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

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Date:	Tue, 2 Feb 2016 17:01:44 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
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

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.

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?

> 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.
> 

Right. Sorry for the garbage; I'll check twice that I trim my replies in
the future.

Best,

- Juri