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

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Message-ID: <557D49A4.2040201@linux.vnet.ibm.com>
Date:	Sun, 14 Jun 2015 15:00:12 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	LKML <linux-kernel@...r.kernel.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 06/12/2015 06:08 PM, Frederic Weisbecker wrote:
> On Fri, Jun 12, 2015 at 09:32:45AM +0200, Peter Zijlstra wrote:
>> On Thu, Jun 11, 2015 at 07:36:05PM +0200, 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
>>> careful review of resched_curr() callers.
>>
>> This seems to assume schedule_ipi() callers use irq_exit(), this is
>> false.
> 
> Indeed there will be that too. Note the current patch doesn't yet rely on
> schedule_ipi(), we are still using the nohz ipis. But introducing the
> tick restart on irq exit prepares for later piggybacking on scheduler_ipi().
> 
> I think this will involve changes on got_nohz_idle_kick(), renamed to
> got_nohz_kick() and include nohz full related checks to trigger the
> irq_enter()/exit() pair.

I maybe saying something obvious here, nevertheless; I am not sure about
other archs, but atleast on powerpc after handling an interrupt, we will
call irq_exit() and reevaluate starting of ticks. So in our case even if
scheduler_ipi() callers do not call irq_exit(), it will be called after
handling the reschedule interrupt.

Regards
Preeti U Murthy
> 

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