Re: [PATCH v3] sched/fair: do not scan twice in detach_tasks()

[Shijie Huang <shijie@amperemail.onmicrosoft.com>]

On 2025/7/19 2:49, Valentin Schneider wrote:
> 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
> """
Okay, I will change the commit message later..
>> 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@os.amperecomputing.com>
> Reviewed-by: Valentin Schneider <vschneid@redhat.com>


Thanks

Huang Shijie