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From: Mark Rutland <>
To: Saravana Kannan <>
Cc: Peter Zijlstra <>,
	Ingo Molnar <>,
	Arnaldo Carvalho de Melo <>,
	Alexander Shishkin <>,
	Jiri Olsa <>, Namhyung Kim <>,,,
Subject: Re: [PATCH v1 2/2] perf/core: Add support for PMUs that can be read from any CPU
Date: Sun, 25 Feb 2018 14:38:03 +0000	[thread overview]
Message-ID: <20180225143802.denbkubqjg2dc7af@salmiak> (raw)
In-Reply-To: <>

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.

I think that if we need to generalize PERF_EV_CAP_READ_ACTIVE_PKG, it would be
better to give events a pointer to a cpumask. That could then cover all cases
quite trivially:

static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
	int local_cpu = smp_processor_id();

	if (event->read_mask &&
	    cpumask_test_cpu(local_cpu, event->read_mask))
		event_cpu = local_cpu;
	return event_cpu;

... in the PERF_EV_CAP_READ_ACTIVE_PKG case, we can use the exiting(?) package
masks, and more generally we can re-use the PMU's affinit mask if it has one.

That said, I see that many pmu::read() implementations have side-effects on
hwc->prev_count, and event->count, so I worry that this won't be sfe in general
(e.g. if we race with the IRQ handler on another CPU).


  parent reply	other threads:[~2018-02-25 14:38 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-24  0:19 [PATCH v1 1/2] perf/core: Add API to look up PMU type by name Saravana Kannan
2018-02-24  0:19 ` [PATCH v1 2/2] perf/core: Add support for PMUs that can be read from any CPU Saravana Kannan
2018-02-24  0:56   ` Saravana Kannan
2018-02-24  8:41   ` Peter Zijlstra
2018-02-27  1:53     ` skannan
2018-02-27 11:52       ` Mark Rutland
2018-03-03 15:41       ` Peter Zijlstra
2018-03-07 16:39         ` Jeremy Linton
2018-02-25 14:38   ` Mark Rutland [this message]
2018-02-27  2:11     ` skannan
2018-02-27 11:43       ` Mark Rutland
2018-02-27 23:15         ` skannan
2018-02-24  8:08 ` [PATCH v1 1/2] perf/core: Add API to look up PMU type by name Peter Zijlstra

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