy_enabled()) {
> >>>>>> new_cpu = find_energy_efficient_cpu(p, prev_cpu);
> >>>>>> if (new_cpu >= 0)
> >>>>>> return new_cpu;
> >>>>>
> >>>>> Super quick testing on pixel6:
> >>>>> for i in $(seq 0 6); do /data/local/tmp/hackbench -l 500 -g 100 | grep Time; sleep 60; done
> >>>>> with patch 5/5 only:
> >>>>> Time: 19.433
> >>>>> Time: 19.657
> >>>>> Time: 19.851
> >>>>> Time: 19.789
> >>>>> Time: 19.857
> >>>>> Time: 20.092
> >>>>> Time: 19.973
> >>>>>
> >>>>> mainline:
> >>>>> Time: 18.836
> >>>>> Time: 18.718
> >>>>> Time: 18.781
> >>>>> Time: 19.015
> >>>>> Time: 19.061
> >>>>> Time: 18.950
> >>>>> Time: 19.166
> >>>>>
> >>>>>
> >>>>> The reason we didn't always have this enabled is the belief that
> >>>>> this costs us too much performance in scenarios we most need it
> >>>>> while at best making subpar EAS decisions anyway (in an
> >>>>> overutilized state).
> >>>>> I'd be open for questioning that, but why the change of mind?
> >>>>> And why is this necessary in your series if the EAS selection
> >>>>> isn't 'final' (until the next sleep) anymore (Patch 5/5)?
> >>>>>