linux-kernel - Re: [PATCH v3] sched/fair: do not scan twice in detach

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Message-ID: <xhsmhy0sle3mf.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Fri, 18 Jul 2025 20:49:12 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Huang Shijie <shijie@...amperecomputing.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org
Cc: patches@...erecomputing.com, cl@...ux.com,
 Shubhang@...amperecomputing.com, dietmar.eggemann@....com,
 rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 linux-kernel@...r.kernel.org, Huang Shijie <shijie@...amperecomputing.com>
Subject: Re: [PATCH v3] sched/fair: do not scan twice in detach_tasks()

On 18/07/25 14:35, Huang Shijie wrote:
> When detach_tasks() scans the src_cpu's task list, the task list
> may shrink during the scanning. For example, the task list
> may have four tasks at the beginning, it may becomes to two
> during the scanning in detach_tasks():
>     Task list at beginning : "ABCD"
>     Task list in scanning  : "CD"
>
>     (ABCD stands for differnt tasks.)
>
> In this scenario, the env->loop_max is still four, so
> detach_tasks() may scan twice for some tasks:
>     the scanning order maybe : "DCDC"
>

How about something like so:
"""
detach_tasks() uses struct lb_env.loop_max as an env.src_rq->cfs_tasks
iteration count limit. It is however set without the source RQ lock held,
and besides detach_tasks() can be re-invoked after releasing and
re-acquiring the RQ lock per LBF_NEED_BREAK.

This means that env.loop_max and the actual length of env.src_rq->cfs_tasks
as observed within detach_tasks() can differ. This can cause some tasks to
be unnecessarily iterated over more than once, for instance:

  env.loop_max := 4
  detach_tasks()
    // Here env.src->cfs_tasks only contains two tasks which can't be
    // migrated anywhere, so they're put back in the list each time.
    env.src->cfs_tasks := [p1, p0]
    // The iteration goes:
    p0; cfs_tasks := [p0, p1]
    p1; cfs_tasks := [p1, p0]
    p0; cfs_tasks := [p0, p1]
    p1; cfs_tasks := [p1, p0]

    // IOW we iterate over each task twice
"""

> In the Specjbb test, this issue can be catched many times.
                                         ^^^^^^^
                                         caught

> (Over 330,000 times in a 30-min Specjbb test)
>
> The patch introduces "first_back" to record the first task which
> is put back to the task list. If we get a task which is equal to
> first_back, we break the loop, and avoid to scan twice for it.
>
> Signed-off-by: Huang Shijie <shijie@...amperecomputing.com>

Reviewed-by: Valentin Schneider <vschneid@...hat.com>