linux-kernel - Re: CPUfreq - udelay() interaction issues

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Message-ID: <20100424055652.GD2290@elf.ucw.cz>
Date:	Sat, 24 Apr 2010 07:56:52 +0200
From:	Pavel Machek <pavel@....cz>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	cpufreq <cpufreq@...r.kernel.org>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Thomas Renninger <trenn@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: CPUfreq - udelay() interaction issues

Hi!

> Seems a bit more complicated than what I had in mind. This is
> touching the scheduler I think we can get away without having to.
> Also, there is no simple implementation for the "slowpath" that can
> guarantee the delay without starting over the loop and hoping not to
> get interrupted or just giving up and doing a massively inaccurate
> delay (like msleep, etc).
> 
> I was thinking of something along the lines of this:
> 
> udelay()
> {
>   if (!is_atomic())
> 	down_read(&freq_sem);
>   /* else
> 	do nothing since cpufreq can't interrupt you.
>   */
> 
>   call usual code since cpufreq is not going to preempt you.
> 
>   if (!is_atomic())
> 	up_read(&freq_sem);
> }

Well, most delays are very short, so...

What about... we decide that cpufreq interruption or switch to
different cpu takes 100usec minimum, and only try to do complex magic
for delays >100usec? Hopefully there's minimum of those :-).

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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