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

From: Peter Zijlstra <peterz@infradead.org>
To: Saravana Kannan <skannan@codeaurora.org>
Cc: mark.rutland@arm.com, Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	avilaj@codeaurora.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 2/2] perf/core: Add support for PMUs that can be read from any CPU
Date: Sat, 24 Feb 2018 09:41:06 +0100	[thread overview]
Message-ID: <20180224084106.GC25201@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1519431578-11995-2-git-send-email-skannan@codeaurora.org>

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.

> Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
> ---
>  include/linux/perf_event.h |  3 +++
>  kernel/events/core.c       | 10 ++++++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 7546822..ee8978f 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -510,9 +510,12 @@ typedef void (*perf_overflow_handler_t)(struct perf_event *,
>   * PERF_EV_CAP_SOFTWARE: Is a software event.
>   * PERF_EV_CAP_READ_ACTIVE_PKG: A CPU event (or cgroup event) that can be read
>   * from any CPU in the package where it is active.
> + * PERF_EV_CAP_READ_ANY_CPU: A CPU event (or cgroup event) that can be read
> + * from any CPU.
>   */
>  #define PERF_EV_CAP_SOFTWARE		BIT(0)
>  #define PERF_EV_CAP_READ_ACTIVE_PKG	BIT(1)
> +#define PERF_EV_CAP_READ_ANY_CPU	BIT(2)
>  
>  #define SWEVENT_HLIST_BITS		8
>  #define SWEVENT_HLIST_SIZE		(1 << SWEVENT_HLIST_BITS)
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 5d3df58..570187b 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3484,6 +3484,10 @@ static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
>  {
>  	u16 local_pkg, event_pkg;
>  
> +	if (event->group_caps & PERF_EV_CAP_READ_ANY_CPU) {
> +		return smp_processor_id();
> +	}
> +
>  	if (event->group_caps & PERF_EV_CAP_READ_ACTIVE_PKG) {
>  		int local_cpu = smp_processor_id();
>  

> @@ -3575,6 +3579,7 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
>  {
>  	unsigned long flags;
>  	int ret = 0;
> +	bool is_any_cpu = !!(event->group_caps & PERF_EV_CAP_READ_ANY_CPU);
>  
>  	/*
>  	 * Disabling interrupts avoids all counter scheduling (context
> @@ -3600,7 +3605,8 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
>  
>  	/* If this is a per-CPU event, it must be for this CPU */
>  	if (!(event->attach_state & PERF_ATTACH_TASK) &&
> -	    event->cpu != smp_processor_id()) {
> +	    event->cpu != smp_processor_id() &&
> +	    !is_any_cpu) {
>  		ret = -EINVAL;
>  		goto out;
>  	}
> @@ -3610,7 +3616,7 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
>  	 * or local to this CPU. Furthermore it means its ACTIVE (otherwise
>  	 * oncpu == -1).
>  	 */
> -	if (event->oncpu == smp_processor_id())
> +	if (event->oncpu == smp_processor_id() || is_any_cpu)
>  		event->pmu->read(event);
>  
>  	*value = local64_read(&event->count);

And why are you modifying read_local for this? That didn't support
ACTIVE_PKG, so why should it support this?

And again, where are the users?