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

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Message-ID: <20200918103521.GE3049@jagdpanzerIV.localdomain>
Date:   Fri, 18 Sep 2020 19:35:21 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        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 (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.

> > (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).

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.

	-ss