Re: [PATCH v2] perf: Rewrite core context handling
From: Peter Zijlstra
Date: Tue Oct 11 2022 - 13:48:25 EST
On Tue, Oct 11, 2022 at 04:02:56PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 11, 2022 at 06:49:55PM +0530, Ravi Bangoria wrote:
> > On 11-Oct-22 4:59 PM, Peter Zijlstra wrote:
> > > On Sat, Oct 08, 2022 at 11:54:24AM +0530, Ravi Bangoria wrote:
> > >
> > >> +static void perf_event_swap_task_ctx_data(struct perf_event_context *prev_ctx,
> > >> + struct perf_event_context *next_ctx)
> > >> +{
> > >> + struct perf_event_pmu_context *prev_epc, *next_epc;
> > >> +
> > >> + if (!prev_ctx->nr_task_data)
> > >> + return;
> > >> +
> > >> + prev_epc = list_first_entry(&prev_ctx->pmu_ctx_list,
> > >> + struct perf_event_pmu_context,
> > >> + pmu_ctx_entry);
> > >> + next_epc = list_first_entry(&next_ctx->pmu_ctx_list,
> > >> + struct perf_event_pmu_context,
> > >> + pmu_ctx_entry);
> > >> +
> > >> + while (&prev_epc->pmu_ctx_entry != &prev_ctx->pmu_ctx_list &&
> > >> + &next_epc->pmu_ctx_entry != &next_ctx->pmu_ctx_list) {
> > >> +
> > >> + WARN_ON_ONCE(prev_epc->pmu != next_epc->pmu);
> > >> +
> > >> + /*
> > >> + * PMU specific parts of task perf context can require
> > >> + * additional synchronization. As an example of such
> > >> + * synchronization see implementation details of Intel
> > >> + * LBR call stack data profiling;
> > >> + */
> > >> + if (prev_epc->pmu->swap_task_ctx)
> > >> + prev_epc->pmu->swap_task_ctx(prev_epc, next_epc);
> > >> + else
> > >> + swap(prev_epc->task_ctx_data, next_epc->task_ctx_data);
> > >
> > > Did I forget to advance the iterators here?
> >
> > Yeah. Seems so. I overlooked it too.
>
> OK; so I'm not slowly going crazy staring at this code ;-) Let me go add
> it now then. :-)
>
> But first I gotta taxi the kids around for a bit, bbl.
OK, so I've been going over the perf_event_pmu_context life-time thing
as well, there were a bunch of XXXs there and I'm not sure Im happy with
things, but I'd also forgotten most of it.
Ideally epc works like it's a regular member of ctx -- locking wise that
is, but I'm not sure we can make that stick -- see the ctx->mutex issues
we have with put_ctx().
As such, I'm going to have to re-audit all the epc usage to see if
pure ctx->lock is sufficient.
I did do make epc RCU freed, because pretty much everything is and that
was easy enough to make happen -- it means we don't need to worry about
that.
But I'm going cross-eyes from staring at this all day, so more tomorrow.
The below is what I currently have.
---
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -833,13 +833,13 @@ struct perf_event {
* `--------[1:n]---------' `-[n:1]-> pmu <-[1:n]-'
*
*
- * XXX destroy epc when empty
- * refcount, !rcu
+ * epc lifetime is refcount based and RCU freed (similar to perf_event_context).
+ * epc locking is as if it were a member of perf_event_context; specifically:
*
- * XXX epc locking
+ * modification, both: ctx->mutex && ctx->lock
+ * reading, either: ctx->mutex || ctx->lock
*
- * event->pmu_ctx ctx->mutex && inactive
- * ctx->pmu_ctx_list ctx->mutex && ctx->lock
+ * XXX except this isn't true ... see put_pmu_ctx().
*
*/
struct perf_event_pmu_context {
@@ -857,6 +857,7 @@ struct perf_event_pmu_context {
unsigned int nr_events;
atomic_t refcount; /* event <-> epc */
+ struct rcu_head rcu_head;
void *task_ctx_data; /* pmu specific data */
/*
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1727,6 +1727,10 @@ perf_event_groups_next(struct perf_event
return NULL;
}
+#define perf_event_groups_for_cpu_pmu(event, groups, cpu, pmu) \
+ for (event = perf_event_groups_first(groups, cpu, pmu, NULL); \
+ event; event = perf_event_groups_next(event, pmu))
+
/*
* Iterate through the whole groups tree.
*/
@@ -3366,6 +3370,14 @@ static void perf_event_sync_stat(struct
}
}
+#define list_for_each_entry_double(pos1, pos2, head1, head2, member) \
+ for (pos1 = list_first_entry(head1, typeof(*pos1), member), \
+ pos2 = list_first_entry(head2, typeof(*pos2), member); \
+ !list_entry_is_head(pos1, head1, member) && \
+ !list_entry_is_head(pos2, head2, member); \
+ pos1 = list_next_entry(pos1, member), \
+ pos2 = list_next_entry(pos2, member))
+
static void perf_event_swap_task_ctx_data(struct perf_event_context *prev_ctx,
struct perf_event_context *next_ctx)
{
@@ -3374,16 +3386,9 @@ static void perf_event_swap_task_ctx_dat
if (!prev_ctx->nr_task_data)
return;
- prev_epc = list_first_entry(&prev_ctx->pmu_ctx_list,
- struct perf_event_pmu_context,
- pmu_ctx_entry);
- next_epc = list_first_entry(&next_ctx->pmu_ctx_list,
- struct perf_event_pmu_context,
- pmu_ctx_entry);
-
- while (&prev_epc->pmu_ctx_entry != &prev_ctx->pmu_ctx_list &&
- &next_epc->pmu_ctx_entry != &next_ctx->pmu_ctx_list) {
-
+ list_for_each_entry_double(prev_epc, next_epc,
+ &prev_ctx->pmu_ctx_list, &next_ctx->pmu_ctx_list,
+ pmu_ctx_entry) {
WARN_ON_ONCE(prev_epc->pmu != next_epc->pmu);
/*
@@ -3706,7 +3711,6 @@ static noinline int visit_groups_merge(s
perf_assert_pmu_disabled((*evt)->pmu_ctx->pmu);
}
-
min_heapify_all(&event_heap, &perf_min_heap);
while (event_heap.nr) {
@@ -3845,7 +3849,6 @@ ctx_sched_in(struct perf_event_context *
/* start ctx time */
__update_context_time(ctx, false);
perf_cgroup_set_timestamp(cpuctx);
- // XXX ctx->task =? task
/*
* CPU-release for the below ->is_active store,
* see __load_acquire() in perf_event_time_now()
@@ -4815,6 +4818,15 @@ find_get_pmu_context(struct pmu *pmu, st
__perf_init_event_pmu_context(new, pmu);
+ /*
+ * XXX
+ *
+ * lockdep_assert_held(&ctx->mutex);
+ *
+ * can't because perf_event_init_task() doesn't actually hold the
+ * child_ctx->mutex.
+ */
+
raw_spin_lock_irq(&ctx->lock);
list_for_each_entry(epc, &ctx->pmu_ctx_list, pmu_ctx_entry) {
if (epc->pmu == pmu) {
@@ -4849,6 +4861,14 @@ static void get_pmu_ctx(struct perf_even
WARN_ON_ONCE(!atomic_inc_not_zero(&epc->refcount));
}
+static void free_epc_rcu(struct rcu_head *head)
+{
+ struct perf_event_pmu_context *epc = container_of(head, typeof(*epc), rcu_head);
+
+ kfree(epc->task_ctx_data);
+ kfree(epc);
+}
+
static void put_pmu_ctx(struct perf_event_pmu_context *epc)
{
unsigned long flags;
@@ -4859,7 +4879,14 @@ static void put_pmu_ctx(struct perf_even
if (epc->ctx) {
struct perf_event_context *ctx = epc->ctx;
- // XXX ctx->mutex
+ /*
+ * XXX
+ *
+ * lockdep_assert_held(&ctx->mutex);
+ *
+ * can't because of the call-site in _free_event()/put_event()
+ * which isn't always called under ctx->mutex.
+ */
WARN_ON_ONCE(list_empty(&epc->pmu_ctx_entry));
raw_spin_lock_irqsave(&ctx->lock, flags);
@@ -4874,17 +4901,15 @@ static void put_pmu_ctx(struct perf_even
if (epc->embedded)
return;
- kfree(epc->task_ctx_data);
- kfree(epc);
+ call_rcu(&epc->rcu_head, free_epc_rcu);
}
static void perf_event_free_filter(struct perf_event *event);
static void free_event_rcu(struct rcu_head *head)
{
- struct perf_event *event;
+ struct perf_event *event = container_of(head, typeof(*event), rcu_head);
- event = container_of(head, struct perf_event, rcu_head);
if (event->ns)
put_pid_ns(event->ns);
perf_event_free_filter(event);
@@ -12643,13 +12668,6 @@ perf_event_create_kernel_counter(struct
goto err_alloc;
}
- pmu_ctx = find_get_pmu_context(pmu, ctx, event);
- if (IS_ERR(pmu_ctx)) {
- err = PTR_ERR(pmu_ctx);
- goto err_ctx;
- }
- event->pmu_ctx = pmu_ctx;
-
WARN_ON_ONCE(ctx->parent_ctx);
mutex_lock(&ctx->mutex);
if (ctx->task == TASK_TOMBSTONE) {
@@ -12657,6 +12675,13 @@ perf_event_create_kernel_counter(struct
goto err_unlock;
}
+ pmu_ctx = find_get_pmu_context(pmu, ctx, event);
+ if (IS_ERR(pmu_ctx)) {
+ err = PTR_ERR(pmu_ctx);
+ goto err_unlock;
+ }
+ event->pmu_ctx = pmu_ctx;
+
if (!task) {
/*
* Check if the @cpu we're creating an event for is online.
@@ -12668,13 +12693,13 @@ perf_event_create_kernel_counter(struct
container_of(ctx, struct perf_cpu_context, ctx);
if (!cpuctx->online) {
err = -ENODEV;
- goto err_unlock;
+ goto err_pmu_ctx;
}
}
if (!exclusive_event_installable(event, ctx)) {
err = -EBUSY;
- goto err_unlock;
+ goto err_pmu_ctx;
}
perf_install_in_context(ctx, event, event->cpu);
@@ -12683,9 +12708,10 @@ perf_event_create_kernel_counter(struct
return event;
+err_pmu_ctx:
+ put_pmu_ctx(pmu_ctx);
err_unlock:
mutex_unlock(&ctx->mutex);
-err_ctx:
perf_unpin_context(ctx);
put_ctx(ctx);
err_alloc:
@@ -12702,9 +12728,7 @@ static void __perf_pmu_remove(struct per
{
struct perf_event *event, *sibling;
- for (event = perf_event_groups_first(groups, cpu, pmu, NULL);
- event; event = perf_event_groups_next(event, pmu)) {
-
+ perf_event_groups_for_cpu_pmu(event, groups, cpu, pmu) {
perf_remove_from_context(event, 0);
unaccount_event_cpu(event, cpu);
put_pmu_ctx(event->pmu_ctx);
@@ -12998,7 +13022,7 @@ void perf_event_free_task(struct task_st
struct perf_event_context *ctx;
struct perf_event *event, *tmp;
- ctx = rcu_dereference(task->perf_event_ctxp);
+ ctx = rcu_access_pointer(task->perf_event_ctxp);
if (!ctx)
return;