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.2.11.1410061726510.4383@nanos>
Date:	Mon, 6 Oct 2014 17:28:47 +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 Mon, 6 Oct 2014, Christoph Lameter wrote:

> On Mon, 6 Oct 2014, Thomas Gleixner wrote:
> 
> > So if you want to fix that as well, you really need to think about the
> > 32 bit case because there is no serialization for the interrupts which
> > are delivered directly from their own vector. And no, we should not
> > diverge 32 and 64 bit artificially here simply because the same 50
> > days wrap applies to both.
> 
> Is it a divergence if both 64bit and 32 bit are unsing unsigned long?

Sigh, yes. Because unsigned long is 32bit on a 32bit architecture. So
the change would be NOP for 32bit and 32bit would still suffer from
the wrap arounds etc.
 
> >
> > I really start to wonder whether all this is worth the trouble. It has
> > been this way forever and 1k timer interrupts per second is not really
> > a new thing either. So we did not change anything which suddenly makes
> > tools confused.
> 
> Tools expect the number of interrupt to increase linearly and not jump by
> 2^32 once in awhile. There are functions in the kernel (/proc/stat) that
> sum up various interrupt counters and that are types unsigned long. These
> larger numbers can suddenly jump by 2^32. Its pretty unusual for a 64 bit
> conter to do that and it requires some head scratching until we figured
> that one out.

I understand that, I just wonder why nobody noticed before. It's been
that way forever :)

Thanks,

	tglx

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