linux-kernel - Re: [uclinux-dist-devel] [linux-pm] freezer: should barriers be smp ?

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Date:	Fri, 15 Apr 2011 19:24:47 -0400
From:	Mike Frysinger <vapier@...too.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Pavel Machek <pavel@....cz>,
	Alan Stern <stern@...land.harvard.edu>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [uclinux-dist-devel] [linux-pm] freezer: should barriers be smp ?

On Fri, Apr 15, 2011 at 19:11, Rafael J. Wysocki wrote:
> On Friday, April 15, 2011, Mike Frysinger wrote:
>> On Fri, Apr 15, 2011 at 12:29, Pavel Machek wrote:
>> >> > > I believe the code is correct as is.
>> >> >
>> >> > that isnt what the code / documentation says.  unless i'm reading them
>> >> > wrong, both seem to indicate that the proposed patch is what we
>> >> > actually want.
>> >>
>> >> The existing code is correct but it isn't optimal.
>> >>
>> >> wmb() and rmb() are heavy-duty operations, and you don't want to call
>> >> them when they aren't needed.  That's exactly what smp_wmb() and
>> >> smp_rmb() are for -- they call wmb() and rmb(), but only in SMP
>> >> kernels.
>> >>
>> >> Unless you need to synchronize with another processor (not necessarily
>> >> a CPU, it could be something embedded within a device), you should
>> >> always use smp_wmb() and smp_rmb() rather than wmb() and rmb().
>> >
>> > Maybe; but this code is not performance critical and I believe being
>> > obvious here is better...
>>
>> isnt it though ?  especially when we talk about suspending/resuming on
>> embedded systems to get more savings over just cpu idle ?  we want
>> that latency to be as low as possible.
>
> I agree, we can switch the freezer to smp_ barriers, but not for the reason
> you gave before. :-)
>
> Care to repost the patch with a suitable changelog?

np

to be clear, what you said wrt the Blackfin smp barriers still holds
true right ?  so this changset i merged doesnt need any tweaking ...
http://git.kernel.org/?p=linux/kernel/git/vapier/blackfin.git;a=commitdiff;h=943aee0c685d0563228d5a2ad9c8394ad0300fb5
-mike
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