Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 27 Jul 2021 11:54:38 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joel Fernandes <joelaf@google.com>
Cc:     "Rajendran, Jaishankar" <jaishankar.rajendran@intel.com>,
        Dario Faggioli <dfaggioli@suse.com>,
        Vineeth Pillai <vineethrp@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Mallikarjun Chegaraddi, Raju" 
        <raju.mallikarjun.chegaraddi@intel.com>
Subject: Re: Core Scheduling - Concurrent VMs
Message-ID: <YP/X3nSGBfkZ1/c3@hirez.programming.kicks-ass.net>
References: <PH0PR11MB51748333769D9002912FF63C84E89@PH0PR11MB5174.namprd11.prod.outlook.com>
 <CAJWu+oq98Yh4kknze7u6SOYZNw6hR5aFkj61aXbTV3JPq=7RNg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAJWu+oq98Yh4kknze7u6SOYZNw6hR5aFkj61aXbTV3JPq=7RNg@mail.gmail.com>
Precedence: bulk

On Mon, Jul 26, 2021 at 11:57:21AM -0400, Joel Fernandes wrote:
> Hi,
> +Dario Faggioli and +Vineeth Pillai as well.
> 
> On Mon, Jul 26, 2021 at 1:41 AM Rajendran, Jaishankar
> <jaishankar.rajendran@intel.com> wrote:
> >
> > Refer to the below experiments performed using Core Scheduling for Concurrent VMs and we found the benchmark scores executed in different VMs are
> > degrading after enablement of Core Scheduling. Both the host and guest are enabled with Core Scheduler
> 
> Why are you running core scheduling within the guest? Only the host
> needs it and you tag vCPU threads in the host. Within the guest you
> don't need it (and it probably doesn't make sense unless you pin the
> vCPU threads to individual physical hardware threads).

Correct, in-guest core-scheduling only works if there's an in-guest
topology, which requires pinning vcpu threads and sacrificing a chicken
on the altar of qemu to get the right incantation such that the guest
actually receives the right topology.

Nested core scheduling hurts my head.

> I don't think these commands can really tell you that core scheduler
> is working or not. They are too coarse grained. You would need to look
> at it through ftrace scheduling events. Core scheduling will still
> allow differently tagged threads to use a core, it is just that it
> wont allow it simultaneously.

Right, you need tracing to verify it actually works, no real alternative
there.