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SubjectRe: [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@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?

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

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