linux-kernel - Re: [Query] Ticks happen in pair for NO_HZ

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Message-ID: <CAKohpo=Btb8ChRJ4Be9gbDE-nOxSaCVkQ_Qg5g7AVRxOMbXEQQ@mail.gmail.com>
Date:	Wed, 18 Dec 2013 10:08:19 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Kevin Hilman <khilman@...aro.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Linaro Networking <linaro-networking@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [Query] Ticks happen in pair for NO_HZ_FULL cores ?

On 17 December 2013 22:05, Kevin Hilman <khilman@...aro.org> wrote:
> For future reference, for generating email friendly trace output for
> discussion like this, you can use something like:
>
>    trace-cmd report --cpu=1 trace.dat

Okay..

>> And after that the next event comes after 5 Seconds.
>>
>> And so I was talking for the Event 41.
>
> That first event (Event 41) is an interrupt, and comes from the
> scheduler tick.  The tick is happening because the writeback workqueue
> just ran and we're not in NO_HZ mode.

This is what I was trying to ask. Why can't we enter in NO_HZ_FULL mode
as soon as writeback workqueue just ran? That way we can go into NOHZ
mode earlier..

> However, as soon as that IRQ (and resulting softirqs) are finished, we
> enter NO_HZ mode again.  But as you mention, it only lasts for ~5 sec
> when the timer fires again.  Once again, it fires because of the
> writeback workqueue, and soon therafter it switches back to NO_HZ mode
> again.

That's fine.. It wasn't part of my query :) .. But yes your trick
would be useful
for my usecase :)
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