perf/core: Verify we have a single perf_hw_context PMU

There should (and can) only be a single PMU for perf_hw_context
events.

This is because of how we schedule events: once a hardware event fails to
schedule (the PMU is 'full') we stop trying to add more. The trivial
'fix' would break the Round-Robin scheduling we do.

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: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vince Weaver <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

Author: Peter Zijlstra

kernel/events/core.c

@@ -7693,6 +7693,15 @@ int perf_pmu_register(struct pmu *pmu, const char *name, int type)
 	}
 
 skip_type:
+	if (pmu->task_ctx_nr == perf_hw_context) {
+		static int hw_context_taken = 0;
+
+		if (WARN_ON_ONCE(hw_context_taken))
+			pmu->task_ctx_nr = perf_invalid_context;
+
+		hw_context_taken = 1;
+	}
+
 	pmu->pmu_cpu_context = find_pmu_context(pmu->task_ctx_nr);
 	if (pmu->pmu_cpu_context)
 		goto got_cpu_context;