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

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Date:	Sun, 5 Oct 2014 23:49:14 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Christoph Lameter <cl@...ux.com>
cc:	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 Fri, 3 Oct 2014, Christoph Lameter wrote:
> On Fri, 3 Oct 2014, Richard Cochran wrote:
> 
> > >  DECLARE_PER_CPU(char *, irq_stack_ptr);
> > > -DECLARE_PER_CPU(unsigned int, irq_count);
> > > +DECLARE_PER_CPU(unsigned long, irq_count);
> >
> > Still 32 bit on 32 bit machines...
> 
> 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.

> I thought I better leave it alone on 32 bit.

And how exactly are we supposed to explain the different behaviour to
users?

Thanks,

	tglx
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