perf/core: Do not set cpuctx->cgrp for unscheduled cgroups

Commit:

  db4a835601b7 ("perf/core: Set cgroup in CPU contexts for new cgroup events")

failed to verify that event->cgrp is actually the scheduled cgroup
in a CPU before setting cpuctx->cgrp. This patch fixes that.

Now that there is a different path for scheduled and unscheduled
cgroup, add a warning to catch when cpuctx->cgrp is still set after
the last cgroup event has been unsheduled.

To verify the bug:

  # Create 2 cgroups.
  mkdir /dev/cgroups/devices/g1
  mkdir /dev/cgroups/devices/g2

  # launch a task, bind it to a cpu and move it to g1
  CPU=2
  while :; do : ; done &
  P=$!

  taskset -pc $CPU $P
  echo $P > /dev/cgroups/devices/g1/tasks

  # monitor g2 (it runs no tasks) and observe output
  perf stat -e cycles -I 1000 -C $CPU -G g2

  #           time             counts unit events
     1.000091408          7,579,527      cycles                    g2
     2.000350111      <not counted>      cycles                    g2
     3.000589181      <not counted>      cycles                    g2
     4.000771428      <not counted>      cycles                    g2

  # note first line that displays that a task run in g2, despite
  # g2 having no tasks. This is because cpuctx->cgrp was wrongly
  # set when context of new event was installed.
  # After applying the fix we obtain the right output:

  perf stat -e cycles -I 1000 -C $CPU -G g2
  #           time             counts unit events
     1.000119615      <not counted>      cycles                    g2
     2.000389430      <not counted>      cycles                    g2
     3.000590962      <not counted>      cycles                    g2

Signed-off-by: David Carrillo-Cisneros <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Nilay Vaish <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vegard Nossum <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 11 insertions, 0 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 0e292132efac..ff230bb4a02e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -902,6 +902,17 @@ list_update_cgroup_event(struct perf_event *event,
 	 * this will always be called from the right CPU.
 	 */
 	cpuctx = __get_cpu_context(ctx);
+
+	/* Only set/clear cpuctx->cgrp if current task uses event->cgrp. */
+	if (perf_cgroup_from_task(current, ctx) != event->cgrp) {
+		/*
+		 * We are removing the last cpu event in this context.
+		 * If that event is not active in this cpu, cpuctx->cgrp
+		 * should've been cleared by perf_cgroup_switch.
+		 */
+		WARN_ON_ONCE(!add && cpuctx->cgrp);
+		return;
+	}
 	cpuctx->cgrp = add ? event->cgrp : NULL;
 }