linux-kernel - Re: [PATCH v1 2/2] perf/core: Add support for PMUs that can be read from any CPU

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Message-ID: <20180227115224.rmtfy4eif3g4hmmr@lakrids.cambridge.arm.com>
Date:   Tue, 27 Feb 2018 11:52:25 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     skannan@...eaurora.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, avilaj@...eaurora.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] perf/core: Add support for PMUs that can be read
 from any CPU

On Mon, Feb 26, 2018 at 05:53:57PM -0800, skannan@...eaurora.org wrote:
> On 2018-02-24 00:41, Peter Zijlstra wrote:
> > On Fri, Feb 23, 2018 at 04:19:38PM -0800, Saravana Kannan wrote:
> > > Some PMUs events can be read from any CPU. So allow the PMU to mark
> > > events as such. For these events, we don't need to reject reads or
> > > make smp calls to the event's CPU and cause unnecessary wake ups.
> > > 
> > > Good examples of such events would be events from caches shared across
> > > all CPUs.
> > 
> > So why would the existing ACTIVE_PKG not work for you? Because clearly
> > your example does not cross a package.
> 
> Because based on testing it on hardware, it looks like the two clusters in
> an ARM DynamIQ design are not considered part of the same "package". 

I don't think we should consider the topology masks at all for system
PMU affinity. Due to the number of ways these can be integrated, and the
lack of a standard(ish) topology across arm platforms.

IIUC, there's ongoing work to try to clean that up, but that won't give
us anything meaningful for PMU affinity.

If we need a mask, that should be something the FW description of the
PMU provides, and the PMU driver provides to the core code.

Thanks,
Mark.