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.2.11.1410061014080.29937@gentwo.org>
Date:	Mon, 6 Oct 2014 10:16:28 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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, 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?

>
> 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.

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