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

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Message-ID: <CAEXW_YQWngifqRFpS_H3RjDBwV_uH_OebbYzB63Q=jSwheqjLA@mail.gmail.com>
Date: Wed, 17 Jul 2024 17:09:20 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Sean Christopherson <seanjc@...gle.com>, 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, Jul 17, 2024 at 5:00 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 17 Jul 2024 16:57:43 -0400
> Joel Fernandes <joel@...lfernandes.org> wrote:
>
> > On Wed, Jul 17, 2024 at 11:20 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> > >
> > > On Wed, 17 Jul 2024 10:52:33 -0400
> > > Steven Rostedt <rostedt@...dmis.org> wrote:
> > >
> > > > We could possibly add a new sched class that has a dynamic priority.
> > >
> > > It wouldn't need to be a new sched class. This could work with just a
> > > task_struct flag.
> > >
> > > It would only need to be checked in pick_next_task() and
> > > try_to_wake_up(). It would require that the shared memory has to be
> > > allocated by the host kernel and always present (unlike rseq). But this
> > > coming from a virtio device driver, that shouldn't be a problem.
> >
> > Problem is its not only about preemption, if we set the vCPU boosted
> > to RT class, and another RT task is already running on the same CPU,
>
> That can only happen on wakeup (interrupt). As the point of lazy
> priority changing, it is only done when the vCPU is running.

True, but I think it will miss stuff related to load balancing, say if
the "boost" is a higher CFS priority. Then someone has to pull the
running vCPU thread to another CPU etc... IMO it is better to set the
priority/class externally and let the scheduler deal with it. Let me
think some more about your idea though..

thanks,

 - Joel