linux-kernel - Re: [PATCH 5/8] nohz: Restart the tick from irq exit

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Date:	Tue, 7 Jul 2015 16:20:01 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 5/8] nohz: Restart the tick from irq exit

On Sun, Jun 14, 2015 at 02:49:16PM +0530, Preeti U Murthy wrote:
> On 06/11/2015 11:06 PM, Frederic Weisbecker wrote:
> > Restart the tick when necessary from the irq exit path. It makes nohz
> > full more flexible and allow it to piggyback the tick restart on the
> > scheduler IPI in the future instead of sending a dedicated IPI that
> > often doubles the scheduler IPI on task wakeup. This will require
> 
> You can piggy back on the scheduler ipi when you add a timer/hrtimer and
> add a new task to the runqueue of the nohz_full cpus, since we call
> resched_curr() in these code paths. But what about the calls to kick
> nohz_full cpus by perf events and posix cpu timers ? These call sites
> seem to be concerned about specifically waking up nohz_full cpus as far
> as I can see. IOW there is no scheduling ipi that we can fall back on in
> these paths.

Sure, those will need to keep the current IPI. They are less of a worry
because they should be rare events compared to the scheduler.

Thanks.
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