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

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Message-ID: <20190209034117.GC21860@bombadil.infradead.org>
Date:   Fri, 8 Feb 2019 19:41:17 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     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, 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

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.

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.