linux-kernel - Re: Why do we still have 32 bit counters? Interrupt counters overflow within 50 days

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Message-ID: <alpine.DEB.2.11.1410061426471.4383@nanos>
Date:	Mon, 6 Oct 2014 14:28:26 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	David Lang <david@...g.hm>
cc:	Christoph Lameter <cl@...ux.com>,
	Richard Cochran <richardcochran@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Why do we still have 32 bit counters? Interrupt counters overflow
 within 50 days

On Mon, 6 Oct 2014, David Lang wrote:
> On Sun, 5 Oct 2014, Thomas Gleixner wrote:
> 
> > > 
> > > 64 bit counters on 32 bit machines are not an easy thing and could be
> > 
> > Whats so hard about 64bit counters on 32bit machines?
> > 
> > > expensive to handle in particular because these counters are used in
> > > performance critical hotpaths.
> > 
> > The expensive overhead is a single "adcl" instruction.
> 
> As I understand it, since the 64 bit math cannot be made atomic, it requires
> protecting the counter with a lock so that it can't be read while half
> updated. Aquiring a lock on every update is an expensive thing to do. It's not
> something people like to see in a fast path, especially for something of as
> low an importance as the counters.

As I said before. Both reader and writer side are already protected by
the irq descriptor lock. We take that lock anyway.

Thanks,

	tglx
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