linux-kernel - Re: [RFC PATCH 0/4] sched: Introduce cfs

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Date:   Fri, 05 Nov 2021 17:00:22 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Yafang Shao <laoar.shao@...il.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org, Yafang Shao <laoar.shao@...il.com>
Subject: Re: [RFC PATCH 0/4] sched: Introduce cfs_migration


Hi,

On 04/11/21 14:57, Yafang Shao wrote:
> The active load balance has a known issue[1][2] that there is a race
> window between waking up the migration thread on the busiest CPU and it
> begins to preempt the current running CFS task. This race window may cause
> unexpected behavior that the current running CFS task may be preempted
> by a RT task first, and then the RT task will be preempted by this
> waked migration thread. Per our tracing, the latency caused by this
> preemption can be greater than 1ms, which is not a small latency for the
> RT tasks.
>
> We'd better set a proper priority to this balance work so that it can
> preempt CFS task only. A new per-cpu thread cfs_migration is introduced
> for this purpose. The cfs_migration thread has a priority FIFO-1,
> which means it can preempt any cfs tasks but can't preempt other FIFO
> tasks.
>
> Besides the active load balance work, the numa balance work also applies
> to CFS tasks only. So we'd better assign cfs_migraion to numa balance
> work as well.
>
> [1]. https://lore.kernel.org/lkml/CAKfTPtBygNcVewbb0GQOP5xxO96am3YeTZNP5dK9BxKHJJAL-g@mail.gmail.com/
> [2]. https://lore.kernel.org/lkml/20210615121551.31138-1-laoar.shao@gmail.com/
>

So overall I quite like the idea, but am not entirely convinced by the
implementation. See comments in rest of the thread - in any case, thanks
for taking a jab at that!

> Yafang Shao (4):
>   stop_machine: Move cpu_stop_done into stop_machine.h
>   sched/fair: Introduce cfs_migration
>   sched/fair: Do active load balance in cfs_migration
>   sched/core: Do numa balance in cfs_migration
>
>  include/linux/stop_machine.h |  12 +++
>  kernel/sched/core.c          |   2 +-
>  kernel/sched/fair.c          | 143 ++++++++++++++++++++++++++++++++++-
>  kernel/sched/sched.h         |   2 +
>  kernel/stop_machine.c        |  14 +---
>  5 files changed, 158 insertions(+), 15 deletions(-)
>
> --
> 2.17.1