[PATCH V2 1/1] perf: event preserve and create across cpu hotplug

[Mukesh Ojha]

Perf framework doesn't allow preserving CPU events across
CPU hotplugs. The events are scheduled out as and when the
CPU walks offline. Moreover, the framework also doesn't
allow the clients to create events on an offline CPU. As
a result, the clients have to keep on monitoring the CPU
state until it comes back online.

Therefore, introducing the perf framework to support creation
and preserving of (CPU) events for offline CPUs. Through
this, the CPU's online state would be transparent to the
client and it not have to worry about monitoring the CPU's
state. Success would be returned to the client even while
creating the event on an offline CPU. If during the lifetime
of the event the CPU walks offline, the event would be
preserved and would continue to count as soon as (and if) the
CPU comes back online.

Co-authored-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxxxxxx>
Signed-off-by: Mukesh Ojha <mojha@xxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
---
Change in V2:

As per long back discussion happened at
https://lkml.org/lkml/2018/2/15/1324

Peter.Z. has suggested to do thing in different way and shared
patch as well. This patch fixes the issue seen while trying
to achieve the purpose.

Fixed issue on top of Peter's patch:
===================================
1. Added a NULL check on task to avoid crash in __perf_install_in_context.

2. while trying to add event to context when cpu is offline.
   Inside add_event_to_ctx() to make consistent state machine while hotplug.

-event->state += PERF_EVENT_STATE_HOTPLUG_OFFSET;
+event->state = PERF_EVENT_STATE_HOTPLUG_OFFSET;

3. In event_function_call(), added a label 'remove_event_from_ctx' to 
   delete events from context list while cpu is offline.


 include/linux/perf_event.h |   1 +
 kernel/events/core.c       | 119 ++++++++++++++++++++++++++++-----------------
 2 files changed, 76 insertions(+), 44 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index e47ef76..31a3a5d 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -510,6 +510,7 @@ enum perf_event_state {
 	PERF_EVENT_STATE_OFF		= -1,
 	PERF_EVENT_STATE_INACTIVE	=  0,
 	PERF_EVENT_STATE_ACTIVE		=  1,
+	PERF_EVENT_STATE_HOTPLUG_OFFSET	= -32,
 };
 
 struct file;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 72d06e30..fbfffca 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -264,17 +264,18 @@ static void event_function_call(struct perf_event *event, event_f func, void *da
 		lockdep_assert_held(&ctx->mutex);
 	}
 
-	if (!task) {
-		cpu_function_call(event->cpu, event_function, &efs);
-		return;
-	}
-
 	if (task == TASK_TOMBSTONE)
 		return;
 
 again:
-	if (!task_function_call(task, event_function, &efs))
-		return;
+	if (task) {
+		if (!task_function_call(task, event_function, &efs))
+			return;
+	} else {
+		if (!cpu_function_call(event->cpu, event_function, &efs))
+			return;
+	}
+
 
 	raw_spin_lock_irq(&ctx->lock);
 	/*
@@ -286,10 +287,16 @@ static void event_function_call(struct perf_event *event, event_f func, void *da
 		raw_spin_unlock_irq(&ctx->lock);
 		return;
 	}
+
+	if (!task)
+		goto remove_event_from_ctx;
+
 	if (ctx->is_active) {
 		raw_spin_unlock_irq(&ctx->lock);
 		goto again;
 	}
+
+remove_event_from_ctx:
 	func(event, NULL, ctx, data);
 	raw_spin_unlock_irq(&ctx->lock);
 }
@@ -2309,7 +2316,7 @@ static void perf_set_shadow_time(struct perf_event *event,
 	struct perf_event *event, *partial_group = NULL;
 	struct pmu *pmu = ctx->pmu;
 
-	if (group_event->state == PERF_EVENT_STATE_OFF)
+	if (group_event->state <= PERF_EVENT_STATE_OFF)
 		return 0;
 
 	pmu->start_txn(pmu, PERF_PMU_TXN_ADD);
@@ -2388,6 +2395,14 @@ static int group_can_go_on(struct perf_event *event,
 static void add_event_to_ctx(struct perf_event *event,
 			       struct perf_event_context *ctx)
 {
+	if (!ctx->task) {
+		struct perf_cpu_context *cpuctx =
+			container_of(ctx, struct perf_cpu_context, ctx);
+
+		if (!cpuctx->online)
+			event->state = PERF_EVENT_STATE_HOTPLUG_OFFSET;
+	}
+
 	list_add_event(event, ctx);
 	perf_group_attach(event);
 }
@@ -2565,11 +2580,6 @@ static int  __perf_install_in_context(void *info)
 	 */
 	smp_store_release(&event->ctx, ctx);
 
-	if (!task) {
-		cpu_function_call(cpu, __perf_install_in_context, event);
-		return;
-	}
-
 	/*
 	 * Should not happen, we validate the ctx is still alive before calling.
 	 */
@@ -2608,8 +2618,14 @@ static int  __perf_install_in_context(void *info)
 	 */
 	smp_mb();
 again:
-	if (!task_function_call(task, __perf_install_in_context, event))
-		return;
+
+	if (task) {
+		if (!task_function_call(task, __perf_install_in_context, event))
+			return;
+	} else {
+		if (!cpu_function_call(cpu, __perf_install_in_context, event))
+			return;
+	}
 
 	raw_spin_lock_irq(&ctx->lock);
 	task = ctx->task;
@@ -2626,7 +2642,7 @@ static int  __perf_install_in_context(void *info)
 	 * If the task is not running, ctx->lock will avoid it becoming so,
 	 * thus we can safely install the event.
 	 */
-	if (task_curr(task)) {
+	if (task && task_curr(task)) {
 		raw_spin_unlock_irq(&ctx->lock);
 		goto again;
 	}
@@ -10967,16 +10983,7 @@ static int perf_event_set_clock(struct perf_event *event, clockid_t clk_id)
 	}
 
 	if (!task) {
-		/*
-		 * Check if the @cpu we're creating an event for is online.
-		 *
-		 * We use the perf_cpu_context::ctx::mutex to serialize against
-		 * the hotplug notifiers. See perf_event_{init,exit}_cpu().
-		 */
-		struct perf_cpu_context *cpuctx =
-			container_of(ctx, struct perf_cpu_context, ctx);
-
-		if (!cpuctx->online) {
+		if (!cpu_possible(cpu)) {
 			err = -ENODEV;
 			goto err_locked;
 		}
@@ -11158,15 +11165,7 @@ struct perf_event *
 	}
 
 	if (!task) {
-		/*
-		 * Check if the @cpu we're creating an event for is online.
-		 *
-		 * We use the perf_cpu_context::ctx::mutex to serialize against
-		 * the hotplug notifiers. See perf_event_{init,exit}_cpu().
-		 */
-		struct perf_cpu_context *cpuctx =
-			container_of(ctx, struct perf_cpu_context, ctx);
-		if (!cpuctx->online) {
+		if (!cpu_possible(cpu)) {
 			err = -ENODEV;
 			goto err_unlock;
 		}
@@ -11894,17 +11893,48 @@ void perf_swevent_init_cpu(unsigned int cpu)
 }
 
 #if defined CONFIG_HOTPLUG_CPU || defined CONFIG_KEXEC_CORE
+static void __perf_event_init_cpu_context(void *__info)
+{
+	struct perf_cpu_context *cpuctx = __info;
+	struct perf_event_context *ctx = &cpuctx->ctx;
+	struct perf_event_context *task_ctx = cpuctx->task_ctx;
+	struct perf_event *event;
+
+	perf_ctx_lock(cpuctx, task_ctx);
+	ctx_sched_out(ctx, cpuctx, EVENT_ALL);
+	if (task_ctx)
+		ctx_sched_out(task_ctx, cpuctx, EVENT_ALL);
+
+	list_for_each_entry_rcu(event, &ctx->event_list, event_entry)
+		perf_event_set_state(event, event->state - PERF_EVENT_STATE_HOTPLUG_OFFSET);
+
+	perf_event_sched_in(cpuctx, task_ctx, current);
+	perf_ctx_unlock(cpuctx, task_ctx);
+}
+
+static void _perf_event_init_cpu_context(int cpu, struct perf_cpu_context *cpuctx)
+{
+	smp_call_function_single(cpu, __perf_event_init_cpu_context, cpuctx, 1);
+}
+
 static void __perf_event_exit_context(void *__info)
 {
-	struct perf_event_context *ctx = __info;
-	struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
+	struct perf_cpu_context *cpuctx = __info;
+	struct perf_event_context *ctx = &cpuctx->ctx;
+	struct perf_event_context *task_ctx = cpuctx->task_ctx;
 	struct perf_event *event;
 
-	raw_spin_lock(&ctx->lock);
-	ctx_sched_out(ctx, cpuctx, EVENT_TIME);
-	list_for_each_entry(event, &ctx->event_list, event_entry)
-		__perf_remove_from_context(event, cpuctx, ctx, (void *)DETACH_GROUP);
-	raw_spin_unlock(&ctx->lock);
+	perf_ctx_lock(cpuctx, task_ctx);
+	ctx_sched_out(ctx, cpuctx, EVENT_ALL);
+	if (task_ctx)
+		ctx_sched_out(task_ctx, cpuctx, EVENT_ALL);
+
+	list_for_each_entry_rcu(event, &ctx->event_list, event_entry)
+		perf_event_set_state(event,
+			event->state + PERF_EVENT_STATE_HOTPLUG_OFFSET);
+
+	perf_event_sched_in(cpuctx, task_ctx, current);
+	perf_ctx_unlock(cpuctx, task_ctx);
 }
 
 static void perf_event_exit_cpu_context(int cpu)
@@ -11919,7 +11949,7 @@ static void perf_event_exit_cpu_context(int cpu)
 		ctx = &cpuctx->ctx;
 
 		mutex_lock(&ctx->mutex);
-		smp_call_function_single(cpu, __perf_event_exit_context, ctx, 1);
+		smp_call_function_single(cpu, __perf_event_exit_context, cpuctx, 1);
 		cpuctx->online = 0;
 		mutex_unlock(&ctx->mutex);
 	}
@@ -11927,7 +11957,7 @@ static void perf_event_exit_cpu_context(int cpu)
 	mutex_unlock(&pmus_lock);
 }
 #else
-
+static void _perf_event_init_cpu_context(int cpu, struct perf_cpu_context *cpuctx) { }
 static void perf_event_exit_cpu_context(int cpu) { }
 
 #endif
@@ -11948,6 +11978,7 @@ int perf_event_init_cpu(unsigned int cpu)
 
 		mutex_lock(&ctx->mutex);
 		cpuctx->online = 1;
+		_perf_event_init_cpu_context(cpu, cpuctx);
 		mutex_unlock(&ctx->mutex);
 	}
 	mutex_unlock(&pmus_lock);
-- 
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