linux-kernel - Re: [PATCH 3/5] mm/vmstat: use cmpxchg loop in cpu_vm_stats

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Message-ID: <47a289f0-ae96-e0e6-c579-70ec61fff384@gentwo.de>
Date:   Mon, 6 Feb 2023 10:42:46 +0100 (CET)
From:   Christoph Lameter <cl@...two.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>
cc:     Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/5] mm/vmstat: use cmpxchg loop in cpu_vm_stats_fold

On Fri, 3 Feb 2023, Marcelo Tosatti wrote:

> > Isnt there some lock/serializtion to stall the kernel until you are done?
>
> Not that i know of. Anyway, an additional datapoint is:
>
> "Software defined PLC"
> (https://www.redhat.com/en/blog/software-defined-programmable-logic-controller-introduction),
> applications
> can perform system calls in their time sensitive loop.
>
> One example of an opensource software is OpenPLC.
>
> One would like to avoid interruptions for those cases as well.

Well allowing sytem calls during "time sensitiveness" implies
it is not that sensitive to vmstat updates which have a smaller impact
than system calls.

Unless we are talking about virtual system calls like gettimeofday or
clock_gettime. These do not enter the kernel if configured correctly.