Re: [PATCH v6 3/4] bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter

[Peter Zijlstra]

On Wed, Aug 05, 2015 at 12:04:25PM +0200, Peter Zijlstra wrote:
> Also, you probably want a WARN_ON(in_nmi()) there, this function is
> _NOT_ NMI safe.

I had a wee think about that, and I think the below is safe.

(with the obvious problem that WARN from NMI context is not safe)

It does not give you up-to-date overcommit times but your version didn't
either so I'm assuming you don't need those, if you do need those it
needs more but we can do that too.

---
 include/linux/perf_event.h |  1 +
 kernel/events/core.c       | 53 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 2027809433b3..64e821dd64f0 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -659,6 +659,7 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr,
 				void *context);
 extern void perf_pmu_migrate_context(struct pmu *pmu,
 				int src_cpu, int dst_cpu);
+extern u64 perf_event_read_local(struct perf_event *event);
 extern u64 perf_event_read_value(struct perf_event *event,
 				 u64 *enabled, u64 *running);
 
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 39753bfd9520..7105d37763c1 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3222,6 +3222,59 @@ static inline u64 perf_event_count(struct perf_event *event)
 	return __perf_event_count(event);
 }
 
+/*
+ * NMI-safe method to read a local event, that is an event that
+ * is:
+ *   - either for the current task, or for this CPU
+ *   - does not have inherit set, for inherited task events
+ *     will not be local and we cannot read them atomically
+ *   - must not have a pmu::count method
+ */
+u64 perf_event_read_local(struct perf_event *event)
+{
+	unsigned long flags;
+	u64 val;
+
+	/*
+	 * Disabling interrupts avoids all counter scheduling (context
+	 * switches, timer based rotation and IPIs).
+	 */
+	local_irq_safe(flags);
+
+	/* If this is a per-task event, it must be for current */
+	WARN_ON_ONCE((event->attach_state & PERF_ATTACH_TASK) &&
+		     event->hw.target != current);
+
+	/* If this is a per-CPU event, it must be for this CPU */
+	WARN_ON_ONCE(!(event->attach_state & PERF_ATTACH_TASK) &&
+		     event->cpu != smp_processor_id());
+
+	/*
+	 * It must not be an event with inherit set, we cannot read
+	 * all child counters from atomic context.
+	 */
+	WARN_ON_ONCE(event->attr.inherit);
+
+	/*
+	 * It must not have a pmu::count method, those are not
+	 * NMI safe.
+	 */
+	WARN_ON_ONCE(event->pmu->count);
+
+	/*
+	 * If the event is currently on this CPU, its either a per-task event,
+	 * or local to this CPU. Furthermore it means its ACTIVE (otherwise
+	 * oncpu == -1).
+	 */
+	if (event->oncpu == smp_processor_id())
+		event->pmu->read(event);
+
+	val = local64_read(&event->count);
+	local_irq_restore(flags);
+
+	return val;
+}
+
 static u64 perf_event_read(struct perf_event *event)
 {
 	/*
--
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