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

From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To: Marc Zyngier <maz@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Suleiman Souhlal <suleiman@google.com>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	John Garry <john.garry@huawei.com>,
	linux-kernel@vger.kernel.org,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Leo Yan <leo.yan@linaro.org>, Namhyung Kim <namhyung@kernel.org>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCHv3] perf kvm: add kvm-stat for arm64
Date: Fri, 18 Sep 2020 19:35:21 +0900	[thread overview]
Message-ID: <20200918103521.GE3049@jagdpanzerIV.localdomain> (raw)
In-Reply-To: <f60758eeacb4e94db698d98d6d447939@kernel.org>

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

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