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.2.11.1410060952030.4383@nanos>
Date:	Mon, 6 Oct 2014 09:54:14 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
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 Sun, 5 Oct 2014, Eric Dumazet wrote:
> On Sun, 2014-10-05 at 23:49 +0200, Thomas Gleixner wrote:
> 
> > Whats so hard about 64bit counters on 32bit machines?
> 
> Not hard, but not trivial either.
> 
> > 
> > > expensive to handle in particular because these counters are used in
> > > performance critical hotpaths.
> > 
> > The expensive overhead is a single "adcl" instruction.
> > 
> 
> Assuming a reader do not care of reading garbage yes, while carry is not
> yet propagated.

Readers and writers are serialized via desc->lock.

Thanks,

	tglx
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