linux-kernel - Re: [Patch] idle governor: Avoid lock acquisition to read pm

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Date:	Thu, 10 Feb 2011 19:50:30 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	James Bottomley <James.Bottomley@...e.de>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	mark gross <markgross@...gnar.org>,
	David Alan Gilbert <linux@...blig.org>,
	linux-kernel@...r.kernel.org, Len <len.brown@...el.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [Patch] idle governor: Avoid lock acquisition to read pm_qos before entering idle

On Thursday, February 10, 2011, James Bottomley wrote:
> On Wed, 2011-02-09 at 17:21 -0800, Tim Chen wrote:
> > I noticed that before entering idle state, the menu idle governor will
> > look up the current pm_qos value according to the list of qos request
> > received.  This look up currently needs the acquisition of a lock to go
> > down a list of qos requests to find the qos value, slowing down the
> > entrance into idle state due to contention by multiple cpus to traverse
> > this list.  The contention is severe when there are a lot of cpus waking
> > and going into idle.  For example, for a simple workload that has 32
> > pair of processes ping ponging messages to each other, where 64 cores
> > cores are active in test system, I see the following profile:
> > 
> > -     37.82%          swapper  [kernel.kallsyms]          [k]
> > _raw_spin_lock_irqsave
> >    - _raw_spin_lock_irqsave
> >       - 95.65% pm_qos_request             
> >            menu_select                                             
> >            cpuidle_idle_call                      
> >          - cpu_idle                                                     
> >               99.98% start_secondary
> > 
> > Perhaps a better approach will be to cache the updated pm_qos value so
> > reading it does not require lock acquisition as in the patch below.   
> > 
> > Tim
> > 
> > Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> 
> What you say is true as long as the value is 32 bits ... perhaps a note
> of that should be made somewhere?
> 
> Otherwise, it looks like a good lockless optimisation on the read path,
> so you can add my ack.

Thanks James, I'm going to take the patch.

Rafael
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