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Date:   Wed, 11 Mar 2020 16:39:29 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     cl@rock-chips.com
Cc:     heiko@sntech.de, mingo@redhat.com, juri.lelli@redhat.com,
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Subject: Re: [PATCH v3 1/1] kthread: do not preempt current task if it is
 going to call schedule()
Message-ID: <20200311153929.GQ12561@hirez.programming.kicks-ass.net>
References: <20200306070133.18335-1-cl@rock-chips.com>
 <20200306070133.18335-2-cl@rock-chips.com>
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On Fri, Mar 06, 2020 at 03:01:33PM +0800, cl@rock-chips.com wrote:
> From: Liang Chen <cl@rock-chips.com>
> 
> when we create a kthread with ktrhead_create_on_cpu(),the child thread
> entry is ktread.c:ktrhead() which will be preempted by the parent after
> call complete(done) while schedule() is not called yet,then the parent
> will call wait_task_inactive(child) but the child is still on the runqueue,
> so the parent will schedule_hrtimeout() for 1 jiffy,it will waste a lot of
> time,especially on startup.
> 
>   parent                             child
> ktrhead_create_on_cpu()
>   wait_fo_completion(&done) -----> ktread.c:ktrhead()
>                              |----- complete(done);--wakeup and preempted by parent
>  kthread_bind() <------------|  |-> schedule();--dequeue here
>   wait_task_inactive(child)     |
>    schedule_hrtimeout(1 jiffy) -|
> 
> So we hope the child just wakeup parent but not preempted by parent, and the
> child is going to call schedule() soon,then the parent will not call
> schedule_hrtimeout(1 jiffy) as the child is already dequeue.
> 
> The same issue for ktrhead_park()&&kthread_parkme().
> This patch can save 120ms on rk312x startup with CONFIG_HZ=300.
> 
> Signed-off-by: Liang Chen <cl@rock-chips.com>

Thanks!