linux-kernel - Re: [PATCH 0/11 v3] sched/fair: Fix statistics with delayed dequeue

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Message-ID: <CAKfTPtD9ZPW3nj9Npf16ynMckKNteiGwn1Xcc_xtQ=vMCpGD7Q@mail.gmail.com>
Date: Thu, 5 Dec 2024 08:23:56 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Luis Machado <luis.machado@....com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org, 
	kprateek.nayak@....com, pauld@...hat.com, efault@....de, tj@...nel.org, 
	void@...ifault.com
Subject: Re: [PATCH 0/11 v3] sched/fair: Fix statistics with delayed dequeue

On Wed, 4 Dec 2024 at 20:10, Luis Machado <luis.machado@....com> wrote:
>
> On 12/2/24 17:45, Vincent Guittot wrote:
> > Delayed dequeued feature keeps a sleeping sched_entitiy enqueued until its
> > lag has elapsed. As a result, it stays also visible in the statistics that
> > are used to balance the system and in particular the field h_nr_running.
> >
> > This serie fixes those metrics by creating a new h_nr_runnable that tracks
> > only tasks that want to run. It renames h_nr_running into h_nr_runnable.
> >
> > h_nr_runnable is used in several places to make decision on load balance:
> >   - PELT runnable_avg
> >   - deciding if a group is overloaded or has spare capacity
> >   - numa stats
> >   - reduced capacity management
> >   - load balance between groups
> >
> > While fixing h_nr_running, some fields have been renamed to follow the
> > same pattern. We now have:
> >   - cfs.h_nr_runnable : running tasks in the hierarchy
> >   - cfs.h_nr_queued : enqueued tasks in the hierarchy either running or
> >       delayed dequeue
> >   - cfs.h_nr_idle : enqueued sched idle tasks in the hierarchy
> >
> > cfs.nr_running has been rename cfs.nr_queued because it includes the
> > delayed dequeued entities
> >
> > The unused cfs.idle_nr_running has been removed
> >
> > Load balance compares the number of running tasks when selecting the
> > busiest group or runqueue and tries to migrate a runnable task and not a
> > sleeping delayed dequeue one. delayed dequeue tasks are considered only
> > when migrating load as they continue to impact it.
> >
> > It should be noticed that this serie doesn't fix the problem of delayed
> > dequeued tasks that can't migrate at wakeup.
> >
> > Some additional cleanups have been added:
> >   - move variable declaration at the beginning of pick_next_entity()
> >     and dequeue_entity()
> >   - sched_can_stop_tick() should use cfs.h_nr_queued instead of
> >     cfs.nr_queued (previously cfs.nr_running) to know how many tasks
> >     are running in the whole hierarchy instead of how many entities at
> >     root level
> >
> > Changes since v2:
> > - Fix h_nr_runnable after removing h_nr_delayed (reported by Mike and Prateek)
> > - Move "sched/fair: Fix sched_can_stop_tick() for fair tasks" at the
> >   beginning of the series so it can be easily backported (asked by Prateek)
> > - Split "sched/fair: Add new cfs_rq.h_nr_runnable" in 2 patches. One
> >   for the creation of h_nr_runnable and one for its use (asked by Peter)
> > - Fix more variable declarations (reported Prateek)
> >
> >
> > Changes since v1:
> > - reorder the patches
> > - rename fields into:
> >   - h_nr_queued for all tasks queued both runnable and delayed dequeue
> >   - h_nr_runnable for all runnable tasks
> >   - h_nr_idle for all tasks with sched_idle policy
> > - Cleanup how h_nr_runnable is updated in enqueue_task_fair() and
> >   dequeue_entities
> >
> > Peter Zijlstra (1):
> >   sched/eevdf: More PELT vs DELAYED_DEQUEUE
> >
> > Vincent Guittot (10):
> >   sched/fair: Fix sched_can_stop_tick() for fair tasks
> >   sched/fair: Rename h_nr_running into h_nr_queued
> >   sched/fair: Add new cfs_rq.h_nr_runnable
> >   sched/fair: Use the new cfs_rq.h_nr_runnable
> >   sched/fair: Removed unsued cfs_rq.h_nr_delayed
> >   sched/fair: Rename cfs_rq.idle_h_nr_running into h_nr_idle
> >   sched/fair: Remove unused cfs_rq.idle_nr_running
> >   sched/fair: Rename cfs_rq.nr_running into nr_queued
> >   sched/fair: Do not try to migrate delayed dequeue task
> >   sched/fair: Fix variable declaration position
> >
> >  kernel/sched/core.c  |   4 +-
> >  kernel/sched/debug.c |  14 ++-
> >  kernel/sched/fair.c  | 240 ++++++++++++++++++++++++-------------------
> >  kernel/sched/pelt.c  |   4 +-
> >  kernel/sched/sched.h |  12 +--
> >  5 files changed, 153 insertions(+), 121 deletions(-)
> >
>
> Sorry, it took me a bit to work the backports for 6.8/Android. I gave this series a try
> on the Pixel 6 with a couple workloads (Speedometer and a UI-based one) and I didn't
> spot any significant performance/frame metric differences or power metric differences compared
> to a 6.8 kernel with this series applied.
>
> Also, checking the statistics for the cores didn't show any numbers that are obviously wrong.
>
> Though the kernel version is a bit behind mainline, I hope that's useful.
>
> Tested-By: Luis Machado <luis.machado@....com>

Thanks