[PATCH] perf: decouple unthrottling and rotating

[Mark Rutland]

Currently the adjusments made as part of perf_event_task_tick use the
percpu rotation lists to iterate over any active PMU contexts, but these
are not used by the context rotation code, having been replaced by
separate (per-context) hrtimer callbacks. However, some manipulation of
the rotation lists (i.e. removal of contexts) has remained in
perf_rotate_context. This leads to the following issues:

* Contexts are not always removed from the rotation lists. Removal of
  PMUs which have been placed in rotation lists, but have not been
  removed by a hrtimer callback can result in corruption of the rotation
  lists (when memory backing the context is freed).

  This has been observed to result in hangs when PMU drivers built as
  modules are inserted and removed around the creation of events for
  said PMUs.

* Contexts which do not require rotation may be removed from the
  rotation lists as a result of a hrtimer, and will not be considered by
  the unthrottling code in perf_event_task_tick.

This patch solves these issues by moving any and all removal of contexts
from rotation lists to only occur when the final event is removed from a
context, mirroring the addition which only occurs when the first event
is added to a context. The vestigal manipulation of the rotation lists
is removed from perf_event_rotate_context.

As the rotation_list variables are not used for rotation, these are
renamed to active_ctx_list, which better matches their current function.
perf_pmu_rotate_{start,stop} are renamed to
perf_pmu_ctx_{activate,deactivate}.

Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Reported-by: Johannes Jensen <johannes.jensen@xxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
 include/linux/perf_event.h |  2 +-
 kernel/events/core.c       | 64 ++++++++++++++++++++--------------------------
 2 files changed, 29 insertions(+), 37 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 486e84c..4ca17f1 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -530,7 +530,7 @@ struct perf_cpu_context {
 	int				exclusive;
 	struct hrtimer			hrtimer;
 	ktime_t				hrtimer_interval;
-	struct list_head		rotation_list;
+	struct list_head		active_ctx_list;
 	struct pmu			*unique_pmu;
 	struct perf_cgroup		*cgrp;
 };
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4faccf3..ac0b9c8 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -872,22 +872,33 @@ void perf_pmu_enable(struct pmu *pmu)
 		pmu->pmu_enable(pmu);
 }
 
-static DEFINE_PER_CPU(struct list_head, rotation_list);
+static DEFINE_PER_CPU(struct list_head, active_ctx_list);
 
 /*
- * perf_pmu_rotate_start() and perf_rotate_context() are fully serialized
- * because they're strictly cpu affine and rotate_start is called with IRQs
- * disabled, while rotate_context is called from IRQ context.
+ * perf_pmu_ctx_activate(), perf_pmu_ctx_deactivate(), and
+ * perf_even_task_tick() are fully serialized because they're strictly cpu
+ * affine and perf_pmu_ctx{activate,deactiveate} are called with IRQs disabled,
+ * while perf_event_task_tick is called from IRQ context.
  */
-static void perf_pmu_rotate_start(struct pmu *pmu)
+static void perf_pmu_ctx_activate(struct pmu *pmu)
 {
 	struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
-	struct list_head *head = this_cpu_ptr(&rotation_list);
+	struct list_head *head = this_cpu_ptr(&active_ctx_list);
 
 	WARN_ON(!irqs_disabled());
 
-	if (list_empty(&cpuctx->rotation_list))
-		list_add(&cpuctx->rotation_list, head);
+	WARN_ON(!list_empty(&cpuctx->active_ctx_list));
+
+	list_add(&cpuctx->active_ctx_list, head);
+}
+
+static void perf_pmu_ctx_deactivate(struct pmu *pmu)
+{
+	struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
+
+	WARN_ON(!irqs_disabled());
+
+	list_del_init(&cpuctx->active_ctx_list);
 }
 
 static void get_ctx(struct perf_event_context *ctx)
@@ -1156,7 +1167,7 @@ list_add_event(struct perf_event *event, struct perf_event_context *ctx)
 
 	list_add_rcu(&event->event_entry, &ctx->event_list);
 	if (!ctx->nr_events)
-		perf_pmu_rotate_start(ctx->pmu);
+		perf_pmu_ctx_activate(ctx->pmu);
 	ctx->nr_events++;
 	if (event->attr.inherit_stat)
 		ctx->nr_stat++;
@@ -1324,6 +1335,8 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
 		ctx->nr_stat--;
 
 	list_del_rcu(&event->event_entry);
+	if (!ctx->nr_events)
+		perf_pmu_ctx_deactivate(ctx->pmu);
 
 	if (event->group_leader == event)
 		list_del_init(&event->group_entry);
@@ -2612,12 +2625,6 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
 
 	perf_pmu_enable(ctx->pmu);
 	perf_ctx_unlock(cpuctx, ctx);
-
-	/*
-	 * Since these rotations are per-cpu, we need to ensure the
-	 * cpu-context we got scheduled on is actually rotating.
-	 */
-	perf_pmu_rotate_start(ctx->pmu);
 }
 
 /*
@@ -2906,24 +2913,22 @@ static void rotate_ctx(struct perf_event_context *ctx)
 }
 
 /*
- * perf_pmu_rotate_start() and perf_rotate_context() are fully serialized
+ * perf_pmu_ctx_activate() and perf_rotate_context() are fully serialized
  * because they're strictly cpu affine and rotate_start is called with IRQs
  * disabled, while rotate_context is called from IRQ context.
  */
 static int perf_rotate_context(struct perf_cpu_context *cpuctx)
 {
 	struct perf_event_context *ctx = NULL;
-	int rotate = 0, remove = 1;
+	int rotate = 0;
 
 	if (cpuctx->ctx.nr_events) {
-		remove = 0;
 		if (cpuctx->ctx.nr_events != cpuctx->ctx.nr_active)
 			rotate = 1;
 	}
 
 	ctx = cpuctx->task_ctx;
 	if (ctx && ctx->nr_events) {
-		remove = 0;
 		if (ctx->nr_events != ctx->nr_active)
 			rotate = 1;
 	}
@@ -2947,8 +2952,6 @@ static int perf_rotate_context(struct perf_cpu_context *cpuctx)
 	perf_pmu_enable(cpuctx->ctx.pmu);
 	perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
 done:
-	if (remove)
-		list_del_init(&cpuctx->rotation_list);
 
 	return rotate;
 }
@@ -2966,7 +2969,7 @@ bool perf_event_can_stop_tick(void)
 
 void perf_event_task_tick(void)
 {
-	struct list_head *head = this_cpu_ptr(&rotation_list);
+	struct list_head *head = this_cpu_ptr(&active_ctx_list);
 	struct perf_cpu_context *cpuctx, *tmp;
 	struct perf_event_context *ctx;
 	int throttled;
@@ -2976,7 +2979,7 @@ void perf_event_task_tick(void)
 	__this_cpu_inc(perf_throttled_seq);
 	throttled = __this_cpu_xchg(perf_throttled_count, 0);
 
-	list_for_each_entry_safe(cpuctx, tmp, head, rotation_list) {
+	list_for_each_entry_safe(cpuctx, tmp, head, active_ctx_list) {
 		ctx = &cpuctx->ctx;
 		perf_adjust_freq_unthr_context(ctx, throttled);
 
@@ -6784,7 +6787,7 @@ skip_type:
 
 		__perf_cpu_hrtimer_init(cpuctx, cpu);
 
-		INIT_LIST_HEAD(&cpuctx->rotation_list);
+		INIT_LIST_HEAD(&cpuctx->active_ctx_list);
 		cpuctx->unique_pmu = pmu;
 	}
 
@@ -8133,7 +8136,7 @@ static void __init perf_event_init_all_cpus(void)
 	for_each_possible_cpu(cpu) {
 		swhash = &per_cpu(swevent_htable, cpu);
 		mutex_init(&swhash->hlist_mutex);
-		INIT_LIST_HEAD(&per_cpu(rotation_list, cpu));
+		INIT_LIST_HEAD(&per_cpu(active_ctx_list, cpu));
 	}
 }
 
@@ -8154,22 +8157,11 @@ static void perf_event_init_cpu(int cpu)
 }
 
 #if defined CONFIG_HOTPLUG_CPU || defined CONFIG_KEXEC
-static void perf_pmu_rotate_stop(struct pmu *pmu)
-{
-	struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
-
-	WARN_ON(!irqs_disabled());
-
-	list_del_init(&cpuctx->rotation_list);
-}
-
 static void __perf_event_exit_context(void *__info)
 {
 	struct remove_event re = { .detach_group = true };
 	struct perf_event_context *ctx = __info;
 
-	perf_pmu_rotate_stop(ctx->pmu);
-
 	rcu_read_lock();
 	list_for_each_entry_rcu(re.event, &ctx->event_list, event_entry)
 		__perf_remove_from_context(&re);
-- 
1.9.1

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