powerpc/perf: Implement a global lock to avoid races between trace, core and thread imc events.

IMC(In-memory Collection Counters) does performance monitoring in
two different modes, i.e accumulation mode(core-imc and thread-imc events),
and trace mode(trace-imc events). A cpu thread can either be in
accumulation-mode or trace-mode at a time and this is done via the LDBAR
register in POWER architecture. The current design does not address the
races between thread-imc and trace-imc events.

Patch implements a global id and lock to avoid the races between
core, trace and thread imc events. With this global id-lock
implementation, the system can either run core, thread or trace imc
events at a time. i.e. to run any core-imc events, thread/trace imc events
should not be enabled/monitored.
Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200313055238.8656-1-anju@linux.vnet.ibm.com

arch/powerpc/perf/imc-pmu.c

@@ -44,6 +44,16 @@ static DEFINE_PER_CPU(u64 *, trace_imc_mem);
 static struct imc_pmu_ref *trace_imc_refc;
 static int trace_imc_mem_size;
 
+/*
+ * Global data structure used to avoid races between thread,
+ * core and trace-imc
+ */
+static struct imc_pmu_ref imc_global_refc = {
+	.lock = __MUTEX_INITIALIZER(imc_global_refc.lock),
+	.id = 0,
+	.refc = 0,
+};
+
 static struct imc_pmu *imc_event_to_pmu(struct perf_event *event)
 {
 	return container_of(event->pmu, struct imc_pmu, pmu);
@@ -698,6 +708,16 @@ static int ppc_core_imc_cpu_offline(unsigned int cpu)
 			return -EINVAL;
 
 		ref->refc = 0;
+		/*
+		 * Reduce the global reference count, if this is the
+		 * last cpu in this core and core-imc event running
+		 * in this cpu.
+		 */
+		mutex_lock(&imc_global_refc.lock);
+		if (imc_global_refc.id == IMC_DOMAIN_CORE)
+			imc_global_refc.refc--;
+
+		mutex_unlock(&imc_global_refc.lock);
 	}
 	return 0;
 }
@@ -710,6 +730,23 @@ static int core_imc_pmu_cpumask_init(void)
 				 ppc_core_imc_cpu_offline);
 }
 
+static void reset_global_refc(struct perf_event *event)
+{
+		mutex_lock(&imc_global_refc.lock);
+		imc_global_refc.refc--;
+
+		/*
+		 * If no other thread is running any
+		 * event for this domain(thread/core/trace),
+		 * set the global id to zero.
+		 */
+		if (imc_global_refc.refc <= 0) {
+			imc_global_refc.refc = 0;
+			imc_global_refc.id = 0;
+		}
+		mutex_unlock(&imc_global_refc.lock);
+}
+
 static void core_imc_counters_release(struct perf_event *event)
 {
 	int rc, core_id;
@@ -759,6 +796,8 @@ static void core_imc_counters_release(struct perf_event *event)
 		ref->refc = 0;
 	}
 	mutex_unlock(&ref->lock);
+
+	reset_global_refc(event);
 }
 
 static int core_imc_event_init(struct perf_event *event)
@@ -819,6 +858,29 @@ static int core_imc_event_init(struct perf_event *event)
 	++ref->refc;
 	mutex_unlock(&ref->lock);
 
+	/*
+	 * Since the system can run either in accumulation or trace-mode
+	 * of IMC at a time, core-imc events are allowed only if no other
+	 * trace/thread imc events are enabled/monitored.
+	 *
+	 * Take the global lock, and check the refc.id
+	 * to know whether any other trace/thread imc
+	 * events are running.
+	 */
+	mutex_lock(&imc_global_refc.lock);
+	if (imc_global_refc.id == 0 || imc_global_refc.id == IMC_DOMAIN_CORE) {
+		/*
+		 * No other trace/thread imc events are running in
+		 * the system, so set the refc.id to core-imc.
+		 */
+		imc_global_refc.id = IMC_DOMAIN_CORE;
+		imc_global_refc.refc++;
+	} else {
+		mutex_unlock(&imc_global_refc.lock);
+		return -EBUSY;
+	}
+	mutex_unlock(&imc_global_refc.lock);
+
 	event->hw.event_base = (u64)pcmi->vbase + (config & IMC_EVENT_OFFSET_MASK);
 	event->destroy = core_imc_counters_release;
 	return 0;
@@ -877,7 +939,23 @@ static int ppc_thread_imc_cpu_online(unsigned int cpu)
 
 static int ppc_thread_imc_cpu_offline(unsigned int cpu)
 {
-	mtspr(SPRN_LDBAR, 0);
+	/*
+	 * Set the bit 0 of LDBAR to zero.
+	 *
+	 * If bit 0 of LDBAR is unset, it will stop posting
+	 * the counter data to memory.
+	 * For thread-imc, bit 0 of LDBAR will be set to 1 in the
+	 * event_add function. So reset this bit here, to stop the updates
+	 * to memory in the cpu_offline path.
+	 */
+	mtspr(SPRN_LDBAR, (mfspr(SPRN_LDBAR) & (~(1UL << 63))));
+
+	/* Reduce the refc if thread-imc event running on this cpu */
+	mutex_lock(&imc_global_refc.lock);
+	if (imc_global_refc.id == IMC_DOMAIN_THREAD)
+		imc_global_refc.refc--;
+	mutex_unlock(&imc_global_refc.lock);
+
 	return 0;
 }
 
@@ -916,7 +994,22 @@ static int thread_imc_event_init(struct perf_event *event)
 	if (!target)
 		return -EINVAL;
 
+	mutex_lock(&imc_global_refc.lock);
+	/*
+	 * Check if any other trace/core imc events are running in the
+	 * system, if not set the global id to thread-imc.
+	 */
+	if (imc_global_refc.id == 0 || imc_global_refc.id == IMC_DOMAIN_THREAD) {
+		imc_global_refc.id = IMC_DOMAIN_THREAD;
+		imc_global_refc.refc++;
+	} else {
+		mutex_unlock(&imc_global_refc.lock);
+		return -EBUSY;
+	}
+	mutex_unlock(&imc_global_refc.lock);
+
 	event->pmu->task_ctx_nr = perf_sw_context;
+	event->destroy = reset_global_refc;
 	return 0;
 }
 
@@ -1063,10 +1156,12 @@ static void thread_imc_event_del(struct perf_event *event, int flags)
 	int core_id;
 	struct imc_pmu_ref *ref;
 
-	mtspr(SPRN_LDBAR, 0);
-
 	core_id = smp_processor_id() / threads_per_core;
 	ref = &core_imc_refc[core_id];
+	if (!ref) {
+		pr_debug("imc: Failed to get event reference count\n");
+		return;
+	}
 
 	mutex_lock(&ref->lock);
 	ref->refc--;
@@ -1082,6 +1177,10 @@ static void thread_imc_event_del(struct perf_event *event, int flags)
 		ref->refc = 0;
 	}
 	mutex_unlock(&ref->lock);
+
+	/* Set bit 0 of LDBAR to zero, to stop posting updates to memory */
+	mtspr(SPRN_LDBAR, (mfspr(SPRN_LDBAR) & (~(1UL << 63))));
+
 	/*
 	 * Take a snapshot and calculate the delta and update
 	 * the event counter values.
@@ -1133,7 +1232,18 @@ static int ppc_trace_imc_cpu_online(unsigned int cpu)
 
 static int ppc_trace_imc_cpu_offline(unsigned int cpu)
 {
-	mtspr(SPRN_LDBAR, 0);
+	/*
+	 * No need to set bit 0 of LDBAR to zero, as
+	 * it is set to zero for imc trace-mode
+	 *
+	 * Reduce the refc if any trace-imc event running
+	 * on this cpu.
+	 */
+	mutex_lock(&imc_global_refc.lock);
+	if (imc_global_refc.id == IMC_DOMAIN_TRACE)
+		imc_global_refc.refc--;
+	mutex_unlock(&imc_global_refc.lock);
+
 	return 0;
 }
 
@@ -1226,15 +1336,14 @@ static int trace_imc_event_add(struct perf_event *event, int flags)
 	local_mem = get_trace_imc_event_base_addr();
 	ldbar_value = ((u64)local_mem & THREAD_IMC_LDBAR_MASK) | TRACE_IMC_ENABLE;
 
-	if (core_imc_refc)
-		ref = &core_imc_refc[core_id];
-	if (!ref) {
-		/* If core-imc is not enabled, use trace-imc reference count */
+	/* trace-imc reference count */
 	if (trace_imc_refc)
 		ref = &trace_imc_refc[core_id];
-		if (!ref)
+	if (!ref) {
+		pr_debug("imc: Failed to get the event reference count\n");
 		return -EINVAL;
 	}
+
 	mtspr(SPRN_LDBAR, ldbar_value);
 	mutex_lock(&ref->lock);
 	if (ref->refc == 0) {
@@ -1242,13 +1351,11 @@ static int trace_imc_event_add(struct perf_event *event, int flags)
 				get_hard_smp_processor_id(smp_processor_id()))) {
 			mutex_unlock(&ref->lock);
 			pr_err("trace-imc: Unable to start the counters for core %d\n", core_id);
-			mtspr(SPRN_LDBAR, 0);
 			return -EINVAL;
 		}
 	}
 	++ref->refc;
 	mutex_unlock(&ref->lock);
-
 	return 0;
 }
 
@@ -1274,16 +1381,13 @@ static void trace_imc_event_del(struct perf_event *event, int flags)
 	int core_id = smp_processor_id() / threads_per_core;
 	struct imc_pmu_ref *ref = NULL;
 
-	if (core_imc_refc)
-		ref = &core_imc_refc[core_id];
-	if (!ref) {
-		/* If core-imc is not enabled, use trace-imc reference count */
 	if (trace_imc_refc)
 		ref = &trace_imc_refc[core_id];
-		if (!ref)
+	if (!ref) {
+		pr_debug("imc: Failed to get event reference count\n");
 		return;
 	}
-	mtspr(SPRN_LDBAR, 0);
+
 	mutex_lock(&ref->lock);
 	ref->refc--;
 	if (ref->refc == 0) {
@@ -1297,6 +1401,7 @@ static void trace_imc_event_del(struct perf_event *event, int flags)
 		ref->refc = 0;
 	}
 	mutex_unlock(&ref->lock);
+
 	trace_imc_event_stop(event, flags);
 }
 
@@ -1314,10 +1419,30 @@ static int trace_imc_event_init(struct perf_event *event)
 	if (event->attr.sample_period == 0)
 		return -ENOENT;
 
+	/*
+	 * Take the global lock, and make sure
+	 * no other thread is running any core/thread imc
+	 * events
+	 */
+	mutex_lock(&imc_global_refc.lock);
+	if (imc_global_refc.id == 0 || imc_global_refc.id == IMC_DOMAIN_TRACE) {
+		/*
+		 * No core/thread imc events are running in the
+		 * system, so set the refc.id to trace-imc.
+		 */
+		imc_global_refc.id = IMC_DOMAIN_TRACE;
+		imc_global_refc.refc++;
+	} else {
+		mutex_unlock(&imc_global_refc.lock);
+		return -EBUSY;
+	}
+	mutex_unlock(&imc_global_refc.lock);
+
 	event->hw.idx = -1;
 	target = event->hw.target;
 
 	event->pmu->task_ctx_nr = perf_hw_context;
+	event->destroy = reset_global_refc;
 	return 0;
 }
 
@@ -1429,10 +1554,10 @@ static void cleanup_all_core_imc_memory(void)
 static void thread_imc_ldbar_disable(void *dummy)
 {
 	/*
-	 * By Zeroing LDBAR, we disable thread-imc
-	 * updates.
+	 * By setting 0th bit of LDBAR to zero, we disable thread-imc
+	 * updates to memory.
 	 */
-	mtspr(SPRN_LDBAR, 0);
+	mtspr(SPRN_LDBAR, (mfspr(SPRN_LDBAR) & (~(1UL << 63))));
 }
 
 void thread_imc_disable(void)