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

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Date:   Mon, 21 Nov 2022 14:52:35 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Sean Christopherson <seanjc@...gle.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 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@...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?
>>>
>>> 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