linux-kernel - Re: [PATCH v2 5/5] cpufreq: Catch double invocations of cpufreq_freq_transition

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Date:	Tue, 29 Apr 2014 12:19:39 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Meelis Roos <mroos@...ux.ee>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/5] cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end

On 29 April 2014 11:46, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> Yes, I'm aware that this corner case doesn't work well with my debug

Don't know if its a corner case, it may be the most obvious case for
some :)

> patch. I tried to avoid this but couldn't think of any solution.

The problem is not that it wouldn't work for these systems, but we will
get WARN_ON() when they shouldn't have come :)

> (One big-hammer way to avoid this is to exclude this infrastructure
> for all ASYNC_NOTIFICATION drivers, but I didn't want to go with that
> approach, since it makes it look ugly). Do you have any better ideas
> to deal with this scenario?

Can't think of anything better than this:

+       WARN_ON(!(cpufreq_driver->flags & CPUFREQ_ASYNC_NOTIFICATION)
                             && (current == policy->transition_task));

which you already mentioned.

> Also, do we really have cases in mind where a single thread does
> multiple frequency transitions in one go? That too in such quick
> successions? Echo's to sysfs, changing of governors from userspace etc
> all do one frequency transition at a time per-task...

Its not really about if we can think of a real use case or not. The point
is, governor is free to call transition calls one after the other (will always
happen from a single thread) and it isn't supposed to wait for drivers
to finish earlier transitions as ->target() has already returned.

--
viresh
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