Subject: Re: [Patch] idle governor: Avoid lock acquisition to read pm_qos
 before entering idle
From: Tim Chen <tim.c.chen@linux.intel.com>
To: markgross@thegnar.org
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>,
        James Bottomley <James.Bottomley@suse.de>,
        David Alan Gilbert <linux@treblig.org>, linux-kernel@vger.kernel.org,
        Len <len.brown@intel.com>, Andi Kleen <ak@linux.intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>
In-Reply-To: <20110210051042.GC4897@gvim.org>
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	 <20110210051042.GC4897@gvim.org>
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Date: Thu, 10 Feb 2011 09:27:24 -0800
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On Wed, 2011-02-09 at 21:10 -0800, mark gross wrote:

> 
> I'm surprised by this as the last update to the pm_qos replaced the
> lists with a O(1) data structure so there was no more walking of pending
> requests.

But you need to acquire a lock before you can read the value on the list
within the function pm_qos_request.  This is a problem if there are a
lot of cpus doing so.

> 
> What is the profile after the patch the Plist should be only one
> dereference and an if instruction slower than a cached value.

After the patch, the acquisition of the lock on plist go away from the
profile, and I see a 12% improvement in throughput to the message
passing benchmark I was running.

> 
> Does your patch remove the need for the locks because if it doesn't I
> don't see how it will make much of a difference?

We still need the lock to update/remove/insert values in the plist and
to update the cached value.  The intention of the patch is to avoid lock
acquisition by reading from a cached value that is up to date. Lock
acquisition is needed --every time-- when a cpu go into idle, which is
bad as you want to let a cpu go to idle ASAP.


Tim

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