[tip: perf/urgent] perf: Tweak perf_event_attr::exclusive semantics

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     1908dc911792067287458fdb0800f036f4f4e0f6
Gitweb:        https://git.kernel.org/tip/1908dc911792067287458fdb0800f036f4f4e0f6
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 29 Oct 2020 16:32:22 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 09 Nov 2020 18:12:36 +01:00

perf: Tweak perf_event_attr::exclusive semantics

Currently perf_event_attr::exclusive can be used to ensure an
event(group) is the sole group scheduled on the PMU. One consequence
is that when you have a pinned event (say the watchdog) you can no
longer have regular exclusive event(group)s.

Inspired by the fact that !pinned events are considered less strict,
allow !pinned,exclusive events to share the PMU with pinned,!exclusive
events.

Pinned,exclusive is still fully exclusive.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20201029162902.105962225@xxxxxxxxxxxxx
---
 kernel/events/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 00be48a..dc568ca 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2637,7 +2637,7 @@ static int group_can_go_on(struct perf_event *event,
 	 * If this group is exclusive and there are already
 	 * events on the CPU, it can't go on.
 	 */
-	if (event->attr.exclusive && cpuctx->active_oncpu)
+	if (event->attr.exclusive && !list_empty(get_event_list(event)))
 		return 0;
 	/*
 	 * Otherwise, try to add it if all previous groups were able