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Tue, 27 May 2025 08:47:36 -0700 (PDT) Date: Tue, 27 May 2025 11:47:34 -0400 From: Yury Norov To: Peter Zijlstra Cc: Shrikanth Hegde , mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, tglx@linutronix.de, maddy@linux.ibm.com, vschneid@redhat.com, dietmar.eggemann@arm.com, rostedt@goodmis.org, jstultz@google.com, kprateek.nayak@amd.com, huschle@linux.ibm.com, srikar@linux.ibm.com, linux-kernel@vger.kernel.org, linux@rasmusvillemoes.dk Subject: Re: [RFC PATCH 0/5] sched: cpu parked and push current task mechanism Message-ID: References: <20250523181448.3777233-1-sshegde@linux.ibm.com> <20250527151020.GV24938@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250527151020.GV24938@noisy.programming.kicks-ass.net> 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