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Subject[[PATCH v6 08/10] Define PERF_PMU_TXN_READ interface
Date
Define a new PERF_PMU_TXN_READ interface to read a group of counters
at once.

pmu->start_txn() // Initialize before first event

for each event in group
pmu->read(event); // Queue each event to be read

rc = pmu->commit_txn() // Read/update all queued counters

Note that we use this interface with all PMUs. PMUs that implement this
interface use the ->read() operation to _queue_ the counters to be read
and use ->commit_txn() to actually read all the queued counters at once.

PMUs that don't implement PERF_PMU_TXN_READ ignore ->start_txn() and
->commit_txn() and continue to read counters one at a time.

Thanks to input from Peter Zijlstra.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
---
Changelog[v4]
- [Peter Zijlstra] Add lockdep_assert_held() in perf_event_read_group().
Make sure the entire transaction happens on the same CPU.
---
include/linux/perf_event.h | 1 +
kernel/events/core.c | 24 +++++++++++++++++++-----
2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 4e8d780..b3435e7 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -169,6 +169,7 @@ struct perf_event;
#define PERF_EVENT_TXN 0x1

#define PERF_PMU_TXN_ADD 0x1 /* txn to add/schedule event on PMU */
+#define PERF_PMU_TXN_READ 0x2 /* txn to read event group from PMU */

/**
* pmu::capabilities flags
diff --git a/kernel/events/core.c b/kernel/events/core.c
index acdd8b5..9335bce 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3199,6 +3199,7 @@ static void __perf_event_read(void *info)
struct perf_event *sub, *event = data->event;
struct perf_event_context *ctx = event->ctx;
struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
+ struct pmu *pmu = event->pmu;

/*
* If this is a task context, we need to check whether it is
@@ -3217,18 +3218,31 @@ static void __perf_event_read(void *info)
}

update_event_times(event);
- if (event->state == PERF_EVENT_STATE_ACTIVE)
- event->pmu->read(event);
+ if (event->state != PERF_EVENT_STATE_ACTIVE)
+ goto unlock;

- if (!data->group)
+ if (!data->group) {
+ pmu->read(event);
+ data->ret = 0;
goto unlock;
+ }
+
+ pmu->start_txn(pmu, PERF_PMU_TXN_READ);
+
+ pmu->read(event);

list_for_each_entry(sub, &event->sibling_list, group_entry) {
update_event_times(sub);
- if (sub->state == PERF_EVENT_STATE_ACTIVE)
+ if (sub->state == PERF_EVENT_STATE_ACTIVE) {
+ /*
+ * Use sibling's PMU rather than @event's since
+ * sibling could be on different (eg: software) PMU.
+ */
sub->pmu->read(sub);
+ }
}
- data->ret = 0;
+
+ data->ret = pmu->commit_txn(pmu);

unlock:
raw_spin_unlock(&ctx->lock);
--
1.7.9.5


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