linux-kernel - RE: [patch V2 1/2] genriq: Avoid summation loops for /proc/stat

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Date:   Wed, 13 Feb 2019 15:55:07 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthew Wilcox' <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Waiman Long <longman@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Kees Cook <keescook@...omium.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Miklos Szeredi" <miklos@...redi.hu>,
        Daniel Colascione <dancol@...gle.com>,
        "Dave Chinner" <david@...morbit.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Marc Zyngier" <marc.zyngier@....com>
Subject: RE: [patch V2 1/2] genriq: Avoid summation loops for /proc/stat

From: Matthew Wilcox
> Sent: 09 February 2019 03:41
> 
> On Fri, Feb 08, 2019 at 03:21:51PM -0800, Andrew Morton wrote:
> > It sounds like it.  A 10khz interrupt will overflow in 4 days...
> 
> If you've got a 10kHz interrupt, you have a bigger problem.  Anything
> happening 10,000 times a second is going to need interrupt mitigation
> to perform acceptably.

Not necessarily - you may want the immediate interrupt for each
received ethernet packet.

> More importantly, userspace can (and must) cope with wrapping.  This isn't
> anything new from Thomas' patch.  As long as userspace is polling more
> often than once a day, it's going to see a wrapped value before it wraps
> again, so it won't miss 4 billion interrupts.

If userspace is expected to detect wraps, making the sum 64bit is
pointless, confusing and stupid.
The code would have to mask off the high bits before determining
that the value has decreased and then adding in 2^32.

	David

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