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

From: Song Liu
Date: Mon Sep 24 2018 - 18:25:29 EST

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@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
Signed-off-by: Song Liu <songliubraving@xxxxxx>
---
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