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Date:	Tue, 14 Oct 2014 19:18:25 +0000
From:	"Elliott, Robert (Server Storage)" <Elliott@...com>
To:	Prarit Bhargava <prarit@...hat.com>,
	Viresh Kumar <viresh.kumar@...aro.org>
CC:	Saravana Kannan <skannan@...eaurora.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Robert Schöne <robert.schoene@...dresden.de>
Subject: RE: Locking issues with cpufreq and sysfs



> -----Original Message-----
> From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-
> owner@...r.kernel.org] On Behalf Of Prarit Bhargava
> Sent: Tuesday, 14 October, 2014 1:24 PM
> To: Viresh Kumar
> Cc: Saravana Kannan; Rafael J. Wysocki; linux-pm@...r.kernel.org; Linux
> Kernel; Robert Schöne
> Subject: Re: Locking issues with cpufreq and sysfs
> 
> On 10/14/2014 03:10 AM, Viresh Kumar wrote:
> > On 13 October 2014 18:41, Prarit Bhargava <prarit@...hat.com> wrote:
> >>
> >> The locking is insufficient here, Viresh.  I no longer believe that fixes
> >> to this locking scheme are the right way to move forward here.  I'm
> wondering
> >> if we can look at other alternatives such as maintaining a refcount or
> >> perhaps using a queuing mechanism for governor and policy related changes.
> >>
...
> So I'm proposing that we move to a single threaded read/write using, if
> possible, a single policy lock for now.  We might transition this back to a
> rwsem later on, however, for the first attempt at cleaning this up I think we
> should just stick with a simple lock.  In doing that, IMO we remove
> cpufreq_rwsem: protects the driver from being unloaded
> cpufreq_governor_lock: protects the current governor
> each policy has a rwsem (policy->rwsem): protects the cpufreq_policy struct
> 
> and potentially
> cpufreq_driver_lock: protects the cpufreq_cpu_data array and cpufreq_driver-
> >boost
> 
> After looking at the way the code would be structured, I'm wondering if
> cpufreq_governor_mutex: protects the cpufreq_governor_list
> is overkill.  The loading of a module should be atomic relative to the
> cpufreq code, so this lock may not be required.  (Admittedly I haven't
> tested that...)
> 
> That would leave:
> global_kobj_lock: protects the "cpufreq" kobject
> each policy has a transition_lock (policy->transition): synchronizes
>                                                         frequency transitions
> and a new lock, perhaps called policy->lock, to serialize all events.
> 

Please keep performance in mind too.  cpufreq_governor_lock
contention is a bit of an issue with heavy IO workloads
as described in:
	http://marc.info/?l=linux-pm&m=140924051503827&w=2


---
Rob Elliott    HP Server Storage



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