linux-kernel - Re: [RFC PATCH v2 0/5] Paravirt Scheduling (Dynamic vcpu priority management)

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Message-ID: <20240717103647.735563af@rorschach.local.home>
Date: Wed, 17 Jul 2024 10:36:47 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Joel Fernandes <joel@...lfernandes.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Vineeth Remanan Pillai
 <vineeth@...byteword.org>, Ben Segall <bsegall@...gle.com>, Borislav Petkov
 <bp@...en8.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, Dave
 Hansen <dave.hansen@...ux.intel.com>, Dietmar Eggemann
 <dietmar.eggemann@....com>, "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar
 <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, Mel Gorman
 <mgorman@...e.de>, Paolo Bonzini <pbonzini@...hat.com>, Andy Lutomirski
 <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner
 <tglx@...utronix.de>, Valentin Schneider <vschneid@...hat.com>, Vincent
 Guittot <vincent.guittot@...aro.org>, Vitaly Kuznetsov
 <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Suleiman Souhlal
 <suleiman@...gle.com>, Masami Hiramatsu <mhiramat@...nel.org>,
 himadrics@...ia.fr, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 x86@...nel.org, graf@...zon.com, drjunior.org@...il.com
Subject: Re: [RFC PATCH v2 0/5] Paravirt Scheduling (Dynamic vcpu priority
 management)

On Wed, 17 Jul 2024 07:14:59 -0700
Sean Christopherson <seanjc@...gle.com> wrote:

> > What you're saying is the scheduler should change the priority of the
> > vCPU thread dynamically. That's really not the job of the scheduler.
> > The user of the scheduler is what changes the priority of threads, not
> > the scheduler itself.  
> 
> No.  If we go the proposed route[*] of adding a data structure that lets userspace
> and/or the guest express/adjust the task's priority, then the scheduler simply
> checks that data structure when querying the priority of a task.

The problem with that is the only use case for such a feature is for
vCPUS. There's no use case for a single thread to up and down its
priority. I work a lot in RT applications (well, not as much anymore,
but my career was heavy into it). And I can't see any use case where a
single thread would bounce its priority around. In fact, if I did see
that, I would complain that it was a poorly designed system.

Now for a guest kernel, that's very different. It has to handle things
like priority inheritance and such, where bouncing a threads (or its
own vCPU thread) priority most definitely makes sense.

So you are requesting that we add a bad user space interface to allow
lazy priority management from a thread so that we can use it in the
proper use case of a vCPU?

-- Steve