linux-kernel - Re: [patch 05/12] timekeeping_notify: use stop_machine

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Date: Thu, 08 Feb 2024 16:23:58 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Marcelo Tosatti <mtosatti@...hat.com>
Cc: linux-kernel@...r.kernel.org, Daniel Bristot de Oliveira
 <bristot@...nel.org>, Juri Lelli <juri.lelli@...hat.com>, Valentin
 Schneider <vschneid@...hat.com>, Frederic Weisbecker
 <frederic@...nel.org>, Leonardo Bras <leobras@...hat.com>, Peter Zijlstra
 <peterz@...radead.org>
Subject: Re: [patch 05/12] timekeeping_notify: use stop_machine_fail when
 appropriate

On Wed, Feb 07 2024 at 09:58, Marcelo Tosatti wrote:
> On Wed, Feb 07, 2024 at 12:57:19PM +0100, Thomas Gleixner wrote:
>> On Tue, Feb 06 2024 at 15:49, Marcelo Tosatti wrote:
>> > Change timekeeping_notify to use stop_machine_fail when appropriate,
>> > which will fail in case the target CPU is tagged as block interference
>> > CPU.
>> 
>> You completely fail to explain 'appropriate'. There is zero reason for
>> this churn, really.
>
> The churn is so that we can return an error to
> current_clocksource_store (sysfs handler for writes to
> /sys/devices/system/clocksource/clocksource0/current_clocksource).

What for? Why?

Writing to that file requires root. Root can rightfully screw up a
system and adding a debugfs based "prevention" mechanism is not making
this any better because root can just clear the CPU mask there and move
on.

So what is the actual real world problem solved by these patches?

All I've seen so far is handwaving about interference prevention and TBH
I can't squint hard enough to believe that.

Thanks,

        tglx