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);