linux-kernel - Re: [RFC PATCH RT 0/2] Add PINNED

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Date:   Wed, 20 Feb 2019 08:47:51 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     tglx@...utronix.de, linux-rt-users@...r.kernel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        bristot@...hat.com, williams@...hat.com
Subject: Re: [RFC PATCH RT 0/2] Add PINNED_HARD mode to hrtimers

On 19/02/19 18:19, Sebastian Andrzej Siewior wrote:
> On 2019-02-14 14:37:14 [+0100], Juri Lelli wrote:
> > Hi,
> Hi,
> 
> > Now, I'm sending this and an RFC, as I'm wondering if the first behavior
> > is actually what we want, and it is not odd at all for reasons that are
> > not evident to me at the moment. In this case this posting might also
> > function as a question: why we need things to work as they are today?
> 
> There is /proc/sys/kernel/timer_migration which should disable this but
> I think you know that already.
> 
> So this is a NO_HZ feature. Basically try to move all the timers to a
> designated CPU so all others can deep idle while one CPU does the work.
> Ideally you have no timer which is pending / will expire if you go idle.
> And then, once the timer fires the housekeeping CPU does the work so
> chances are that the CPU, that programmed the timer, may remain idle.

Right.

> In this case you prepare the wakeup and then wake the CPU anyway. There
> should be no downside to this unless the housekeeping CPU is busy and in
> irq-off regions which would increase the latency. Also in case of
> 	cyclictest -d0
> 
> the one CPU would have to process all timers. So the latency will be
> worse compared to every CPU does its own wakeup. And on RT you probably
> do not want to do deep idle anyway.

Mmm, right. But, still very much dependent on the workload, I understand
you are saying? So, no one size fits all solution.

Thanks,

- Juri