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: <7cab1b91545e81e4b6b09e85c2f81d7e@codeaurora.org>
Date:   Mon, 26 Feb 2018 17:53:57 -0800
From:   skannan@...eaurora.org
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mark.rutland@....com, 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 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". 
When I say clusters, I using the more common interpretation of 
"homogeneous CPUs running on the same clock"/CPUs in a cpufreq policy 
and not ARM's new redefinition of cluster. So, on a SoC with 4 little 
and 4 big cores, it'll still trigger a lot of unnecessary smp calls/IPIs 
that cause unnecessary wakeups.

Although, I like Mark's suggestion of just giving a cpumask for every 
event and using that instead. Because the meaning of "active package" is 
very ambiguous. For example if a SoC has 2 DynamIQ blocks (not sure if 
that's possible), what's considered a package? CPUs that are sitting on 
one L3 can't read the PMU counters of a different L3. In that case, 
neither "Any CPU" nor "Active Package" is correct/usable for reducing 
IPIs.

> 
>> Signed-off-by: Saravana Kannan <skannan@...eaurora.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?

Maybe I'll make a separate patch to first have perf_event_read_local() 
also handle ACTIVE_PACKAGE? Because in those cases, the smp call made by 
read_local is unnecessary too.

> 
> And again, where are the users?

The DynamIQ PMU driver would be the user.

-Saravana