Re: Xen on ARM IRQ latency and scheduler overhead

From: Dario Faggioli <dario.faggioli@citrix.com>
To: Julien Grall <julien.grall@arm.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	xen-devel@lists.xen.org
Cc: george.dunlap@eu.citrix.com, edgar.iglesias@xilinx.com, nd@arm.com
Subject: Re: Xen on ARM IRQ latency and scheduler overhead
Date: Sat, 18 Feb 2017 01:59:11 +0100	[thread overview]
Message-ID: <1487379551.6732.117.camel@citrix.com> (raw)
In-Reply-To: <69783c6d-1e65-3fa4-8b71-8c99d9e23eb6@arm.com>

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On Fri, 2017-02-17 at 19:44 +0000, Julien Grall wrote:
> On 02/17/2017 06:40 PM, Dario Faggioli wrote:
> > Does ARM
> > have frequency scaling (I did remember something on xen-devel, but
> > I am
> > not sure whether it landed upstream)?
> 
> I guess you mean the series sent by globallogic ([1])? If so, it was 
> never upstreamed.
> 
Yep, that one. I was quite sure it did not went in, but was in hurry,
and could not double check the code. Sorry.

> However the frequency scaling may depend on the processor used and 
> sometimes implemented using big.LITTLE (e.g the task has to switch).
> 
> I am not sure why it would matter in this case.
> 
It's hard to tell whether or not (and if yes, how much) it matter in
this specific case. In general, when I was working on squeezing latency
on bare-metal Linux, disabling scaling was one of the steps.

It depends on quite a few different factors, but, roughly, during the
time you're idle waiting for the interrupt, you're running at low
frequency.

When the interrupt comes, what happens depends on the governor, but you
usually don't immediately jump to the highest frequency, because that
happens gradually. And if all you do is something quick, in response to
the timer interrupt (like in this and similar cases), you most likely
end up doing that running at low frequency, which may impact latency
(at least the latency of the completion of the work that the timer
triggered).

But I was also referring to the fact that the app is using some
hardware counters, and a frequency value for updating them, and for
converting their value to time. So I was wondering how the fact that
frequency may be changing dynamically may interact with that (and
that's because I'm pretty ignorant about ARM internals, I know :-/).

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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