linux-kernel - Re: [patch v12 00/13] extensible prctl task isolation interface and vmstat sync

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Date:   Wed, 04 May 2022 15:20:03 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Christoph Lameter <cl@...two.de>
Cc:     linux-kernel@...r.kernel.org, Nitesh Lal <nilal@...hat.com>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alex Belits <abelits@...its.com>, Peter Xu <peterx@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Oscar Shiang <oscar0225@...email.tw>,
        linux-rdma@...r.kernel.org
Subject: Re: [patch v12 00/13] extensible prctl task isolation interface and
 vmstat sync

On Tue, May 03 2022 at 15:57, Marcelo Tosatti wrote:
> On Wed, Apr 27, 2022 at 11:19:02AM +0200, Christoph Lameter wrote:
>> I could modify busyloop() in ib2roce.c to use the oneshot mode via prctl
>> provided by this patch instead of the NOHZ_FULL.
>> 
>> What kind of metric could I be using to show the difference in idleness of
>> the quality of the cpu isolation?
>
> Interruption length and frequencies:
>
> -------|xxxxx|---------------|xxx|---------
> 	 5us 		      3us
>
> which is what should be reported by oslat ?

How is oslat helpful there? That's running artifical workload benchmarks
which are not necessarily representing the actual
idle->interrupt->idle... timing sequence of the real world usecase.

> Inheritance is an attempt to support unmodified binaries like so:
>
> 1) configure task isolation parameters (eg sync per-CPU vmstat to global
> stats on system call returns).
> 2) enable inheritance (so that task isolation configuration and
> activation states are copied across to child processes).
> 3) enable task isolation.
> 4) execv(binary, params)

What for? If an application has isolation requirements, then the
specific requirements are part of the application design and not of some
arbitrary wrapper. Can we please focus on the initial problem of
providing a sensible isolation mechanism with well defined semantics?

Inheritance is an orthogonal problem and there is no reason to have this
initially.

>> Special handling when the scheduler
>> switches a task? If tasks are being switched that requires them to be low
>> latency and undisturbed then something went very very wrong with the
>> system configuration and the only thing I would suggest is to issue some
>> kernel warning that this is not the way one should configure the system.
>
> Trying to provide mechanisms, not policy? 

This preemption notifier is not a mechanism, it's simply mindless
hackery as I told you already.

Thanks,

        tglx