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

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Message-ID: <BANLkTikaymG-5-YoTnG2SPJCnBtXPpfHBQ@mail.gmail.com>
Date:	Fri, 15 Apr 2011 12:33:12 -0400
From:	Mike Frysinger <vapier@...too.org>
To:	Pavel Machek <pavel@....cz>
Cc:	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 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.
-mike
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