[PATCH 19/32] perf/core: introduce PMU event flag PERF_CGROUP_NO_RECURSION

[David Carrillo-Cisneros <davidcc@google.com>]

Some events, such as Intel's CQM llc_occupancy, need small deviations
from the traditional behavior in the generic code in a way that depends
on the event itself (and known by the PMU) and not in a field of
perf_event_attrs.

An example is the recursive scope for cgroups: The generic code handles
cgroup hierarchy for a cgroup C by simultaneously adding to the PMU
the events of all cgroups that are ancestors of C. This approach is
incompatible with the CQM hw that only allows one RMID per virtual core
at a time. CQM's PMU work-arounds this limitation by internally
maintaining the hierarchical dependency between monitored cgroups and
only requires that the generic code adds current cgroup's event to
the PMU.

The introduction of the flag PERF_CGROUP_NO_RECURSION allows the PMU to
signal the generic code to avoid using recursive cgroup scope for
llc_occupancy events, preventing an undesired overwrite of RMIDs.

The PERF_CGROUP_NO_RECURSION, introduced in this patch, is the first flag
of this type, more will be added in this patch series.
To keep things tidy, this patch introduces the flag field pmu_event_flag,
intended to contain all flags that:
  - Are not user-configurable event attributes (not suitable for
    perf_event_attributes).
  - Are known by the PMU during initialization of struct perf_event.
  - Signal something to the generic code.

Reviewed-by: Stephane Eranian <eranian@google.com>
Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
---
 include/linux/perf_event.h | 10 ++++++++++
 kernel/events/core.c       |  3 +++
 2 files changed, 13 insertions(+)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 81e29c6..e4c58b0 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -594,9 +594,19 @@ struct perf_event {
 #endif
 
 	struct list_head		sb_list;
+
+	/* Flags to generic code set by PMU. */
+	int				pmu_event_flags;
+
 #endif /* CONFIG_PERF_EVENTS */
 };
 
+/*
+ * Possible flags for mpu_event_flags.
+ */
+/* Do not enable cgroup events in descendant cgroups. */
+#define PERF_CGROUP_NO_RECURSION		(1 << 0)
+
 /**
  * struct perf_event_context - event context structure
  *
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2a868a6..33961ec 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -545,6 +545,9 @@ perf_cgroup_match(struct perf_event *event)
 	if (!cpuctx->cgrp)
 		return false;
 
+	if (event->pmu_event_flags & PERF_CGROUP_NO_RECURSION)
+		return cpuctx->cgrp->css.cgroup == event->cgrp->css.cgroup;
+
 	/*
 	 * Cgroup scoping is recursive.  An event enabled for a cgroup is
 	 * also enabled for all its descendant cgroups.  If @cpuctx's
-- 
2.8.0.rc3.226.g39d4020