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

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Date:   Fri, 3 Feb 2023 15:52:14 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Christoph Lameter <cl@...two.de>
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, Feb 03, 2023 at 10:34:22AM +0100, Christoph Lameter wrote:
> On Thu, 2 Feb 2023, Marcelo Tosatti wrote:
> 
> > > I thought you would only run this while the kernel is not active on the
> > > remote cpu? Then you dont need any cmpxchg and you can leave the function
> > > as is.
> >
> > The remote cpu can enter kernel mode while this function executes.
> 
> 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.

> > There is no mode which indicates userspace cannot enter the kernel.
> 
> There are lot of thinngs that happen upon entry to the kernel. I would
> hope that you can do something there. Scheduler?

The use-case in question is with isolation, where a CPU is dedicated
to a single task. So the scheduler should not be an issue.