linux-kernel - Re: [PATCH V2 0/7] cpufreq: governors: Fix ABBA lockups

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Message-ID: <20160205024428.GB3068@vireshk>
Date:	Fri, 5 Feb 2016 08:14:28 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	Shilpa Bhat <shilpabhatppc@...il.com>,
	Juri Lelli <juri.lelli@....com>,
	Rafael Wysocki <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Turquette <mturquette@...libre.com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	dietmar.eggemann@....com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 0/7] cpufreq: governors: Fix ABBA lockups

On 04-02-16, 19:18, Rafael J. Wysocki wrote:
> On Thu, Feb 4, 2016 at 6:44 PM, Saravana Kannan <skannan@...eaurora.org> wrote:
> > On 02/04/2016 09:43 AM, Saravana Kannan wrote:

> >> No no no no! Let's not open up this can of worms of queuing up the work
> >> to handle a write to a sysfs file. It *MIGHT* work for this specific
> >> tunable (I haven't bothered to analyze), but this makes it impossible to
> >> return a useful/proper error value.
> >
> >
> > Sent too soon. Not only that, but it can also cause the writes to the sysfs
> > files to get processed in a different order and I don't know what other
> > issues/races THAT will open up.
> 
> Well, I don't like this too.

I expected similar responses only, so no surprises for me :)

Though there are few things I would like to tell here:
- There wouldn't be any race for updating the file, as that is done
  directly from store_sampling_rate(). It updates the *real* file we
  wanted to.

- What's offloaded to the work-handler is something very special about
  ondemand governor and sampling rate. The same is not done for
  conservative governor as well, don't know why though.

- After updating the sampling rate, we assess if we need to reschedule
  the timers/workqueue to a different time for better efficiency. I
  don't think there can be a race there and it can be safely done in a
  work..

> I actually do have an idea about how to fix these deadlocks, but it is
> on top of my cleanup series.

I would like to hear that, if you can, to save your time. I have tried
so many different ways of fixing this yesterday and found issue
everywhere :(

-- 
viresh