Re: [PATCH 00/10] perf/core: Generalise event exclusion checking

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 16, 2018 at 10:24:03AM +0000, Andrew Murray wrote:
> Many PMU drivers do not have the capability to exclude counting events
> that occur in specific contexts such as idle, kernel, guest, etc. These
> drivers indicate this by returning an error in their event_init upon
> testing the events attribute flags.
> 
> However this approach requires that each time a new event modifier is
> added to perf, all the perf drivers need to be modified to indicate that
> they don't support the attribute. This results in additional boiler-plate
> code common to many drivers that needs to be maintained. An example of
> this is the addition of exclude_host and exclude_guest in 2011 yet many
> PMU drivers do not support this or indicate an error on events that make
> use of it.
> 
> This patch generalises the test for exclusion and updates PMU drivers to
> use it. This is a functional change as some PMU drivers will now correctly
> report that they don't support certain events whereas they previously did.

Right, I like that idea, and yes, there's a lot of fail around there :/

> A longer term approach may instead be for PMU's to advertise their
> capabilities on registration.

This I think is the better approach. We already have the
PERF_PMU_CAP_flags that can be used to advertise various PMU
capabilities.

Something along these lines I suppose; then every PMU that actually
checks the flags, needs to set the flag, otherwise it'll fail.

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 53c500f0ca79..de15723ea52a 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -244,6 +244,7 @@ struct perf_event;
 #define PERF_PMU_CAP_EXCLUSIVE			0x10
 #define PERF_PMU_CAP_ITRACE			0x20
 #define PERF_PMU_CAP_HETEROGENEOUS_CPUS		0x40
+#define PERF_PMU_CAP_EXCLUDE			0x80
 
 /**
  * struct pmu - generic performance monitoring unit
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 84530ab358c3..d76b724177b9 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9772,6 +9772,14 @@ static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
 	if (ctx)
 		perf_event_ctx_unlock(event->group_leader, ctx);
 
+	if (!ret) {
+		if ((pmu->capabilities & PERF_PMU_CAP_EXCLUDE) ||
+		    event_has_exclude_flags(event)) {
+			event->destroy(event);
+			ret = -EINVAL;
+		}
+	}
+
 	if (ret)
 		module_put(pmu->module);