linux-kernel - Re: [PATCH v2 0/5] Defer throttle when task exits to user

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Message-ID: <9eab6c1f-31e0-40c7-8737-499976c0526a@amd.com>
Date: Wed, 2 Jul 2025 09:55:19 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Aaron Lu <ziqianlu@...edance.com>,
 Valentin Schneider <vschneid@...hat.com>, Ben Segall <bsegall@...gle.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Chengming Zhou <chengming.zhou@...ux.dev>, Josh Don <joshdon@...gle.com>,
 Ingo Molnar <mingo@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, Xi Wang <xii@...gle.com>
Cc: 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>, Jan Kiszka <jan.kiszka@...mens.com>,
 Florian Bezdeka <florian.bezdeka@...mens.com>
Subject: Re: [PATCH v2 0/5] Defer throttle when task exits to user

Hello Aaron,

On 6/18/2025 1:49 PM, Aaron Lu wrote:
> v2:
> - Re-org the patchset to use a single patch to implement throttle
>    related changes, suggested by Chengming;
> - Use check_cfs_rq_runtime()'s return value in pick_task_fair() to
>    decide if throttle task work is needed instead of checking
>    throttled_hierarchy(), suggested by Peter;
> - Simplify throttle_count check in tg_throtthe_down() and
>    tg_unthrottle_up(), suggested by Peter;
> - Add enqueue_throttled_task() to speed up enqueuing a throttled task to
>    a throttled cfs_rq, suggested by Peter;
> - Address the missing of detach_task_cfs_rq() for throttled tasks that
>    get migrated to a new rq, pointed out by Chengming;
> - Remove cond_resched_tasks_rcu_qs() in throttle_cfs_rq_work() as
>    cond_resched*() is going away, pointed out by Peter.
> I hope I didn't miss any comments and suggestions for v1 and if I do,
> please kindly let me know, thanks!
> 
> Base: tip/sched/core commit dabe1be4e84c("sched/smp: Use the SMP version
> of double_rq_clock_clear_update()")

Sorry for the delay! I gave this a spin with my nested hierarchy stress
test with sched-messaging as well as with Jan's reproducer from [1] and
I didn't see anything unexpected.

A 2 vCPU VM running vanilla tip:sched/core (PREEMPT_RT) hangs within a
few seconds when the two tasks from Jan's reproducer are pinned to the
same CPU as the bandwidth timer.

I haven't seen any hangs / rcu-stalls with this series applied on top of
tip:sched/core. Feel free to include:

Tested-by: K Prateek Nayak <kprateek.nayak@....com>

[1] https://lore.kernel.org/all/7483d3ae-5846-4067-b9f7-390a614ba408@siemens.com/

-- 
Thanks and Regards,
Prateek