Re: [RFC PATCH] perf: honoring cpuid for number of fixed counters

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 04, 2015 at 02:30:37PM +0200, Imre Palik wrote:
> The trouble is that the number of fixed counters is not taken into
> account when scheduling the events, and the cpu model based event
> constraints will favour fixed counters.  So perf tries to use them.

Ah! so that is what your hunk below does. Tricky, and without comment
that.

---

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 87848eb..eaa0b5e 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -647,6 +647,8 @@ static void perf_sched_init(struct perf_sched *sched, struct perf_event **events
        sched->state.event      = idx;          /* start with min weight */
        sched->state.weight     = wmin;
        sched->state.unassigned = num;
+       sched->state.used[0]    =
+               ~0UL << (INTEL_PMC_IDX_FIXED + x86_pmu.num_counters_fixed);
 }

 static void perf_sched_save_state(struct perf_sched *sched)

---

Please change the FIXED_EVENT constraints init instead; that way
validate_event() will actually work too, otherwise it thinks it can
schedule the fixed function only events.

That is, change the below loop from intel_pmu_init():

	if (x86_pmu.event_constraints) {
		/*
		 * event on fixed counter2 (REF_CYCLES) only works on this
		 * counter, so do not extend mask to generic counters
		 */
		for_each_event_constraint(c, x86_pmu.event_constraints) {
			if (c->cmask != FIXED_EVENT_FLAGS
			    || c->idxmsk64 == INTEL_PMC_MSK_FIXED_REF_CYCLES) {
				continue;
			}

			c->idxmsk64 |= (1ULL << x86_pmu.num_counters) - 1;
			c->weight += x86_pmu.num_counters;
		}
	}

To clear all counters that are not in fact present, that way we keep the
event constraints correct instead of working around invalid constraints.