Re: [PATCH 4/6] watchdog: Boot-disable by default on full dynticks

[Peter Zijlstra]

On Thu, Jun 13, 2013 at 10:02:07AM -0400, Don Zickus wrote:
> On Thu, Jun 13, 2013 at 03:10:59PM +0200, Frederic Weisbecker wrote:
> > On Wed, Jun 12, 2013 at 01:03:16PM -0400, Don Zickus wrote:
> > > On Wed, Jun 12, 2013 at 04:02:36PM +0200, Frederic Weisbecker wrote:
> > > > When the watchdog runs, it prevents the full dynticks
> > > > CPUs from stopping their tick because the hard lockup
> > > > detector uses perf events internally, which in turn
> > > > rely on the periodic tick.
> > > > 
> > > > Since this is a rather confusing behaviour that is not
> > > > easy to track down and identify for those who want to
> > > > test CONFIG_NO_HZ_FULL, let's default disable the
> > > > watchdog on boot time when full dynticks is enabled.
> > > > 
> > > > The user can still enable it later on runtime using
> > > > proc or sysctl.
> > > 
> > > I thought we had a conversation awhile ago, where we agreed this was going
> > > to be fixed for 3.11?  Didn't Peter find the patch and apply it to his
> > > tree?  I am confused why this is still needed?
> > 
> > We agreed on the patch but it hasn't been applied yet. I'm trying to get
> > a sane series of nohz patches before sending to Ingo.
> 
> Peter,
> 
> Where is this patch?
> 

9e6302056f8029f438e853432a856b9f13de26a6
62b8563979273424d6ebe9201e34d1acc133ad4f

made it in, it does need a little additional work; something like the
below might be a start:

Aside from doing the NOHZ thing there's also a partial fix for event migration
since I now noticed we have per-cpu counters for various event types.

Very much needs more work but shows what would be needed

---
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -139,8 +139,11 @@ enum event_type_t {
  * perf_cgroup_events: >0 per-cpu cgroup events exist on this cpu
  */
 struct static_key_deferred perf_sched_events __read_mostly;
+
+/* do we really need the atomic thing? */
 static DEFINE_PER_CPU(atomic_t, perf_cgroup_events);
 static DEFINE_PER_CPU(atomic_t, perf_branch_stack_events);
+static DEFINE_PER_CPU(atomic_t, perf_freq_events);
 
 static atomic_t nr_mmap_events __read_mostly;
 static atomic_t nr_comm_events __read_mostly;
@@ -2791,6 +2794,17 @@ bool perf_event_can_stop_tick(void)
 }
 #endif
 
+void perf_kick_nohz_cpu(void)
+{
+	/* magic to drop out of NOHZ mode */
+}
+
+bool perf_event_needs_cpu(void)
+{
+	return atomic_read(&__get_cpu_var(perf_freq_events)) || 
+	       __this_cpu_read(perf_throttled_count);
+}
+
 void perf_event_task_tick(void)
 {
 	struct list_head *head = &__get_cpu_var(rotation_list);
@@ -3101,6 +3115,28 @@ static void free_event_rcu(struct rcu_he
 static void ring_buffer_put(struct ring_buffer *rb);
 static void ring_buffer_detach(struct perf_event *event, struct ring_buffer *rb);
 
+static void __account_event(struct perf_event *event)
+{
+	if (is_cgroup_event(event))
+		atomic_inc(&per_cpu(perf_cgroup_events, event->cpu));
+	if (has_branch_stack(event) && !(event->attach_state & PERF_ATTACH_TASK))
+		atomic_inc(&per_cpu(perf_branch_stack_events, event->cpu));
+	if (event->attr.freq) {
+		atomic_inc(&per_cpu(perf_freq_events, event->cpu));
+		perf_kick_nohz_cpu();
+	}
+}
+
+static void __unaccount_event(struct perf_event *event)
+{
+	if (is_cgroup_event(event))
+		atomic_dec(&per_cpu(perf_cgroup_events, event->cpu));
+	if (has_branch_stack(event) && !(event->attach_state & PERF_ATTACH_TASK))
+		atomic_dec(&per_cpu(perf_branch_stack_events, event->cpu));
+	if (event->attr.freq)
+		atomic_dec(&per_cpu(perf_freq_events, event->cpu));
+}
+
 static void free_event(struct perf_event *event)
 {
 	irq_work_sync(&event->pending);
@@ -3116,19 +3152,12 @@ static void free_event(struct perf_event
 			atomic_dec(&nr_task_events);
 		if (event->attr.sample_type & PERF_SAMPLE_CALLCHAIN)
 			put_callchain_buffers();
-		if (is_cgroup_event(event)) {
-			atomic_dec(&per_cpu(perf_cgroup_events, event->cpu));
+		if (is_cgroup_event(event))
 			static_key_slow_dec_deferred(&perf_sched_events);
-		}
-
-		if (has_branch_stack(event)) {
+		if (has_branch_stack(event))
 			static_key_slow_dec_deferred(&perf_sched_events);
-			/* is system-wide event */
-			if (!(event->attach_state & PERF_ATTACH_TASK)) {
-				atomic_dec(&per_cpu(perf_branch_stack_events,
-						    event->cpu));
-			}
-		}
+
+		__unaccount_event(event);
 	}
 
 	if (event->rb) {
@@ -5149,6 +5178,7 @@ static int __perf_event_overflow(struct
 		if (unlikely(throttle
 			     && hwc->interrupts >= max_samples_per_tick)) {
 			__this_cpu_inc(perf_throttled_count);
+			perf_kick_nohz_cpu(); /* XXX we're in NMI context */
 			hwc->interrupts = MAX_INTERRUPTS;
 			perf_log_throttle(event, 0);
 			ret = 1;
@@ -6544,6 +6574,7 @@ perf_event_alloc(struct perf_event_attr
 			atomic_inc(&nr_task_events);
 		if (event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) {
 			err = get_callchain_buffers();
+			/* XXX this error patch leaks the above counts */
 			if (err) {
 				free_event(event);
 				return ERR_PTR(err);
@@ -6845,11 +6876,20 @@ SYSCALL_DEFINE5(perf_event_open,
 		 * one more event:
 		 * - that has cgroup constraint on event->cpu
 		 * - that may need work on context switch
+		 *
+		 * assumes inherited counters don't have flags set.
 		 */
-		atomic_inc(&per_cpu(perf_cgroup_events, event->cpu));
 		static_key_slow_inc(&perf_sched_events.key);
 	}
 
+	if (!event->parent) {
+		/* 
+		 * XXX the above err_alloc will dec below zero for
+		 * perf_branch_stack_events.
+		 */ 
+		__account_event(event);
+	}
+
 	/*
 	 * Special case software events and allow them to be part of
 	 * any hardware group.
@@ -7083,6 +7123,7 @@ void perf_pmu_migrate_context(struct pmu
 		perf_remove_from_context(event);
 		put_ctx(src_ctx);
 		list_add(&event->event_entry, &events);
+		__unaccount_event(event);
 	}
 	mutex_unlock(&src_ctx->mutex);
 
@@ -7091,6 +7132,8 @@ void perf_pmu_migrate_context(struct pmu
 	mutex_lock(&dst_ctx->mutex);
 	list_for_each_entry_safe(event, tmp, &events, event_entry) {
 		list_del(&event->event_entry);
+		event->cpu = dst_cpu; /* XXX perf_install_in_context already does this; do we really need __account_event() _before_ that? */
+		__account_event(event);
 		if (event->state >= PERF_EVENT_STATE_OFF)
 			event->state = PERF_EVENT_STATE_INACTIVE;
 		perf_install_in_context(dst_ctx, event, dst_cpu);

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