Re: [PATCH 00/24] Complete EEVDF

Saravana Kannan
2024-11-08T07:08:25Z

On Wed, Nov 6, 2024 at 4:07 AM Luis Machado <luis.machado@arm.com> wrote:
>
> Hi,
>
> On 11/6/24 11:09, Peter Zijlstra wrote:
> > On Wed, Nov 06, 2024 at 11:49:00AM +0530, K Prateek Nayak wrote:
> >
> >> Since delayed entities are still on the runqueue, they can affect PELT
> >> calculation. Vincent and Dietmar have both noted this and Peter posted
> >> https://lore.kernel.org/lkml/172595576232.2215.18027704125134691219.tip-bot2@tip-bot2/
> >> in response but it was pulled out since Luis reported observing -ve
> >> values for h_nr_delayed on his setup. A lot has been fixed around
> >> delayed dequeue since and I wonder if now would be the right time to
> >> re-attempt h_nr_delayed tracking.
> >
> > Yeah, it's something I meant to get back to. I think the patch as posted
> > was actually right and it didn't work for Luis because of some other,
> > since fixed issue.
> >
> > But I might be misremembering things. I'll get to it eventually :/
>
> Sorry for the late reply, I got sidetracked on something else.
>
> There have been a few power regressions (based on our Pixel6-based testing) due
> to the delayed-dequeue series.
>
> The main one drove the frequencies up due to an imbalance in the uclamp inc/dec
> handling. That has since been fixed by "[PATCH 10/24] sched/uclamg: Handle delayed dequeue". [1]
>
> The bug also made it so disabling DELAY_DEQUEUE at runtime didn't fix things, because the
> imbalance/stale state would be perpetuated. Disabling DELAY_DEQUEUE before boot did fix things.
>
> So power use was brought down by the above fix, but some issues still remained, like the
> accounting issues with h_nr_running and not taking sched_delayed tasks into account.
>
> Dietmar addressed some of it with "kernel/sched: Fix util_est accounting for DELAY_DEQUEUE". [2]
>
> Peter sent another patch to add accounting for sched_delayed tasks [3]. Though the patch was
> mostly correct, under some circumstances [4] we spotted imbalances in the sched_delayed
> accounting that slowly drove frequencies up again.
>
> If I recall correctly, Peter has pulled that particular patch from the tree, but we should
> definitely revisit it with a proper fix for the imbalance. Suggestion in [5].
>
> [1] https://lore.kernel.org/lkml/20240727105029.315205425@infradead.org/
> [2] https://lore.kernel.org/lkml/c49ef5fe-a909-43f1-b02f-a765ab9cedbf@arm.com/
> [3] https://lore.kernel.org/lkml/172595576232.2215.18027704125134691219.tip-bot2@tip-bot2/
> [4] https://lore.kernel.org/lkml/6df12fde-1e0d-445f-8f8a-736d11f9ee41@arm.com/
> [5] https://lore.kernel.org/lkml/6df12fde-1e0d-445f-8f8a-736d11f9ee41@arm.com/

Thanks for the replies. We are trying to disable DELAY_DEQUEUE and
recollect the data to see if that's the cause. We'll get back to this
thread once we have some data.

-Saravana