perf: Fix event multiplexing for exclusive groups

Commit 9e630205 ("perf: Use hrtimers for event multiplexing")
placed the hrtimer (re)start call in the wrong place.  Instead of
capturing all scheduling failures, it only considered the PMU failure.

The result is that groups using perf_event_attr::exclusive are no
longer rotated.

Fixes: 9e630205 ("perf: Use hrtimers for event multiplexing")
Reported-by: NAndi Kleen <ak@linux.intel.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201029162902.038667689@infradead.org

kernel/events/core.c

@@ -2612,7 +2612,6 @@ group_sched_in(struct perf_event *group_event,
 
 error:
 	pmu->cancel_txn(pmu);
-	perf_mux_hrtimer_restart(cpuctx);
 	return -EAGAIN;
 }
 
@@ -3672,6 +3671,7 @@ static int merge_sched_in(struct perf_event *event, void *data)
 
 		*can_add_hw = 0;
 		ctx->rotate_necessary = 1;
+		perf_mux_hrtimer_restart(cpuctx);
 	}
 
 	return 0;