linux-kernel - Re: [RFC PATCH 0/5] sched: cpu parked and push current task mechanism

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Message-ID: <aDXelhCbIvKjZyqG@yury>
Date: Tue, 27 May 2025 11:47:34 -0400
From: Yury Norov <yury.norov@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Shrikanth Hegde <sshegde@...ux.ibm.com>, mingo@...hat.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	tglx@...utronix.de, maddy@...ux.ibm.com, vschneid@...hat.com,
	dietmar.eggemann@....com, rostedt@...dmis.org, jstultz@...gle.com,
	kprateek.nayak@....com, huschle@...ux.ibm.com, srikar@...ux.ibm.com,
	linux-kernel@...r.kernel.org, linux@...musvillemoes.dk
Subject: Re: [RFC PATCH 0/5] sched: cpu parked and push current task mechanism

On Tue, May 27, 2025 at 05:10:20PM +0200, Peter Zijlstra wrote:
> On Fri, May 23, 2025 at 11:44:43PM +0530, Shrikanth Hegde wrote:
> > In a para-virtualised environment, there could be multiple
> > overcommitted VMs. i.e sum of virtual CPUs(vCPU) > physical CPU(pCPU). 
> > When all such VMs request for cpu cycles at the same, it is not possible
> > to serve all of them. This leads to VM level preemptions and hence the
> > steal time. 
> > 
> > Bring the notion of CPU parked state which implies underlying pCPU may
> > not be available for use at this time. This means it is better to avoid
> > this vCPU. So when a CPU is marked as parked, one should vacate it as
> > soon as it can. So it is going to dynamic at runtime and can change
> > often.
> 
> You've lost me here already. Why would pCPU not be available? Simply
> because it is running another vCPU? I would say this means the pCPU is
> available, its just doing something else.
> 
> Not available to me means it is going offline or something like that.
> 
> > In general, task level preemption(driven by VM) is less expensive than VM
> > level preemption(driven by hypervisor). So pack to less CPUs helps to
> > improve the overall workload throughput/latency. 
> 
> This seems to suggest you're 'parking' vCPUs, while above you seemed to
> suggest pCPU. More confusion.
> 
> > cpu parking and need for cpu parking has been explained here as well [1]. Much
> > of the context explained in the cover letter there applies to this
> > problem context as well. 
> > [1]: https://lore.kernel.org/all/20250512115325.30022-1-huschle@linux.ibm.com/
> 
> Yeah, totally not following any of that either :/
> 
> 
> Mostly I have only confusion and no idea what you're actually wanting to
> do.

My wild guess is that the idea is to not preempt the pCPU while running
a particular vCPU workload. But I agree, this should all be reworded and
explained better. I didn't understand this, either.

Thanks,
YUry