[PATCH/RFC v4 0/2] perf: Sharing PMU counters across compatible events

[PATCH/RFC v4 0/2] perf: Sharing PMU counters across compatible events

[Song Liu]

Changes v3 -> v4:
  1. In __perf_install_in_context() always reprogram ctx for cpu and cgroup
     events to make sure event duplications set up properly.
  2. Add a prototype (2/2) to show potential solution of sharing PMU among
     CPU events and task events.

Changes v2 -> v3:
  1. Simplified event_pmu_add() as Jiri Olsa suggested

Changes v1 -> v2:
  1. Fixed a bug reported by 0day kernel testing robot

This is to follow up earlier discussion on sharing hardware PMU counters
across compatible events:

  https://marc.info/?t=151213803600016
  https://marc.info/?t=152547569200001

This version limits PMU sharing to events within same ctx. As a result,
compatible events are not evaluated during task schedule. This vesion also
limits PMU sharing to hardware counting events. From our discussion with
internal users, this is sufficient for most cases.

This version introduces virtual master event. The virtual master event
does require careful handling. But it is makes the logic of event add/del
cleaner.

Cc: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jiri Olsa <jolsa@kernel.org>

Song Liu (2):
  perf: Sharing PMU counters across compatible events
  perf tools: monitoring per task counter with per cgroup event

 include/linux/perf_event.h |  61 ++++++++
 kernel/events/core.c       | 294 ++++++++++++++++++++++++++++++++++---
 tools/perf/builtin-stat.c  |  26 ++++
 tools/perf/util/cgroup.c   |  76 ++++++++++
 tools/perf/util/cgroup.h   |   5 +
 tools/perf/util/target.h   |   1 +
 6 files changed, 440 insertions(+), 23 deletions(-)

--
2.17.1

[PATCH/RFC v4 1/2] perf: Sharing PMU counters across compatible events

[Song Liu]

This patch tries to enable PMU sharing. To make perf event scheduling
fast, we use special data structures.

An array of "struct perf_event_dup" is added to the perf_event_context,
to remember all the duplicated events under this ctx. All the events
under this ctx has a "dup_id" pointing to its perf_event_dup. Compatible
events under the same ctx share the same perf_event_dup. The following
figure shows a simplified version of the data structure.

      ctx ->  perf_event_dup -> master
                     ^
                     |
         perf_event /|
                     |
         perf_event /

Connection among perf_event and perf_event_dup are built when events are
added or removed from the ctx. So these are not on the critical path of
schedule or perf_rotate_context().

On the critical paths (add, del read), sharing PMU counters doesn't
increase the complexity. Helper functions event_pmu_[add|del|read]() are
introduced to cover these cases. All these functions have O(1) time
complexity.

We allocate a separate perf_event for perf_event_dup->master. This needs
extra attention, because perf_event_alloc() may sleep. To allocate the
master event properly, a new pointer, tmp_master, is added to perf_event.
tmp_master carries a separate perf_event into list_[add|del]_event().
The master event has valid ->ctx and holds ctx->refcount.

Details about the handling of the master event is added to
include/linux/perf_event.h, before struct perf_event_dup.

Cc: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Song Liu <songliubraving@fb.com>
---
 include/linux/perf_event.h |  61 ++++++++
 kernel/events/core.c       | 294 ++++++++++++++++++++++++++++++++++---
 2 files changed, 332 insertions(+), 23 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 53c500f0ca79..d092378a3eca 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -695,6 +695,12 @@ struct perf_event {
 #endif

 	struct list_head		sb_list;
+
+	/* for PMU sharing */
+	struct perf_event		*tmp_master;
+	u64				dup_id;
+	u64				dup_base_count;
+	u64				dup_base_child_count;
 #endif /* CONFIG_PERF_EVENTS */
 };

@@ -704,6 +710,58 @@ struct perf_event_groups {
 	u64		index;
 };

+/*
+ * Sharing PMU across compatible events
+ *
+ * If two perf_events in the same perf_event_context are counting same
+ * hardware events (instructions, cycles, etc.), they could share the
+ * hardware PMU counter.
+ *
+ * When a perf_event is added to the ctx (list_add_event), it is compared
+ * against other events in the ctx. If they can share the PMU counter,
+ * a perf_event_dup is allocated to represent the sharing.
+ *
+ * Each perf_event_dup has a virtual master event, which is called by
+ * pmu->add() and pmu->del(). We cannot call perf_event_alloc() in
+ * list_add_event(), so it is allocated and carried by event->tmp_master
+ * into list_add_event().
+ *
+ * Virtual master in different cases/paths:
+ *
+ * < I > perf_event_open() -> close() path:
+ *
+ * 1. Allocated by perf_event_alloc() in sys_perf_event_open();
+ * 2. event->tmp_master->ctx assigned in perf_install_in_context();
+ * 3.a. if used by ctx->dup_events, freed in perf_event_release_kernel();
+ * 3.b. if not used by ctx->dup_events, freed in perf_event_open().
+ *
+ * < II > inherit_event() path:
+ *
+ * 1. Allocated by perf_event_alloc() in inherit_event();
+ * 2. tmp_master->ctx assigned in inherit_event();
+ * 3.a. if used by ctx->dup_events, freed in perf_event_release_kernel();
+ * 3.b. if not used by ctx->dup_events, freed in inherit_event().
+ *
+ * < III > perf_pmu_migrate_context() path:
+ * all dup_events removed during migration (no sharing after the move).
+ *
+ * < IV > perf_event_create_kernel_counter() path:
+ * not supported yet.
+ */
+struct perf_event_dup {
+	/*
+	 * master event being called by pmu->add() and pmu->del().
+	 * This event is allocated with perf_event_alloc(). When
+	 * attached to a ctx, this event should hold ctx->refcount.
+	 */
+	struct perf_event       *master;
+	/* number of events in the ctx that shares the master */
+	int			total_event_count;
+	/* number of active events of the master */
+	int			active_event_count;
+};
+
+#define MAX_PERF_EVENT_DUP_PER_CTX 4
 /**
  * struct perf_event_context - event context structure
  *
@@ -759,6 +817,9 @@ struct perf_event_context {
 #endif
 	void				*task_ctx_data; /* pmu specific data */
 	struct rcu_head			rcu_head;
+
+	/* for PMU sharing. array is needed for O(1) access */
+	struct perf_event_dup		dup_events[MAX_PERF_EVENT_DUP_PER_CTX];
 };

 /*
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5a97f34bc14c..e75e90a7f069 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1655,6 +1655,142 @@ perf_event_groups_next(struct perf_event *event)
 		event = rb_entry_safe(rb_next(&event->group_node),	\
 				typeof(*event), group_node))

+static void _free_event(struct perf_event *event);
+
+/* free event->tmp_master */
+static inline void perf_event_free_tmp_master(struct perf_event *event)
+{
+	if (event->tmp_master) {
+		_free_event(event->tmp_master);
+		event->tmp_master = NULL;
+	}
+}
+
+static inline u64 dup_read_count(struct perf_event_dup *dup)
+{
+	return local64_read(&dup->master->count);
+}
+
+static inline u64 dup_read_child_count(struct perf_event_dup *dup)
+{
+	return atomic64_read(&dup->master->child_count);
+}
+
+/* Returns whether a perf_event can share PMU counter with other events */
+static inline bool perf_event_can_share(struct perf_event *event)
+{
+	/* only do sharing for hardware events */
+	if (is_software_event(event))
+		return false;
+
+	/*
+	 * limit sharing to counting events.
+	 * perf-stat sets PERF_SAMPLE_IDENTIFIER for counting events, so
+	 * let that in.
+	 */
+	if (event->attr.sample_type & ~PERF_SAMPLE_IDENTIFIER)
+		return false;
+
+	return true;
+}
+
+/*
+ * Returns whether the two events can share a PMU counter.
+ *
+ * Note: This function does NOT check perf_event_can_share() for
+ * the two events, they should be checked before this function
+ */
+static inline bool perf_event_compatible(struct perf_event *event_a,
+					 struct perf_event *event_b)
+{
+	return event_a->attr.type == event_b->attr.type &&
+		event_a->attr.config == event_b->attr.config &&
+		event_a->attr.config1 == event_b->attr.config1 &&
+		event_a->attr.config2 == event_b->attr.config2;
+}
+
+/*
+ * After adding a event to the ctx, try find compatible event(s).
+ *
+ * This function should only be called at the end of list_add_event().
+ * Master event cannot be allocated or freed within this function. To add
+ * new master event, the event should already have a master event
+ * allocated (event->tmp_master).
+ */
+static inline void perf_event_setup_dup(struct perf_event *event,
+					struct perf_event_context *ctx)
+
+{
+	struct perf_event *tmp;
+	int empty_slot = -1;
+	int match;
+	int i;
+
+	if (!perf_event_can_share(event))
+		return;
+
+	/* look for sharing with existing dup events */
+	for (i = 0; i < MAX_PERF_EVENT_DUP_PER_CTX; i++) {
+		if (ctx->dup_events[i].master == NULL) {
+			if (empty_slot == -1)
+				empty_slot = i;
+			continue;
+		}
+
+		if (perf_event_compatible(event, ctx->dup_events[i].master)) {
+			event->dup_id = i;
+			ctx->dup_events[i].total_event_count++;
+			return;
+		}
+	}
+
+	if (!event->tmp_master ||  /* perf_event_alloc() failed */
+	    empty_slot == -1)      /* no available dup_event */
+		return;
+
+	match = 0;
+	/* look for dup with other events */
+	list_for_each_entry(tmp, &ctx->event_list, event_entry) {
+		if (tmp == event || tmp->dup_id != -1 ||
+		    !perf_event_can_share(tmp) ||
+		    !perf_event_compatible(event, tmp))
+			continue;
+
+		tmp->dup_id = empty_slot;
+		match++;
+	}
+
+	/* found at least one dup */
+	if (match) {
+		ctx->dup_events[empty_slot].master = event->tmp_master;
+		ctx->dup_events[empty_slot].total_event_count = match + 1;
+		event->dup_id = empty_slot;
+		event->tmp_master = NULL;
+		return;
+	}
+}
+
+/*
+ * Remove the event from ctx->dup_events.
+ * This function should only be called from list_del_event(). Similar to
+ * perf_event_setup_dup(), we cannot call _free_event(master). If a master
+ * event should be freed, it is carried out of this function by the event
+ * (event->tmp_master).
+ */
+static void perf_event_remove_dup(struct perf_event *event,
+				  struct perf_event_context *ctx)
+
+{
+	if (event->dup_id == -1)
+		return;
+
+	if (--ctx->dup_events[event->dup_id].total_event_count == 0) {
+		event->tmp_master = ctx->dup_events[event->dup_id].master;
+		ctx->dup_events[event->dup_id].master = NULL;
+	}
+	event->dup_id = -1;
+}
+
 /*
  * Add an event from the lists for its context.
  * Must be called with ctx->mutex and ctx->lock held.
@@ -1687,6 +1823,7 @@ list_add_event(struct perf_event *event, struct perf_event_context *ctx)
 		ctx->nr_stat++;

 	ctx->generation++;
+	perf_event_setup_dup(event, ctx);
 }

 /*
@@ -1853,6 +1990,7 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
 	WARN_ON_ONCE(event->ctx != ctx);
 	lockdep_assert_held(&ctx->lock);

+	perf_event_remove_dup(event, ctx);
 	/*
 	 * We can have double detach due to exit/hot-unplug + close.
 	 */
@@ -1982,6 +2120,84 @@ event_filter_match(struct perf_event *event)
 	       perf_cgroup_match(event) && pmu_filter_match(event);
 }

+/* PMU sharing aware version of event->pmu->add() */
+static int event_pmu_add(struct perf_event *event,
+			 struct perf_event_context *ctx)
+{
+	struct perf_event_dup *dup;
+	int ret;
+
+	/* no sharing, just do event->pmu->add() */
+	if (event->dup_id == -1)
+		return event->pmu->add(event, PERF_EF_START);
+
+	dup = &ctx->dup_events[event->dup_id];
+
+	if (!dup->active_event_count) {
+		ret = event->pmu->add(dup->master, PERF_EF_START);
+		if (ret)
+			return ret;
+	}
+
+	dup->active_event_count++;
+	dup->master->pmu->read(dup->master);
+	event->dup_base_count = dup_read_count(dup);
+	event->dup_base_child_count = dup_read_child_count(dup);
+	return 0;
+}
+
+/*
+ * sync data count from dup->master to event, called on event_pmu_read()
+ * and event_pmu_del()
+ */
+static void event_sync_dup_count(struct perf_event *event,
+				 struct perf_event_dup *dup)
+{
+	u64 new_count;
+	u64 new_child_count;
+
+	event->pmu->read(dup->master);
+	new_count = dup_read_count(dup);
+	new_child_count = dup_read_child_count(dup);
+
+	local64_add(new_count - event->dup_base_count, &event->count);
+	atomic64_add(new_child_count - event->dup_base_child_count,
+		     &event->child_count);
+
+	event->dup_base_count = new_count;
+	event->dup_base_child_count = new_child_count;
+}
+
+/* PMU sharing aware version of event->pmu->del() */
+static void event_pmu_del(struct perf_event *event,
+			  struct perf_event_context *ctx)
+{
+	struct perf_event_dup *dup;
+
+	if (event->dup_id == -1) {
+		event->pmu->del(event, 0);
+		return;
+	}
+
+	dup = &ctx->dup_events[event->dup_id];
+	event_sync_dup_count(event, dup);
+	if (--dup->active_event_count == 0)
+		event->pmu->del(dup->master, 0);
+}
+
+/* PMU sharing aware version of event->pmu->read() */
+static void event_pmu_read(struct perf_event *event)
+{
+	struct perf_event_dup *dup;
+
+	if (event->dup_id == -1) {
+		event->pmu->read(event);
+		return;
+	}
+	dup = &event->ctx->dup_events[event->dup_id];
+	event_sync_dup_count(event, dup);
+}
+
 static void
 event_sched_out(struct perf_event *event,
 		  struct perf_cpu_context *cpuctx,
@@ -2004,7 +2220,7 @@ event_sched_out(struct perf_event *event,

 	perf_pmu_disable(event->pmu);

-	event->pmu->del(event, 0);
+	event_pmu_del(event, ctx);
 	event->oncpu = -1;

 	if (event->pending_disable) {
@@ -2276,7 +2492,7 @@ event_sched_in(struct perf_event *event,

 	perf_log_itrace_start(event);

-	if (event->pmu->add(event, PERF_EF_START)) {
+	if (event_pmu_add(event, ctx)) {
 		perf_event_set_state(event, PERF_EVENT_STATE_INACTIVE);
 		event->oncpu = -1;
 		ret = -EAGAIN;
@@ -2514,20 +2730,9 @@ static int  __perf_install_in_context(void *info)
 		raw_spin_lock(&task_ctx->lock);
 	}

-#ifdef CONFIG_CGROUP_PERF
-	if (is_cgroup_event(event)) {
-		/*
-		 * If the current cgroup doesn't match the event's
-		 * cgroup, we should not try to schedule it.
-		 */
-		struct perf_cgroup *cgrp = perf_cgroup_from_task(current, ctx);
-		reprogram = cgroup_is_descendant(cgrp->css.cgroup,
-					event->cgrp->css.cgroup);
-	}
-#endif
-
 	if (reprogram) {
-		ctx_sched_out(ctx, cpuctx, EVENT_TIME);
+		/* schedule out all events to set up dup properly */
+		ctx_sched_out(ctx, cpuctx, EVENT_ALL);
 		add_event_to_ctx(event, ctx);
 		ctx_resched(cpuctx, task_ctx, get_event_type(event));
 	} else {
@@ -2561,6 +2766,10 @@ perf_install_in_context(struct perf_event_context *ctx,
 	 * Ensures that if we can observe event->ctx, both the event and ctx
 	 * will be 'complete'. See perf_iterate_sb_cpu().
 	 */
+	if (event->tmp_master) {
+		event->tmp_master->ctx = ctx;
+		get_ctx(ctx);
+	}
 	smp_store_release(&event->ctx, ctx);

 	if (!task) {
@@ -3006,7 +3215,7 @@ static void __perf_event_sync_stat(struct perf_event *event,
 	 * don't need to use it.
 	 */
 	if (event->state == PERF_EVENT_STATE_ACTIVE)
-		event->pmu->read(event);
+		event_pmu_read(event);

 	perf_event_update_time(event);

@@ -3862,14 +4071,14 @@ static void __perf_event_read(void *info)
 		goto unlock;

 	if (!data->group) {
-		pmu->read(event);
+		event_pmu_read(event);
 		data->ret = 0;
 		goto unlock;
 	}

 	pmu->start_txn(pmu, PERF_PMU_TXN_READ);

-	pmu->read(event);
+	event_pmu_read(event);

 	for_each_sibling_event(sub, event) {
 		if (sub->state == PERF_EVENT_STATE_ACTIVE) {
@@ -3877,7 +4086,7 @@ static void __perf_event_read(void *info)
 			 * Use sibling's PMU rather than @event's since
 			 * sibling could be on different (eg: software) PMU.
 			 */
-			sub->pmu->read(sub);
+			event_pmu_read(sub);
 		}
 	}

@@ -3947,7 +4156,7 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
 	 * oncpu == -1).
 	 */
 	if (event->oncpu == smp_processor_id())
-		event->pmu->read(event);
+		event_pmu_read(event);

 	*value = local64_read(&event->count);
 	if (enabled || running) {
@@ -4470,6 +4679,7 @@ static void free_event(struct perf_event *event)
 		return;
 	}

+	perf_event_free_tmp_master(event);
 	_free_event(event);
 }

@@ -4529,6 +4739,7 @@ static void put_event(struct perf_event *event)
 	if (!atomic_long_dec_and_test(&event->refcount))
 		return;

+	perf_event_free_tmp_master(event);
 	_free_event(event);
 }

@@ -6092,7 +6303,7 @@ static void perf_output_read_group(struct perf_output_handle *handle,

 	if ((leader != event) &&
 	    (leader->state == PERF_EVENT_STATE_ACTIVE))
-		leader->pmu->read(leader);
+		event_pmu_read(leader);

 	values[n++] = perf_event_count(leader);
 	if (read_format & PERF_FORMAT_ID)
@@ -6105,7 +6316,7 @@ static void perf_output_read_group(struct perf_output_handle *handle,

 		if ((sub != event) &&
 		    (sub->state == PERF_EVENT_STATE_ACTIVE))
-			sub->pmu->read(sub);
+			event_pmu_read(sub);

 		values[n++] = perf_event_count(sub);
 		if (read_format & PERF_FORMAT_ID)
@@ -9116,7 +9327,7 @@ static enum hrtimer_restart perf_swevent_hrtimer(struct hrtimer *hrtimer)
 	if (event->state != PERF_EVENT_STATE_ACTIVE)
 		return HRTIMER_NORESTART;

-	event->pmu->read(event);
+	event_pmu_read(event);

 	perf_sample_data_init(&data, 0, event->hw.last_period);
 	regs = get_irq_regs();
@@ -9969,6 +10180,7 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
 	event->id		= atomic64_inc_return(&perf_event_id);

 	event->state		= PERF_EVENT_STATE_INACTIVE;
+	event->dup_id		= -1;

 	if (task) {
 		event->attach_state = PERF_ATTACH_TASK;
@@ -10506,6 +10718,14 @@ SYSCALL_DEFINE5(perf_event_open,
 		goto err_cred;
 	}

+	if (perf_event_can_share(event)) {
+		event->tmp_master = perf_event_alloc(&event->attr, cpu,
+						     task, NULL, NULL,
+						     NULL, NULL, -1);
+		if (IS_ERR(event->tmp_master))
+			event->tmp_master = NULL;
+	}
+
 	if (is_sampling_event(event)) {
 		if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
 			err = -EOPNOTSUPP;
@@ -10711,9 +10931,17 @@ SYSCALL_DEFINE5(perf_event_open,
 		perf_remove_from_context(group_leader, 0);
 		put_ctx(gctx);

+		/*
+		 * move_group only happens to sw events, from sw ctx to hw
+		 * ctx. The sw events should not have valid dup_id. So it
+		 * is not necessary to handle dup_events.
+		 */
+		WARN_ON_ONCE(group_leader->dup_id != -1);
+
 		for_each_sibling_event(sibling, group_leader) {
 			perf_remove_from_context(sibling, 0);
 			put_ctx(gctx);
+			WARN_ON_ONCE(sibling->dup_id != -1);
 		}

 		/*
@@ -10771,6 +10999,9 @@ SYSCALL_DEFINE5(perf_event_open,
 		put_task_struct(task);
 	}

+	/* if event->tmp_master is not used by ctx->dup_events, free it */
+	perf_event_free_tmp_master(event);
+
 	mutex_lock(&current->perf_event_mutex);
 	list_add_tail(&event->owner_entry, &current->perf_event_list);
 	mutex_unlock(&current->perf_event_mutex);
@@ -10915,6 +11146,7 @@ void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
 		perf_remove_from_context(event, 0);
 		unaccount_event_cpu(event, src_cpu);
 		put_ctx(src_ctx);
+		perf_event_free_tmp_master(event);
 		list_add(&event->migrate_entry, &events);
 	}

@@ -11273,6 +11505,14 @@ inherit_event(struct perf_event *parent_event,
 	if (IS_ERR(child_event))
 		return child_event;

+	if (perf_event_can_share(child_event)) {
+		child_event->tmp_master = perf_event_alloc(&parent_event->attr,
+							   parent_event->cpu,
+							   child, NULL, NULL,
+							   NULL, NULL, -1);
+		if (IS_ERR(child_event->tmp_master))
+			child_event->tmp_master = NULL;
+	}

 	if ((child_event->attach_state & PERF_ATTACH_TASK_DATA) &&
 	    !child_ctx->task_ctx_data) {
@@ -11327,6 +11567,10 @@ inherit_event(struct perf_event *parent_event,
 	child_event->overflow_handler = parent_event->overflow_handler;
 	child_event->overflow_handler_context
 		= parent_event->overflow_handler_context;
+	if (child_event->tmp_master) {
+		child_event->tmp_master->ctx = child_ctx;
+		get_ctx(child_ctx);
+	}

 	/*
 	 * Precalculate sample_data sizes
@@ -11341,6 +11585,7 @@ inherit_event(struct perf_event *parent_event,
 	add_event_to_ctx(child_event, child_ctx);
 	raw_spin_unlock_irqrestore(&child_ctx->lock, flags);

+	perf_event_free_tmp_master(child_event);
 	/*
 	 * Link this into the parent event's child list
 	 */
@@ -11612,6 +11857,7 @@ static void perf_event_exit_cpu_context(int cpu)
 {
 	struct perf_cpu_context *cpuctx;
 	struct perf_event_context *ctx;
+	struct perf_event *event;
 	struct pmu *pmu;

 	mutex_lock(&pmus_lock);
@@ -11623,6 +11869,8 @@ static void perf_event_exit_cpu_context(int cpu)
 		smp_call_function_single(cpu, __perf_event_exit_context, ctx, 1);
 		cpuctx->online = 0;
 		mutex_unlock(&ctx->mutex);
+		list_for_each_entry(event, &ctx->event_list, event_entry)
+			perf_event_free_tmp_master(event);
 	}
 	cpumask_clear_cpu(cpu, perf_online_mask);
 	mutex_unlock(&pmus_lock);
--
2.17.1

[PATCH/RFC v4 2/2] perf tools: monitoring per task counter with per cgroup event

[Song Liu]

This is just a prototype.

Previous patch enables sharing hardware PMU among perf_events within same
perf_event_context. This sharing comes with limitation that per CPU event
cannot share hardware PMU with per task event. This limitation becomes
a blocker when certain events could only use a specific PMU, for example,
ref-cycles in some Intel CPUs. The following two commands will not share
the PMU (when run in parallel):

   perf stat -e ref-cycles -I 1000
   perf stat -e ref-cycles -I 1000 --pid <pid>

This patch shows a prototype that solves this problem with cgroup events.
With this patch, the following two commands can share the PMU:

   perf stat -e ref-cycles -I 1000
   perf stat -e ref-cycles -I 1000 --pid <pid> --create-cgroup

The second command creates a cgroup for the pid, and move the pid to
that cgroup. Then, a cgroup event (instead of task event) is created
to monitor the process.

Alternatively, we can also create a mechanism in the kernel that is very
similar to cgroup perf events. I am also open to other suggestions.

Signed-off-by: Song Liu <songliubraving@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jiri Olsa <jolsa@kernel.org>
---
 tools/perf/builtin-stat.c | 26 ++++++++++++++
 tools/perf/util/cgroup.c  | 76 +++++++++++++++++++++++++++++++++++++++
 tools/perf/util/cgroup.h  |  5 +++
 tools/perf/util/target.h  |  1 +
 4 files changed, 108 insertions(+)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index b86aba1c8028..66a4da2d506e 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -646,6 +646,17 @@ static void print_counters(struct timespec *ts, int argc, const char **argv)
 				    ts, argc, argv);
 }

+static void cleanup(void)
+{
+	/* clean up cgroups */
+	if (target.create_cgroup) {
+		char name[32];
+
+		scnprintf(name, 31, "perf.%u", getpid());
+		cgroup__cleanup(name);
+	}
+}
+
 static volatile int signr = -1;

 static void skip_signal(int signo)
@@ -661,6 +672,7 @@ static void skip_signal(int signo)
 	 * and fast PID recycling
 	 */
 	child_pid = -1;
+	cleanup();
 }

 static void sig_atexit(void)
@@ -725,6 +737,8 @@ static const struct option stat_options[] = {
 		   "stat events on existing process id"),
 	OPT_STRING('t', "tid", &target.tid, "tid",
 		   "stat events on existing thread id"),
+	OPT_BOOLEAN(0, "create-cgroup", &target.create_cgroup,
+		    "create a cgroup for the pid/tid"),
 	OPT_BOOLEAN('a', "all-cpus", &target.system_wide,
 		    "system-wide collection from all CPUs"),
 	OPT_BOOLEAN('g', "group", &group,
@@ -1607,6 +1621,17 @@ int cmd_stat(int argc, const char **argv)
 	perf_stat__collect_metric_expr(evsel_list);
 	perf_stat__init_shadow_stats();

+	if (target.create_cgroup) {
+		char name[32];
+
+		scnprintf(name, 31, "perf.%u", getpid());
+		cgroup__create(name);
+		cgroup__add_pid(name, strtoul(target.pid, NULL, 0));
+
+		cgroup__add_evlist(name, evsel_list);
+		target.pid = NULL;
+	}
+
 	if (stat_config.csv_sep) {
 		stat_config.csv_output = true;
 		if (!strcmp(stat_config.csv_sep, "\\t"))
@@ -1906,5 +1931,6 @@ int cmd_stat(int argc, const char **argv)

 	runtime_stat_delete(&stat_config);

+	cleanup();
 	return status;
 }
diff --git a/tools/perf/util/cgroup.c b/tools/perf/util/cgroup.c
index ccd02634a616..f3e706f6fa96 100644
--- a/tools/perf/util/cgroup.c
+++ b/tools/perf/util/cgroup.c
@@ -249,3 +249,79 @@ int parse_cgroups(const struct option *opt, const char *str,
 	}
 	return 0;
 }
+
+int cgroup__add_evlist(const char *name, struct perf_evlist *evlist)
+{
+	return add_cgroup(evlist, name);
+}
+
+int cgroup__create(const char *name)
+{
+	char path[PATH_MAX + 1];
+	char mnt[PATH_MAX + 1];
+
+	if (cgroupfs_find_mountpoint(mnt, PATH_MAX + 1))
+		return -1;
+
+	scnprintf(path, PATH_MAX, "%s/%s", mnt, name);
+
+	return mkdir(path, 0755);
+}
+
+int cgroup__add_pid(const char *name, pid_t pid)
+{
+	char path[PATH_MAX + 1];
+	char mnt[PATH_MAX + 1];
+	char buf[32];
+	int fd;
+
+	if (cgroupfs_find_mountpoint(mnt, PATH_MAX + 1))
+		return -1;
+
+	scnprintf(path, PATH_MAX, "%s/%s/cgroup.procs", mnt, name);
+	fd = open(path, O_WRONLY);
+	if (fd < 0)
+		return -1;
+	scnprintf(buf, 31, "%u", pid);
+	if (write(fd, buf, strlen(buf)) < 0)
+		fprintf(stderr, "Error writing %s to %s\n", buf, path);
+
+	close(fd);
+	return 0;
+}
+
+int cgroup__cleanup(const char *name)
+{
+	char path[PATH_MAX + 1];
+	char mnt[PATH_MAX + 1];
+	char *line;
+	size_t len = 0;
+	FILE *fp;
+	int fd;
+
+	if (cgroupfs_find_mountpoint(mnt, PATH_MAX + 1))
+		return -1;
+
+	scnprintf(path, PATH_MAX, "%s/%s/cgroup.procs", mnt, name);
+	fp = fopen(path, "r");
+
+	if (fp == NULL)
+		return -1;
+
+	scnprintf(path, PATH_MAX, "%s/cgroup.procs", mnt);
+	fd = open(path, O_WRONLY);
+	if (fd < 0) {
+		fclose(fp);
+		return -1;
+	}
+
+	while (getline(&line, &len, fp) != -1) {
+		if (write(fd, line, strlen(line)) < 0)
+			fprintf(stderr, "Error writing %s to %s\n", line, path);
+	}
+	close(fd);
+	fclose(fp);
+
+	scnprintf(path, PATH_MAX, "%s/%s", mnt, name);
+	return rmdir(path);
+}
diff --git a/tools/perf/util/cgroup.h b/tools/perf/util/cgroup.h
index f033a80c1b14..7bdd8d99d130 100644
--- a/tools/perf/util/cgroup.h
+++ b/tools/perf/util/cgroup.h
@@ -26,4 +26,9 @@ void evlist__set_default_cgroup(struct perf_evlist *evlist, struct cgroup *cgrou

 int parse_cgroups(const struct option *opt, const char *str, int unset);

+int cgroup__create(const char *name);
+int cgroup__cleanup(const char *name);
+int cgroup__add_pid(const char *name, pid_t pid);
+int cgroup__add_evlist(const char *name, struct perf_evlist *evlist);
+
 #endif /* __CGROUP_H__ */
diff --git a/tools/perf/util/target.h b/tools/perf/util/target.h
index 6ef01a83b24e..03c9ac06660a 100644
--- a/tools/perf/util/target.h
+++ b/tools/perf/util/target.h
@@ -15,6 +15,7 @@ struct target {
 	bool	     uses_mmap;
 	bool	     default_per_cpu;
 	bool	     per_thread;
+	bool	     create_cgroup;
 };

 enum target_errno {
--
2.17.1