On Tue, Mar 23, 2021 at 11:32:03AM +0100, Peter Zijlstra wrote:
> And at that point there's very little value in still using
> perf_event_exit_event()... let me see if there's something to be done
> about that.
I ended up with something like the below. Which then simplifies
remove_on_exec() to:
static void perf_event_remove_on_exec(int ctxn)
{
struct perf_event_context *ctx, *clone_ctx = NULL;
struct perf_event *event, *next;
bool modified = false;
unsigned long flags;
ctx = perf_pin_task_context(current, ctxn);
if (!ctx)
return;
mutex_lock(&ctx->mutex);
if (WARN_ON_ONCE(ctx->task != current))
goto unlock;
list_for_each_entry_safe(event, next, &ctx->event_list, event_entry) {
if (!event->attr.remove_on_exec)
continue;
if (!is_kernel_event(event))
perf_remove_from_owner(event);
modified = true;
perf_event_exit_event(event, ctx);
}
raw_spin_lock_irqsave(&ctx->lock, flags);
if (modified)
clone_ctx = unclone_ctx(ctx);
--ctx->pin_count;
raw_spin_unlock_irqrestore(&ctx->lock, flags);
unlock:
mutex_unlock(&ctx->mutex);
put_ctx(ctx);
if (clone_ctx)
put_ctx(clone_ctx);
}
Very lightly tested with that {1..1000} thing.
---
Subject: perf: Rework perf_event_exit_event()
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Tue Mar 23 15:16:06 CET 2021
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.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
include/linux/perf_event.h | 1
kernel/events/core.c | 144 +++++++++++++++++++++++++--------------------
2 files changed, 81 insertions(+), 64 deletions(-)
--- 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;
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2210,6 +2210,26 @@ static void perf_group_detach(struct per
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;
@@ -2317,6 +2337,7 @@ group_sched_out(struct perf_event *group
}
#define DETACH_GROUP 0x01UL
+#define DETACH_CHILD 0x02UL
/*
* Cross CPU call to remove a performance event
@@ -2340,6 +2361,8 @@ __perf_remove_from_context(struct perf_e
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) {
@@ -2368,25 +2391,21 @@ static void perf_remove_from_context(str
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.
- */
- 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);
+ * 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.
+ */
+ 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);
}
/*
@@ -12379,14 +12398,17 @@ void perf_pmu_migrate_context(struct pmu
}
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)
+ perf_event_read_event(child_event, task);
+ }
child_val = perf_event_count(child_event);
@@ -12401,60 +12423,53 @@ static void sync_child_event(struct perf
}
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)
@@ -12511,7 +12526,7 @@ static void perf_event_exit_task_context
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);
@@ -12771,6 +12786,7 @@ inherit_event(struct perf_event *parent_
*/
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);
/*
