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Date: Wed, 11 Mar 2020 16:39:29 +0100 From: Peter Zijlstra <peterz@...radead.org> To: cl@...k-chips.com Cc: heiko@...ech.de, mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org, dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, akpm@...ux-foundation.org, tglx@...utronix.de, mpe@...erman.id.au, surenb@...gle.com, ben.dooks@...ethink.co.uk, anshuman.khandual@....com, catalin.marinas@....com, will@...nel.org, keescook@...omium.org, luto@...capital.net, wad@...omium.org, mark.rutland@....com, geert+renesas@...der.be, george_davis@...tor.com, sudeep.holla@....com, linux@...linux.org.uk, gregkh@...uxfoundation.org, info@...ux.net, kstewart@...uxfoundation.org, allison@...utok.net, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, huangtao@...k-chips.com Subject: Re: [PATCH v3 1/1] kthread: do not preempt current task if it is going to call schedule() On Fri, Mar 06, 2020 at 03:01:33PM +0800, cl@...k-chips.com wrote: > From: Liang Chen <cl@...k-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@...k-chips.com> Thanks!
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