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

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Date:	Tue, 2 Feb 2016 17:35:22 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	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 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.

>
> Best,
>
> - Juri

BTW, you could have dropped the stuff below this line from your reply
message.  That at least would have prevented tools like Patchwork from
storing useless garbage.

Thanks,
Rafael