[tip: perf/core] perf: Rework perf_event_exit_event()

From: tip-bot2 for Peter Zijlstra
Date: Fri Apr 16 2021 - 11:02:34 EST


The following commit has been merged into the perf/core branch of tip:

Commit-ID: ef54c1a476aef7eef26fe13ea10dc090952c00f8
Gitweb: https://git.kernel.org/tip/ef54c1a476aef7eef26fe13ea10dc090952c00f8
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Thu, 08 Apr 2021 12:35:56 +02:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 16 Apr 2021 16:32:40 +02:00

perf: Rework perf_event_exit_event()

Make perf_event_exit_event() more robust, such that we can use it from
other contexts. Specifically the up and coming remove_on_exec.

For this to work we need to address a few issues. Remove_on_exec will
not destroy the entire context, so we cannot rely on TASK_TOMBSTONE to
disable event_function_call() and we thus have to use
perf_remove_from_context().

When using perf_remove_from_context(), there's two races to consider.
The first is against close(), where we can have concurrent tear-down
of the event. The second is against child_list iteration, which should
not find a half baked event.

To address this, teach perf_remove_from_context() to special case
!ctx->is_active and about DETACH_CHILD.

[ elver@xxxxxxxxxx: fix racing parent/child exit in sync_child_event(). ]
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210408103605.1676875-2-elver@xxxxxxxxxx
---
include/linux/perf_event.h | 1 +-
kernel/events/core.c | 142 ++++++++++++++++++++----------------
2 files changed, 80 insertions(+), 63 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 3f7f89e..3d478ab 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -607,6 +607,7 @@ struct swevent_hlist {
#define PERF_ATTACH_TASK_DATA 0x08
#define PERF_ATTACH_ITRACE 0x10
#define PERF_ATTACH_SCHED_CB 0x20
+#define PERF_ATTACH_CHILD 0x40

struct perf_cgroup;
struct perf_buffer;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index f079431..318ff7b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2205,6 +2205,26 @@ out:
perf_event__header_size(leader);
}

+static void sync_child_event(struct perf_event *child_event);
+
+static void perf_child_detach(struct perf_event *event)
+{
+ struct perf_event *parent_event = event->parent;
+
+ if (!(event->attach_state & PERF_ATTACH_CHILD))
+ return;
+
+ event->attach_state &= ~PERF_ATTACH_CHILD;
+
+ if (WARN_ON_ONCE(!parent_event))
+ return;
+
+ lockdep_assert_held(&parent_event->child_mutex);
+
+ sync_child_event(event);
+ list_del_init(&event->child_list);
+}
+
static bool is_orphaned_event(struct perf_event *event)
{
return event->state == PERF_EVENT_STATE_DEAD;
@@ -2312,6 +2332,7 @@ group_sched_out(struct perf_event *group_event,
}

#define DETACH_GROUP 0x01UL
+#define DETACH_CHILD 0x02UL

/*
* Cross CPU call to remove a performance event
@@ -2335,6 +2356,8 @@ __perf_remove_from_context(struct perf_event *event,
event_sched_out(event, cpuctx, ctx);
if (flags & DETACH_GROUP)
perf_group_detach(event);
+ if (flags & DETACH_CHILD)
+ perf_child_detach(event);
list_del_event(event, ctx);

if (!ctx->nr_events && ctx->is_active) {
@@ -2363,25 +2386,21 @@ static void perf_remove_from_context(struct perf_event *event, unsigned long fla

lockdep_assert_held(&ctx->mutex);

- event_function_call(event, __perf_remove_from_context, (void *)flags);
-
/*
- * The above event_function_call() can NO-OP when it hits
- * TASK_TOMBSTONE. In that case we must already have been detached
- * from the context (by perf_event_exit_event()) but the grouping
- * might still be in-tact.
+ * Because of perf_event_exit_task(), perf_remove_from_context() ought
+ * to work in the face of TASK_TOMBSTONE, unlike every other
+ * event_function_call() user.
*/
- WARN_ON_ONCE(event->attach_state & PERF_ATTACH_CONTEXT);
- if ((flags & DETACH_GROUP) &&
- (event->attach_state & PERF_ATTACH_GROUP)) {
- /*
- * Since in that case we cannot possibly be scheduled, simply
- * detach now.
- */
- raw_spin_lock_irq(&ctx->lock);
- perf_group_detach(event);
+ raw_spin_lock_irq(&ctx->lock);
+ if (!ctx->is_active) {
+ __perf_remove_from_context(event, __get_cpu_context(ctx),
+ ctx, (void *)flags);
raw_spin_unlock_irq(&ctx->lock);
+ return;
}
+ raw_spin_unlock_irq(&ctx->lock);
+
+ event_function_call(event, __perf_remove_from_context, (void *)flags);
}

/*
@@ -12377,14 +12396,17 @@ void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
}
EXPORT_SYMBOL_GPL(perf_pmu_migrate_context);

-static void sync_child_event(struct perf_event *child_event,
- struct task_struct *child)
+static void sync_child_event(struct perf_event *child_event)
{
struct perf_event *parent_event = child_event->parent;
u64 child_val;

- if (child_event->attr.inherit_stat)
- perf_event_read_event(child_event, child);
+ if (child_event->attr.inherit_stat) {
+ struct task_struct *task = child_event->ctx->task;
+
+ if (task && task != TASK_TOMBSTONE)
+ perf_event_read_event(child_event, task);
+ }

child_val = perf_event_count(child_event);

@@ -12399,60 +12421,53 @@ static void sync_child_event(struct perf_event *child_event,
}

static void
-perf_event_exit_event(struct perf_event *child_event,
- struct perf_event_context *child_ctx,
- struct task_struct *child)
+perf_event_exit_event(struct perf_event *event, struct perf_event_context *ctx)
{
- struct perf_event *parent_event = child_event->parent;
+ struct perf_event *parent_event = event->parent;
+ unsigned long detach_flags = 0;

- /*
- * Do not destroy the 'original' grouping; because of the context
- * switch optimization the original events could've ended up in a
- * random child task.
- *
- * If we were to destroy the original group, all group related
- * operations would cease to function properly after this random
- * child dies.
- *
- * Do destroy all inherited groups, we don't care about those
- * and being thorough is better.
- */
- raw_spin_lock_irq(&child_ctx->lock);
- WARN_ON_ONCE(child_ctx->is_active);
+ if (parent_event) {
+ /*
+ * Do not destroy the 'original' grouping; because of the
+ * context switch optimization the original events could've
+ * ended up in a random child task.
+ *
+ * If we were to destroy the original group, all group related
+ * operations would cease to function properly after this
+ * random child dies.
+ *
+ * Do destroy all inherited groups, we don't care about those
+ * and being thorough is better.
+ */
+ detach_flags = DETACH_GROUP | DETACH_CHILD;
+ mutex_lock(&parent_event->child_mutex);
+ }

- if (parent_event)
- perf_group_detach(child_event);
- list_del_event(child_event, child_ctx);
- perf_event_set_state(child_event, PERF_EVENT_STATE_EXIT); /* is_event_hup() */
- raw_spin_unlock_irq(&child_ctx->lock);
+ perf_remove_from_context(event, detach_flags);
+
+ raw_spin_lock_irq(&ctx->lock);
+ if (event->state > PERF_EVENT_STATE_EXIT)
+ perf_event_set_state(event, PERF_EVENT_STATE_EXIT);
+ raw_spin_unlock_irq(&ctx->lock);

/*
- * Parent events are governed by their filedesc, retain them.
+ * Child events can be freed.
*/
- if (!parent_event) {
- perf_event_wakeup(child_event);
+ if (parent_event) {
+ mutex_unlock(&parent_event->child_mutex);
+ /*
+ * Kick perf_poll() for is_event_hup();
+ */
+ perf_event_wakeup(parent_event);
+ free_event(event);
+ put_event(parent_event);
return;
}
- /*
- * Child events can be cleaned up.
- */
-
- sync_child_event(child_event, child);

/*
- * Remove this event from the parent's list
- */
- WARN_ON_ONCE(parent_event->ctx->parent_ctx);
- mutex_lock(&parent_event->child_mutex);
- list_del_init(&child_event->child_list);
- mutex_unlock(&parent_event->child_mutex);
-
- /*
- * Kick perf_poll() for is_event_hup().
+ * Parent events are governed by their filedesc, retain them.
*/
- perf_event_wakeup(parent_event);
- free_event(child_event);
- put_event(parent_event);
+ perf_event_wakeup(event);
}

static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
@@ -12509,7 +12524,7 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
perf_event_task(child, child_ctx, 0);

list_for_each_entry_safe(child_event, next, &child_ctx->event_list, event_entry)
- perf_event_exit_event(child_event, child_ctx, child);
+ perf_event_exit_event(child_event, child_ctx);

mutex_unlock(&child_ctx->mutex);

@@ -12769,6 +12784,7 @@ inherit_event(struct perf_event *parent_event,
*/
raw_spin_lock_irqsave(&child_ctx->lock, flags);
add_event_to_ctx(child_event, child_ctx);
+ child_event->attach_state |= PERF_ATTACH_CHILD;
raw_spin_unlock_irqrestore(&child_ctx->lock, flags);

/*