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Sender: Tejun Heo <htejun@gmail.com>
Date: Fri, 19 Jan 2024 13:57:48 -1000
From: Tejun Heo <tj@kernel.org>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Frederic Weisbecker <frederic@kernel.org>,
	Joe Mario <jmario@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mark power efficient workqueue as unbounded if nohz_full
 enabled
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On Fri, Jan 19, 2024 at 12:54:39PM -0300, Marcelo Tosatti wrote:
> 
> A customer using nohz_full has experienced the following interruption:
> 
> oslat-1004510 [018] timer_cancel:         timer=0xffff90a7ca663cf8
> oslat-1004510 [018] timer_expire_entry:   timer=0xffff90a7ca663cf8 function=delayed_work_timer_fn now=4709188240 baseclk=4709188240
> oslat-1004510 [018] workqueue_queue_work: work struct=0xffff90a7ca663cd8 function=fb_flashcursor workqueue=events_power_efficient req_cpu=8192 cpu=18
> oslat-1004510 [018] workqueue_activate_work: work struct 0xffff90a7ca663cd8
> oslat-1004510 [018] sched_wakeup:         kworker/18:1:326 [120] CPU:018
> oslat-1004510 [018] timer_expire_exit:    timer=0xffff90a7ca663cf8
> oslat-1004510 [018] irq_work_entry:       vector=246
> oslat-1004510 [018] irq_work_exit:        vector=246
> oslat-1004510 [018] tick_stop:            success=0 dependency=SCHED
> oslat-1004510 [018] hrtimer_start:        hrtimer=0xffff90a70009cb00 function=tick_sched_timer/0x0 ...
> oslat-1004510 [018] softirq_exit:         vec=1 [action=TIMER]
> oslat-1004510 [018] softirq_entry:        vec=7 [action=SCHED]
> oslat-1004510 [018] softirq_exit:         vec=7 [action=SCHED]
> oslat-1004510 [018] tick_stop:            success=0 dependency=SCHED
> oslat-1004510 [018] sched_switch:         oslat:1004510 [120] R ==> kworker/18:1:326 [120]
> kworker/18:1-326 [018] workqueue_execute_start: work struct 0xffff90a7ca663cd8: function fb_flashcursor
> kworker/18:1-326 [018] workqueue_queue_work: work struct=0xffff9078f119eed0 function=drm_fb_helper_damage_work workqueue=events req_cpu=8192 cpu=18
> kworker/18:1-326 [018] workqueue_activate_work: work struct 0xffff9078f119eed0
> kworker/18:1-326 [018] timer_start:          timer=0xffff90a7ca663cf8 function=delayed_work_timer_fn ...
> 
> Set wq_power_efficient to true, in case nohz_full is enabled. 
> This makes the power efficient workqueue be unbounded, which allows
> workqueue items there to be moved to HK CPUs.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Applied to wq/for-6.9.

A side note: with the recent affinity improvements to unbound workqueues, I
wonder whether we'd be able to drop wq_power_efficient and just use
system_unbound_wq instead without noticeable perf difference.

Thanks.

-- 
tejun