linux-kernel - Re: [RFC PATCH 2/7] sched/fair: Handle throttle path for task based throttle

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Message-ID: <a257205f-525f-48cf-b8d9-101ad8a95081@linux.dev>
Date: Fri, 14 Mar 2025 19:07:10 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Aaron Lu <ziqianlu@...edance.com>
Cc: Valentin Schneider <vschneid@...hat.com>, Ben Segall
 <bsegall@...gle.com>, K Prateek Nayak <kprateek.nayak@....com>,
 Peter Zijlstra <peterz@...radead.org>, Josh Don <joshdon@...gle.com>,
 Ingo Molnar <mingo@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, linux-kernel@...r.kernel.org,
 Juri Lelli <juri.lelli@...hat.com>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Mel Gorman <mgorman@...e.de>,
 Chuyi Zhou <zhouchuyi@...edance.com>
Subject: Re: [RFC PATCH 2/7] sched/fair: Handle throttle path for task based
 throttle

On 2025/3/14 17:42, Aaron Lu wrote:
> On Fri, Mar 14, 2025 at 04:39:41PM +0800, Chengming Zhou wrote:
>> On 2025/3/13 15:21, Aaron Lu wrote:
>>> From: Valentin Schneider <vschneid@...hat.com>
>>>
>>> Once a cfs_rq gets throttled, for all tasks belonging to this cfs_rq,
>>> add a task work to them so that when those tasks return to user, the
>>> actual throttle/dequeue can happen.
>>>
>>> Note that since the throttle/dequeue always happens on a task basis when
>>> it returns to user, it's no longer necessary for check_cfs_rq_runtime()
>>> to return a value and pick_task_fair() acts differently according to that
>>> return value, so check_cfs_rq_runtime() is changed to not return a
>>> value.
>>
>> Previously with the per-cfs_rq throttling, we use update_curr() -> put() path
>> to throttle the cfs_rq and dequeue it from the cfs_rq tree.
>>
>> Now with your per-task throttling, maybe things can become simpler. That we
>> can just throttle_cfs_rq() (cfs_rq subtree) when curr accouting to mark these
>> throttled.
> 
> Do I understand correctly that now in throttle_cfs_rq(), we just mark
> this hierarchy as throttled, but do not add any throttle work to these
> tasks in this hierarchy and leave the throttle work add job to pick
> time?

Right, we can move throttle_cfs_rq() forward to the curr accouting time, which
just mark these throttled.

And move setup_task_work() afterward to the pick task time, which make that task
dequeue when ret2user.

> 
>> Then then if we pick a task from a throttled cfs_rq subtree, we can setup task work
>> for it, so we don't botter with the delayed_dequeue task case that Prateek mentioned.
> 
> If we add a check point in pick time, maybe we can also avoid the check
> in enqueue time. One thing I'm thinking is, for a task, it may be picked
> multiple times with only a single enqueue so if we do the check in pick,
> the overhead can be larger?

As Prateek already mentioned, this check cost is negligeable.

> 
>> WDYT?
> 
> Thanks for your suggestion. I'll try this approach and see how it turned
> out.