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

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Message-ID: <CAKohpon7Vf=zHt5OBpf=LCaj9GVW8F34Tx3ypEbP6DDOHD+ugA@mail.gmail.com>
Date:	Tue, 7 Jan 2014 14:25:44 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Kevin Hilman <khilman@...aro.org>, Ingo Molnar <mingo@...hat.com>,
	Tejun Heo <tj@...nel.org>,
	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>
Subject: Re: [LNG] Re: [Query] Ticks happen in pair for NO_HZ_FULL cores ?

On 7 January 2014 14:17, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Dec 23, 2013 at 01:48:02PM +0530, Viresh Kumar wrote:
>> *But wouldn't it make sense if we can tell scheduler that don't queue
>> these works on a CPU that is running in NO_HZ_FULL mode?*
>
> No,.. that's the wrong way around.

Hmm.. Just to make it clear I didn't meant that any input from workqueue
code should go to scheduler but something like this:

Scheduler will check following before pushing a task on any CPU:
- If that CPU is part of NO_HZ_FULL cpu list?
- If yes, is that CPU running only one task for now? i.e. running task for
best performance case?
- If yes, then don't queue new task to that CPU, whether task belongs to
workqueue or not doesn't matter.

> That looks to be a normal unpinned timer, it should migrate to a 'busy'
> cpu once the one its running on it going idle.
>
> ISTR people trying to make that active and also migrating on nohz full
> or somesuch, just like the workqueues. Forgot what happened with that;
> if it got dropped it should probably be ressurected.

I will search for that in archives..
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