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SubjectRe: [PATCHv3] perf kvm: add kvm-stat for arm64
On 2020-09-18 11:35, Sergey Senozhatsky wrote:
> On (20/09/18 09:20), Marc Zyngier wrote:
>> On 2020-09-18 01:32, Sergey Senozhatsky wrote:
>> > On (20/09/17 12:53), Marc Zyngier wrote:
>> > > Feel free to add a *new* tracepoint instead.
>> >
>> > Wouldn't we want a whole bunch of new tracepoints in this case?
>>
>> Yes. I don't have a better solution as long as tracepoints are ABI.
>
> Well, no one does.
>
>> Get someone to sign-off on it, and I'll happily change them.
>
> Sorry, I'm not sure I understand this sentence.

What I meant is that the only way to make changes to existing
tracepoints
would be to get someone like Linus to approve them.

It's all rhetorical anyway, so let's move on.

>
>> > (almost all of the existing ones with the extra vcpu_id field).
>> > Right now we have 3 types of events:
>> > - events with no vcpu at all // nil
>> > - events with vcpu_pc // "0x%016lx", __entry->vcpu_pc
>> > - events with (void *)vcpu // "vcpu: %p", __entry->vcpu
>> >
>> > It might be helpful if we could filter out events by vcpu_id.
>> > But this, basically, doubles the number of events in the ringbuffer.
>>
>> Only if you enable them both, right?
> [..]
>> How would that double the number of events in the buffer?
>
> Yes. I assume that many scripts do something like "capture kvm:*
> events",
> so new and old events are enabled. Unless we want to keep new events in
> something like kvm2:* namespace (which is unlikely to happen, I guess).

I really don't mind. I actually like the namespacing, as it gives us
a notion of versioning, something tracepoints lack.. And it gives an
opportunity to argue about the name of the namespace.

>
> And `sudo ./perf stat -e 'kvm:*'` is not unseen. In fact, this is
> literally the first thing mentioned at
> https://www.linux-kvm.org/page/Perf_events
>
> So if we'll have something like
>
> trace_kvm_foo(vcpu);
> + trace_kvm_foo2(vcpu->id, vcpu);
>
> for all arm64 kvm events, then we double the number of arm64 kvm:*
> events
> in the ringbuffer, don't we? Maybe this is not a gigantic issue, but
> who
> knows.

I don't think it's a problem, but I'm more in favour of the namespace
approach.

M.
--
Jazz is not dead. It just smells funny...

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