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

[Mathieu Desnoyers]

On 2022-11-21 14:00, Mathieu Desnoyers wrote:
> On 2022-11-17 16:15, Sean Christopherson wrote:
> > On Thu, Nov 17, 2022, Mathieu Desnoyers wrote:
> > > On 2022-11-17 14:10, Sean Christopherson wrote:
> > > > 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@xxxxxxxxxxxx> 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?
> > >
> > > Considering that the shorthand for "memory space" is "VM" in e.g. 
> > > "CLONE_VM"
> >
> > No objection from me for "vm", I've already had to untrain myself and 
> > remember
> > that "vm" doesn't always mean "virtual machine" :-)
> >
> > > clone(2) flags, perhaps "vm-cpu-id", "vm-local-cpu-id" or 
> > > "per-vm-cpu-id" ?
> >
> > I have a slight preference for vm-local-cpu-id, but any of 'em work 
> > for me.
>
> Taking a step back wrt naming (because if I do a name change across the 
> series, I want it to be the last time I do it):
>
> - VM (kvm) vs vm_ (rseq) is confusing.
> - vCPU (kvm) vs vcpu (rseq) is confusing.
>
> I propose "Address Space Concurrency ID". This indicates that those IDs 
> are really just tags assigned uniquely within an address space for each 
> thread running concurrently (and only while they are running).
>
> Then the question that arises is whether the abbreviation presented to 
> user-space should be "mm_cid" (as would be expected from an internal 
> implementation perspective) or "as_cid" (which would match the name 
> exposed to user-space) ?

Or it could be "Memory Map Concurrency ID" (mm_cid) to have matching 
abbreviation and naming. The notion of a "memory map" seems to be seen 
in a few places in man pages, and there are event tools to explore 
process memory maps (pmap(1)).

Thanks,

Mathieu



> Thanks,
>
> Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com