linux-kernel - Re: [PATCH 2/5] cpufreq: governor: Create separate sysfs-ops

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Date:	Tue, 2 Feb 2016 20:40:31 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rafael Wysocki <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Saravana Kannan <skannan@...eaurora.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 2/5] cpufreq: governor: Create separate sysfs-ops

On Tue, Feb 2, 2016 at 6:01 PM, Juri Lelli <juri.lelli@....com> wrote:
> Hi Rafael,
>
> On 02/02/16 17:35, Rafael J. Wysocki wrote:
>> On Tue, Feb 2, 2016 at 4:47 PM, Juri Lelli <juri.lelli@....com> wrote:
>> > Hi Viresh,
>> >
>> > On 02/02/16 16:27, Viresh Kumar wrote:
>> >> Until now, governors (ondemand/conservative) were using the
>> >> 'global-attr' or 'freq-attr', depending on the sysfs location where we
>> >> want to create governor's directory.
>> >>
>> >> The problem is that, in case of 'freq-attr', we are forced to use
>> >> show()/store() present in cpufreq.c, which always take policy->rwsem.
>> >>
>> >> And because of that we were facing some ABBA lockups during governor
>> >> callback event CPUFREQ_GOV_POLICY_EXIT. And so we were dropping the
>> >> rwsem right before calling governor callback for CPUFREQ_GOV_POLICY_EXIT
>> >> event.
>> >>
>> >> That caused further problems and it never worked perfectly.
>> >>
>> >> This patch attempts to fix that by creating separate sysfs-ops for
>> >> cpufreq governors.
>> >>
>> >> Because things got much simplified now, we don't need separate
>> >> show/store callbacks for governor-for-system and governor-per-policy
>> >> cases.
>> >>
>> >> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
>> >
>> > This patch cleans things up a lot, that's good.
>> >
>> > One thing I'm still concerned about, though: don't we need some locking
>> > in place for some of the store operations on governors attributes? Are
>> > store_{ignore_nice_load, sampling_down_fact, etc} safe without locking?
>>
>> That would require some investigation I suppose.
>>
>> > It seems that we can call them from different cpus concurrently.
>>
>> Yes, we can.
>>
>> One quick-and-dirty way of dealing with that might be to introduce a
>> "sysfs lock" into struct dbs_data and hold that around the invocation
>> of gattr->store() in the sysfs_ops's ->store callback.
>>
>
> There is value in trying to solve this issue by using some of the
> existing locks, IMHO.

Some value - maybe.  I'm not sure how much of it, though.

Finer-grained locking is generally easier to follow, because the locks
tend to be used for specific purposes only.

> Can't we actually try to use the policy->rwsem (or one of the core
> locks) + wait_for_completion approach as we do in cpufreq core?

No.  Too many things depend on that lock already and some of them work
by accident rather than by design.

Thanks,
Rafael