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

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Message-Id: <201104160111.49409.rjw@sisk.pl>
Date:	Sat, 16 Apr 2011 01:11:49 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Mike Frysinger <vapier@...too.org>
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 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?

Rafael
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