linux-kernel - Re: [PATCHv3] perf kvm: add kvm-stat for arm64

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Date:   Fri, 18 Sep 2020 11:55:56 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        John Garry <john.garry@...wei.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [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...