linux-kernel - Re: [RFC PATCH 3/7] sched/fair: Handle unthrottle path for task based throttle

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Message-ID: <5dc150a8-8c96-446e-aa6b-f5c1e46b5b2a@amd.com>
Date: Wed, 2 Apr 2025 22:54:55 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Aaron Lu <ziqianlu@...edance.com>
CC: Valentin Schneider <vschneid@...hat.com>, Ben Segall <bsegall@...gle.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>, Chengming Zhou <chengming.zhou@...ux.dev>,
	Chuyi Zhou <zhouchuyi@...edance.com>
Subject: Re: [RFC PATCH 3/7] sched/fair: Handle unthrottle path for task based
 throttle

Hello Aaron,

On 4/2/2025 2:55 PM, Aaron Lu wrote:
> Hi Prateek,
> 
> On Fri, Mar 14, 2025 at 11:22:20PM +0530, K Prateek Nayak wrote:
> ... ...
>> but with per-task model, it is probably the amount of time that
>> "throttled_limbo_list" has a task on it since they are runnable
>> but are in-fact waiting for an unthrottle.
> 
> I tried this way of accounting and realized a problem. Assume a
> hierarchy like this: /sys/fs/cgroup/1/1_1, quota configured at
> /sys/fs/cgroup/1 level. When throttle happend and tasks of 1_1 get
> throttled, the limbo list of /sys/fs/cgroup/1 will always be empty so
> its "throttled_clock_self_time" is always 0...This doesn't match
> throttled_clock_self_time's semantic. "throttled_time" is similar.
> 
> I suppose we can somehow fix this by introducing something like
> h_nr_throttled, but I feel that's an overkill. So I'll probabaly just
> keep the current accounting as is in the next version, feel free to let
> me know if you have other thoughts.

I agree it might an overkill. We can discuss this more on v2.

-- 
Thanks and Regards,
Prateek

> 
> Thanks.
> 
>> If a task enqueues itself on a throttled hierarchy and then blocks
>> again before exiting to the userspace, it should not count in
>> "throttled_clock_self_time" since the task was runnable the whole
>> time despite the hierarchy being frozen.