linux-kernel - Re: [PATCH v5 08/24] sched: Introduce per memory space current virtual cpu id

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Message-ID: <Y3aHMTNv0yiyHCun@google.com>
Date:   Thu, 17 Nov 2022 19:10:41 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org, Christian Brauner <brauner@...nel.org>,
        Florian Weimer <fw@...eb.enyo.de>, David.Laight@...lab.com,
        carlos@...hat.com, Peter Oskolkov <posk@...k.io>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Chris Kennelly <ckennelly@...gle.com>
Subject: Re: [PATCH v5 08/24] sched: Introduce per memory space current
 virtual cpu id

On Thu, Nov 17, 2022, Mathieu Desnoyers wrote:
> On 2022-11-14 15:49, Sean Christopherson wrote:
> > On Fri, Nov 11, 2022, Mathieu Desnoyers wrote:
> > > On 2022-11-10 23:41, Andy Lutomirski wrote:
> > > > On Thu, Nov 3, 2022 at 1:05 PM Mathieu Desnoyers
> > > > <mathieu.desnoyers@...icios.com> wrote:
> > > > Also, in my mind "virtual cpu" is vCPU, which this isn't.  Maybe
> > > > "compacted cpu" or something?  It's a strange sort of concept.
> > > 
> > > I've kept the same wording that has been introduced in 2011 by Paul Turner
> > > and used internally at Google since then, although it may be confusing if
> > > people expect kvm-vCPU and rseq-vcpu to mean the same thing. Both really end
> > > up providing the semantic of a virtually assigned cpu id (in opposition to
> > > the logical cpu id on the system), but this is much more involved in the
> > > case of KVM.
> > 
> > I had the same reaction as Andy.  The rseq concepts don't worry me so much as the
> > existence of "vcpu" in mm_struct/task_struct, e.g. switch_mm_vcpu() when switching
> > between KVM vCPU tasks is going to be super confusing.  Ditto for mm_vcpu_get()
> > and mm_vcpu_put() in the few cases where KVM currently does mmget()/mmput().
> 
> I'm fine with changing the wording if it helps make things less confusing.
> 
> Should we go for "compact-cpu-id" ? "packed-cpu-id" ? Other ideas ?

What about something like "process-local-cpu-id" to capture that the ID has meaning
only within the associated address space / process?