Re: [RFC][PATCH 08/10] sched/fair: Implement delayed dequeue

Luis Machado
2024-05-02T10:26:32Z

On 4/29/24 15:33, Luis Machado wrote:
> Hi Peter,
>
> On 4/26/24 10:32, Peter Zijlstra wrote:
>> On Thu, Apr 25, 2024 at 01:49:49PM +0200, Peter Zijlstra wrote:
>>> On Thu, Apr 25, 2024 at 12:42:20PM +0200, Peter Zijlstra wrote:
>>>
>>>>> I wonder if the delayed dequeue logic is having an unwanted effect on the calculation of
>>>>> utilization/load of the runqueue and, as a consequence, we're scheduling things to run on
>>>>> higher OPP's in the big cores, leading to poor decisions for energy efficiency.
>>>>
>>>> Notably util_est_update() gets delayed. Given we don't actually do an
>>>> enqueue when a delayed task gets woken, it didn't seem to make sense to
>>>> update that sooner.
>>>
>>> The PELT runnable values will be inflated because of delayed dequeue.
>>> cpu_util() uses those in the @boost case, and as such this can indeed
>>> affect things.
>>>
>>> This can also slightly affect the cgroup case, but since the delay goes
>>> away as contention goes away, and the cgroup case must already assume
>>> worst case overlap, this seems limited.
>>>
>>> /me goes ponder things moar.
>>
>> First order approximation of a fix would be something like the totally
>> untested below I suppose...
>
> I gave this a try on the Pixel 6, and I noticed some improvement (see below), but not
> enough to bring it back to the original levels.
>
> (1) m6.6-stock - Basic EEVDF with wakeup preemption fix (baseline)
> (2) m6.6-eevdf-complete: m6.6-stock plus this series.
> (3) m6.6-eevdf-complete-no-delay-dequeue: (2) + NO_DELAY_DEQUEUE
> (4) m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero: (2) + NO_DELAY_DEQUEUE + NO_DELAY_ZERO
> (5) m6.6-eevdf-complete-no-delay-zero: (2) + NO_DELAY_ZERO
> (6) m6.6-eevdf-complete-pelt-fix: (2) + the proposed load_avg update patch.
>
> I included (3), (4) and (5) to exercise the impact of disabling the individual
> scheduler features.
>
>
> Energy use.
>
> +------------+------------------------------------------------------+-----------+
> | cluster | tag | perc_diff |
> +------------+------------------------------------------------------+-----------+
> | CPU | m6.6-stock | 0.0% |
> | CPU-Big | m6.6-stock | 0.0% |
> | CPU-Little | m6.6-stock | 0.0% |
> | CPU-Mid | m6.6-stock | 0.0% |
> | GPU | m6.6-stock | 0.0% |
> | Total | m6.6-stock | 0.0% |
> | CPU | m6.6-eevdf-complete | 114.51% |
> | CPU-Big | m6.6-eevdf-complete | 90.75% |
> | CPU-Little | m6.6-eevdf-complete | 98.74% |
> | CPU-Mid | m6.6-eevdf-complete | 213.9% |
> | GPU | m6.6-eevdf-complete | -7.04% |
> | Total | m6.6-eevdf-complete | 100.92% |
> | CPU | m6.6-eevdf-complete-no-delay-dequeue | 117.77% |
> | CPU-Big | m6.6-eevdf-complete-no-delay-dequeue | 113.79% |
> | CPU-Little | m6.6-eevdf-complete-no-delay-dequeue | 97.47% |
> | CPU-Mid | m6.6-eevdf-complete-no-delay-dequeue | 189.0% |
> | GPU | m6.6-eevdf-complete-no-delay-dequeue | -6.74% |
> | Total | m6.6-eevdf-complete-no-delay-dequeue | 103.84% |
> | CPU | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | 120.45% |
> | CPU-Big | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | 113.65% |
> | CPU-Little | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | 99.04% |
> | CPU-Mid | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | 201.14% |
> | GPU | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | -5.37% |
> | Total | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | 106.38% |
> | CPU | m6.6-eevdf-complete-no-delay-zero | 119.05% |
> | CPU-Big | m6.6-eevdf-complete-no-delay-zero | 107.55% |
> | CPU-Little | m6.6-eevdf-complete-no-delay-zero | 98.66% |
> | CPU-Mid | m6.6-eevdf-complete-no-delay-zero | 206.58% |
> | GPU | m6.6-eevdf-complete-no-delay-zero | -5.25% |
> | Total | m6.6-eevdf-complete-no-delay-zero | 105.14% |
> | CPU | m6.6-eevdf-complete-pelt-fix | 105.56% |
> | CPU-Big | m6.6-eevdf-complete-pelt-fix | 100.45% |
> | CPU-Little | m6.6-eevdf-complete-pelt-fix | 94.4% |
> | CPU-Mid | m6.6-eevdf-complete-pelt-fix | 150.94% |
> | GPU | m6.6-eevdf-complete-pelt-fix | -3.96% |
> | Total | m6.6-eevdf-complete-pelt-fix | 93.31% |
> +------------+------------------------------------------------------+-----------+
>
> Utilization and load levels.
>
> +---------+------------------------------------------------------+----------+-----------+
> | cluster | tag | variable | perc_diff |
> +---------+------------------------------------------------------+----------+-----------+
> | little | m6.6-stock | load | 0.0% |
> | little | m6.6-stock | util | 0.0% |
> | little | m6.6-eevdf-complete | load | 29.56% |
> | little | m6.6-eevdf-complete | util | 55.4% |
> | little | m6.6-eevdf-complete-no-delay-dequeue | load | 42.89% |
> | little | m6.6-eevdf-complete-no-delay-dequeue | util | 69.47% |
> | little | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | load | 51.05% |
> | little | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | util | 76.55% |
> | little | m6.6-eevdf-complete-no-delay-zero | load | 34.51% |
> | little | m6.6-eevdf-complete-no-delay-zero | util | 72.53% |
> | little | m6.6-eevdf-complete-pelt-fix | load | 29.96% |
> | little | m6.6-eevdf-complete-pelt-fix | util | 59.82% |
> | mid | m6.6-stock | load | 0.0% |
> | mid | m6.6-stock | util | 0.0% |
> | mid | m6.6-eevdf-complete | load | 29.37% |
> | mid | m6.6-eevdf-complete | util | 75.22% |
> | mid | m6.6-eevdf-complete-no-delay-dequeue | load | 36.4% |
> | mid | m6.6-eevdf-complete-no-delay-dequeue | util | 80.28% |
> | mid | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | load | 30.35% |
> | mid | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | util | 90.2% |
> | mid | m6.6-eevdf-complete-no-delay-zero | load | 37.83% |
> | mid | m6.6-eevdf-complete-no-delay-zero | util | 93.79% |
> | mid | m6.6-eevdf-complete-pelt-fix | load | 33.57% |
> | mid | m6.6-eevdf-complete-pelt-fix | util | 67.83% |
> | big | m6.6-stock | load | 0.0% |
> | big | m6.6-stock | util | 0.0% |
> | big | m6.6-eevdf-complete | load | 97.39% |
> | big | m6.6-eevdf-complete | util | 12.63% |
> | big | m6.6-eevdf-complete-no-delay-dequeue | load | 139.69% |
> | big | m6.6-eevdf-complete-no-delay-dequeue | util | 22.58% |
> | big | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | load | 125.36% |
> | big | m6.6-eevdf-complete-no-delay-dequeue-no-delay-zero | util | 23.15% |
> | big | m6.6-eevdf-complete-no-delay-zero | load | 128.56% |
> | big | m6.6-eevdf-complete-no-delay-zero | util | 25.03% |
> | big | m6.6-eevdf-complete-pelt-fix | load | 130.73% |
> | big | m6.6-eevdf-complete-pelt-fix | util | 17.52% |
> +---------+------------------------------------------------------+----------+-----------+

Going through the code, my understanding is that the util_est functions seem to be getting
called correctly, and in the right order. That is, we first util_est_enqueue, then util_est_dequeue
and finally util_est_update. So the stats *should* be correct.

On dequeuing (dequeue_task_fair), we immediately call util_est_dequeue, even for the case of
a DEQUEUE_DELAYED task, since we're no longer going to run the dequeue_delayed task for now, even
though it is still in the rq.

We delay the util_est_update of dequeue_delayed tasks until a later time in dequeue_entities.

Eventually the dequeue_delayed task will have its lag zeroed when it becomes eligible again,
(requeue_delayed_entity) while still being in the rq. It will then get dequeued/enqueued (requeued),
and marked as a non-dequeue-delayed task.

Next time we attempt to enqueue such a task (enqueue_task_fair), it will skip the ENQUEUE_DELAYED
block and call util_est_enqueue.

Still, something seems to be signalling that util/load is high, and causing migration to the big cores.

Maybe we're not decaying the util/load properly at some point, and inflated numbers start to happen.

I'll continue investigating.