From: Valentin Schneider <valentin.schneider@arm.com>
To: Yafang Shao <laoar.shao@gmail.com>,
mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
bristot@redhat.com
Cc: linux-kernel@vger.kernel.org, Yafang Shao <laoar.shao@gmail.com>
Subject: Re: [RFC PATCH 0/4] sched: Introduce cfs_migration
Date: Fri, 05 Nov 2021 17:00:22 +0000 [thread overview]
Message-ID: <87cznetu55.mognet@arm.com> (raw)
In-Reply-To: <20211104145713.4419-1-laoar.shao@gmail.com>
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
prev parent reply other threads:[~2021-11-05 17:00 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-04 14:57 [RFC PATCH 0/4] sched: Introduce cfs_migration Yafang Shao
2021-11-04 14:57 ` [RFC PATCH 1/4] stop_machine: Move cpu_stop_done into stop_machine.h Yafang Shao
2021-11-05 17:00 ` Valentin Schneider
2021-11-06 7:30 ` Yafang Shao
2021-11-04 14:57 ` [RFC PATCH 2/4] sched/fair: Introduce cfs_migration Yafang Shao
2021-11-05 17:01 ` Valentin Schneider
2021-11-06 7:40 ` Yafang Shao
2021-11-09 10:47 ` Valentin Schneider
2021-11-10 14:17 ` Yafang Shao
2021-11-07 9:38 ` [sched/fair] 64228563c2: WARNING:at_kernel/kthread.c:#__kthread_bind_mask kernel test robot
2021-11-08 3:53 ` Yafang Shao
2021-11-04 14:57 ` [RFC PATCH 3/4] sched/fair: Do active load balance in cfs_migration Yafang Shao
2021-11-04 14:57 ` [RFC PATCH 4/4] sched/core: Do numa " Yafang Shao
2021-11-05 17:02 ` Valentin Schneider
2021-11-06 7:40 ` Yafang Shao
2021-11-05 17:00 ` Valentin Schneider [this message]
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