Message-Id: <20250512175542.2000708-1-kan.liang@linux.intel.com>
Date: Mon, 12 May 2025 10:55:42 -0700
From: kan.liang@...ux.intel.com
To: peterz@...radead.org,
	mingo@...hat.com,
	acme@...nel.org,
	namhyung@...nel.org,
	irogers@...gle.com,
	linux-kernel@...r.kernel.org
Cc: Kan Liang <kan.liang@...ux.intel.com>
Subject: [RFC PATCH] perf/x86/intel: Add a check for dynamic constraints

From: Kan Liang <kan.liang@...ux.intel.com>

The current event scheduler has a limit. If the counter constraint of an
event is not a subset of any other counter constraint with an equal or
higher weight. The counters may not be fully utilized.

To workaround it, the commit bc1738f6ee83 ("perf, x86: Fix event
scheduler for constraints with overlapping counters") introduced an
overlap flag, which is hardcoded to the event constraint that may
trigger the limit. It only works for static constraints.

Many features on and after Intel PMON v6 require dynamic constraints. An
event constraint is decided by both static and dynamic constraints at
runtime. See commit 4dfe3232cc04 ("perf/x86: Add dynamic constraint").
The dynamic constraints are from CPUID enumeration. It's impossible to
hardcode it in advance. It's not practical to set the overlap flag to all
events. It's harmful to the scheduler.

For the existing Intel platforms, the dynamic constraints don't trigger
the limit. A real fix is not required.

However, for virtualization, VMM may give a weird CPUID enumeration to a
guest. It's impossible to indicate what the weird enumeration is. A
check is introduced, which can list the possible breaks if a weird
enumeration is used.

Check the dynamic constraints enumerated for normal, branch counters
logging, and auto-counter reload.
Check both PEBS and non-PEBS constratins.

Closes: https://lore.kernel.org/lkml/20250416195610.GC38216@noisy.programming.kicks-ass.net/
Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
---
 arch/x86/events/intel/core.c | 156 +++++++++++++++++++++++++++++++++--
 1 file changed, 148 insertions(+), 8 deletions(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 3a319cf6d364..5928523dc96f 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -5257,6 +5257,151 @@ static void intel_pmu_check_event_constraints(struct event_constraint *event_con
 					      u64 fixed_cntr_mask,
 					      u64 intel_ctrl);
 
+enum dyn_constr_type {
+	DYN_CONSTR_NONE,
+	DYN_CONSTR_BR_CNTR,
+	DYN_CONSTR_ACR_CNTR,
+	DYN_CONSTR_ACR_CAUSE,
+
+	DYN_CONSTR_MAX,
+};
+
+static const char * const dyn_constr_type_name[] = {
+	[DYN_CONSTR_NONE] = "a normal event",
+	[DYN_CONSTR_BR_CNTR] = "a branch counter logging event",
+	[DYN_CONSTR_ACR_CNTR] = "an auto-counter reload event",
+	[DYN_CONSTR_ACR_CAUSE] = "an auto-counter reload cause event",
+};
+
+static void __intel_pmu_check_dyn_constr(struct event_constraint *constr,
+					 enum dyn_constr_type type, u64 mask)
+{
+	struct event_constraint *c1, *c2;
+	int new_weight, check_weight;
+	u64 new_mask, check_mask;
+
+	for_each_event_constraint(c1, constr) {
+		new_mask = c1->idxmsk64 & mask;
+		new_weight = hweight64(new_mask);
+
+		/* ignore topdown perf metrics event */
+		if (c1->idxmsk64 & INTEL_PMC_MSK_TOPDOWN)
+			continue;
+
+		if (!new_weight && fls64(c1->idxmsk64) < INTEL_PMC_IDX_FIXED) {
+			pr_info("The event 0x%llx is not supported as %s.\n",
+				c1->code, dyn_constr_type_name[type]);
+		}
+
+		if (new_weight <= 1)
+			continue;
+
+		for_each_event_constraint(c2, c1 + 1) {
+			bool check_fail = false;
+
+			check_mask = c2->idxmsk64 & mask;
+			check_weight = hweight64(check_mask);
+
+			if (c2->idxmsk64 & INTEL_PMC_MSK_TOPDOWN ||
+			    !check_weight)
+				continue;
+
+			/* The same constraints or no overlap */
+			if (new_mask == check_mask ||
+			    (new_mask ^ check_mask) == (new_mask | check_mask))
+				continue;
+
+			/*
+			 * A scheduler issue may be triggered in the following cases.
+			 * - Two overlap constraints have the same weight.
+			 *   E.g., A constraints: 0x3, B constraints: 0x6
+			 *   event	counter		failure case
+			 *   B		PMC[2:1]	1
+			 *   A		PMC[1:0]	0
+			 *   A		PMC[1:0]	FAIL
+			 * - Two overlap constraints have different weight.
+			 *   The constraint has a low weight, but has high last bit.
+			 *   E.g., A constraints: 0x7, B constraints: 0xC
+			 *   event	counter		failure case
+			 *   B		PMC[3:2]	2
+			 *   A		PMC[2:0]	0
+			 *   A		PMC[2:0]	1
+			 *   A		PMC[2:0]	FAIL
+			 */
+			if (new_weight == check_weight) {
+				check_fail = true;
+			} else if (new_weight < check_weight) {
+				if ((new_mask | check_mask) != check_mask &&
+				    fls64(new_mask) > fls64(check_mask))
+					check_fail = true;
+			} else {
+				if ((new_mask | check_mask) != new_mask &&
+				    fls64(new_mask) < fls64(check_mask))
+					check_fail = true;
+			}
+
+			if (check_fail) {
+				pr_info("The two events 0x%llx and 0x%llx may not be "
+					"fully scheduled under some circumstances as "
+					"%s.\n",
+					c1->code, c2->code, dyn_constr_type_name[type]);
+			}
+		}
+	}
+}
+
+static void intel_pmu_check_dyn_constr(struct pmu *pmu,
+				       struct event_constraint *constr,
+				       u64 cntr_mask)
+{
+	enum dyn_constr_type i;
+	u64 mask;
+
+	for (i = DYN_CONSTR_NONE; i < DYN_CONSTR_MAX; i++) {
+		mask = 0;
+		switch (i) {
+		case DYN_CONSTR_NONE:
+			mask = cntr_mask;
+			break;
+		case DYN_CONSTR_BR_CNTR:
+			if (x86_pmu.flags & PMU_FL_BR_CNTR)
+				mask = x86_pmu.lbr_counters;
+			break;
+		case DYN_CONSTR_ACR_CNTR:
+			mask = hybrid(pmu, acr_cntr_mask64) & GENMASK_ULL(INTEL_PMC_MAX_GENERIC - 1, 0);
+			break;
+		case DYN_CONSTR_ACR_CAUSE:
+			if (hybrid(pmu, acr_cntr_mask64) == hybrid(pmu, acr_cause_mask64))
+				continue;
+			mask = hybrid(pmu, acr_cause_mask64) & GENMASK_ULL(INTEL_PMC_MAX_GENERIC - 1, 0);
+			break;
+		default:
+			pr_warn("Unsupported dynamic constraint type %d\n", i);
+		}
+
+		if (mask)
+			__intel_pmu_check_dyn_constr(constr, i, mask);
+	}
+}
+
+static void intel_pmu_check_event_constraints_all(struct pmu *pmu)
+{
+	struct event_constraint *event_constraints = hybrid(pmu, event_constraints);
+	struct event_constraint *pebs_constraints = hybrid(pmu, pebs_constraints);
+	u64 cntr_mask = hybrid(pmu, cntr_mask64);
+	u64 fixed_cntr_mask = hybrid(pmu, fixed_cntr_mask64);
+	u64 intel_ctrl = hybrid(pmu, intel_ctrl);
+
+	intel_pmu_check_event_constraints(event_constraints, cntr_mask,
+					  fixed_cntr_mask, intel_ctrl);
+
+	if (event_constraints)
+		intel_pmu_check_dyn_constr(pmu, event_constraints, cntr_mask);
+
+	if (pebs_constraints)
+		intel_pmu_check_dyn_constr(pmu, pebs_constraints, cntr_mask);
+}
+
 static void intel_pmu_check_extra_regs(struct extra_reg *extra_regs);
 
 static inline bool intel_pmu_broken_perf_cap(void)
@@ -5319,10 +5464,7 @@ static void intel_pmu_check_hybrid_pmus(struct x86_hybrid_pmu *pmu)
 	else
 		pmu->intel_ctrl &= ~(1ULL << GLOBAL_CTRL_EN_PERF_METRICS);
 
-	intel_pmu_check_event_constraints(pmu->event_constraints,
-					  pmu->cntr_mask64,
-					  pmu->fixed_cntr_mask64,
-					  pmu->intel_ctrl);
+	intel_pmu_check_event_constraints_all(&pmu->pmu);
 
 	intel_pmu_check_extra_regs(pmu->extra_regs);
 }
@@ -7734,10 +7876,8 @@ __init int intel_pmu_init(void)
 	if (x86_pmu.intel_cap.anythread_deprecated)
 		x86_pmu.format_attrs = intel_arch_formats_attr;
 
-	intel_pmu_check_event_constraints(x86_pmu.event_constraints,
-					  x86_pmu.cntr_mask64,
-					  x86_pmu.fixed_cntr_mask64,
-					  x86_pmu.intel_ctrl);
+	intel_pmu_check_event_constraints_all(NULL);
+
 	/*
 	 * Access LBR MSR may cause #GP under certain circumstances.
 	 * Check all LBR MSR here.
-- 
2.38.1

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