[PATCH v3 0/1] perf: Sharing PMU counters across compatible events

[PATCH v3 0/1] perf: Sharing PMU counters across compatible events

[Song Liu]

Chnages 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>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>

Song Liu (1):
  perf: Sharing PMU counters across compatible events

 include/linux/perf_event.h |  61 ++++++++
 kernel/events/core.c       | 281 +++++++++++++++++++++++++++++++++++--
 2 files changed, 332 insertions(+), 10 deletions(-)

--
2.17.1

[PATCH v3 1/1] 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>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
---
 include/linux/perf_event.h |  61 ++++++++
 kernel/events/core.c       | 281 +++++++++++++++++++++++++++++++++++--
 2 files changed, 332 insertions(+), 10 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 abaed4f8bb7f..44af93edc81e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1655,6 +1655,145 @@ 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))
+		goto not_dup;
+
+	/* 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 */
+		goto not_dup;
+
+	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;
+	}
+
+not_dup:
+	event->dup_id = -1;
+}
+
+/*
+ * 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 +1826,7 @@ 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 +1993,7 @@ 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 +2123,84 @@ 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 +2223,7 @@ 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 +2495,7 @@ 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;
@@ -2561,6 +2780,10 @@ 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 +3229,7 @@ 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 +4085,14 @@ 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 +4100,7 @@ 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);
 		}
 	}
 
@@ -3941,7 +4164,7 @@ 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) {
@@ -4464,6 +4687,7 @@ static void free_event(struct perf_event *event)
 		return;
 	}
 
+	perf_event_free_tmp_master(event);
 	_free_event(event);
 }
 
@@ -4523,6 +4747,7 @@ 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);
 }
 
@@ -6082,7 +6307,7 @@ 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)
@@ -6095,7 +6320,7 @@ 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)
@@ -9104,7 +9329,7 @@ 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();
@@ -10499,6 +10724,14 @@ 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;
@@ -10704,9 +10937,17 @@ 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);
 		}
 
 		/*
@@ -10764,6 +11005,9 @@ 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);
@@ -10908,6 +11152,7 @@ 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);
 	}
 
@@ -11266,6 +11511,14 @@ 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) {
@@ -11320,6 +11573,10 @@ 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
@@ -11334,6 +11591,7 @@ 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
 	 */
@@ -11605,6 +11863,7 @@ 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);
@@ -11616,6 +11875,8 @@ 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

Re: [PATCH v3 1/1] perf: Sharing PMU counters across compatible events

[Ravi Bangoria]

Hi Song,

On 09/25/2018 03:55 AM, Song Liu wrote:
> 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 /
> 

I've not gone through the patch in detail, but I was specifically
interested in scenarios where one perf instance is counting event
systemwide and thus other perf instance fails to count the same
event for a specific workload because that event can be counted
in one hw counter only.

Ex: https://lkml.org/lkml/2018/3/12/1011

Seems this patch does not solve this issue. Please let me know if
I'm missing anything.

Thanks,
Ravi

Re: [PATCH v3 1/1] perf: Sharing PMU counters across compatible events

[Song Liu]

Hi Ravi,

> On Sep 27, 2018, at 9:33 PM, Ravi Bangoria <ravi.bangoria@linux.ibm.com> wrote:
> 
> Hi Song,
> 
> On 09/25/2018 03:55 AM, Song Liu wrote:
>> 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 /
>> 
> 
> I've not gone through the patch in detail, but I was specifically
> interested in scenarios where one perf instance is counting event
> systemwide and thus other perf instance fails to count the same
> event for a specific workload because that event can be counted
> in one hw counter only.
> 
> Ex: https://lkml.org/lkml/2018/3/12/1011
> 
> Seems this patch does not solve this issue. Please let me know if
> I'm missing anything.
> 

In this case, unfortunately, these two events cannot share the same
counter, because one of them is in cpu ctx; while the other belongs 
to the task ctx. They have to go through the rotation, that each 
event counts 50% of the time. However, if you have 2 events in cpu 
ctx and 2 events in task ctx on the same counter, this patch will 
help each event to count 50% of time, instead of 25%. 

Another potential solution is to create a cgroup for the workload, 
and attach perf event to the cgroup. Since cgroup events are added
to the cpu ctx, they can share counters with the system wide events. 

I made this trade-off for O(1) time context switch. If we share 
hw counter between cpu ctx and task ctx, we have to do linear time 
comparison to identify events that can share the counter.  

Thanks,
Song

Re: [PATCH v3 1/1] perf: Sharing PMU counters across compatible events

[Song Liu]

> On Sep 27, 2018, at 9:53 PM, Song Liu <songliubraving@fb.com> wrote:
> 
> Hi Ravi,
> 
>> On Sep 27, 2018, at 9:33 PM, Ravi Bangoria <ravi.bangoria@linux.ibm.com> wrote:
>> 
>> Hi Song,
>> 
>> On 09/25/2018 03:55 AM, Song Liu wrote:
>>> 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 /
>>> 
>> 
>> I've not gone through the patch in detail, but I was specifically
>> interested in scenarios where one perf instance is counting event
>> systemwide and thus other perf instance fails to count the same
>> event for a specific workload because that event can be counted
>> in one hw counter only.
>> 
>> Ex: https://lkml.org/lkml/2018/3/12/1011
>> 
>> Seems this patch does not solve this issue. Please let me know if
>> I'm missing anything.
>> 
> 
> In this case, unfortunately, these two events cannot share the same
> counter, because one of them is in cpu ctx; while the other belongs 
> to the task ctx. They have to go through the rotation, that each 
> event counts 50% of the time. However, if you have 2 events in cpu 
> ctx and 2 events in task ctx on the same counter, this patch will 
> help each event to count 50% of time, instead of 25%. 

I was wrong here. The CPU flexible events have higher priority than
task flexible events, so the per task event will never get scheduled. 

On the other hand, I think it is possible to put per task event on 
per CPU ctx, and handle the event just like per cgroup events. Once we
get this part work, system wide events will be able to share PMU with
per task events. Let me try prototype it. 

Thanks,
Song