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Diffstat (limited to 'kernel/events/core.c')
-rw-r--r--kernel/events/core.c70
1 files changed, 48 insertions, 22 deletions
diff --git a/kernel/events/core.c b/kernel/events/core.c
index ab15509fab8c..72ce7d63e561 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2249,7 +2249,7 @@ static int __perf_install_in_context(void *info)
struct perf_event_context *ctx = event->ctx;
struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
struct perf_event_context *task_ctx = cpuctx->task_ctx;
- bool activate = true;
+ bool reprogram = true;
int ret = 0;
raw_spin_lock(&cpuctx->ctx.lock);
@@ -2257,27 +2257,26 @@ static int __perf_install_in_context(void *info)
raw_spin_lock(&ctx->lock);
task_ctx = ctx;
- /* If we're on the wrong CPU, try again */
- if (task_cpu(ctx->task) != smp_processor_id()) {
- ret = -ESRCH;
- goto unlock;
- }
+ reprogram = (ctx->task == current);
/*
- * If we're on the right CPU, see if the task we target is
- * current, if not we don't have to activate the ctx, a future
- * context switch will do that for us.
+ * If the task is running, it must be running on this CPU,
+ * otherwise we cannot reprogram things.
+ *
+ * If its not running, we don't care, ctx->lock will
+ * serialize against it becoming runnable.
*/
- if (ctx->task != current)
- activate = false;
- else
- WARN_ON_ONCE(cpuctx->task_ctx && cpuctx->task_ctx != ctx);
+ if (task_curr(ctx->task) && !reprogram) {
+ ret = -ESRCH;
+ goto unlock;
+ }
+ WARN_ON_ONCE(reprogram && cpuctx->task_ctx && cpuctx->task_ctx != ctx);
} else if (task_ctx) {
raw_spin_lock(&task_ctx->lock);
}
- if (activate) {
+ if (reprogram) {
ctx_sched_out(ctx, cpuctx, EVENT_TIME);
add_event_to_ctx(event, ctx);
ctx_resched(cpuctx, task_ctx);
@@ -2328,13 +2327,36 @@ perf_install_in_context(struct perf_event_context *ctx,
/*
* Installing events is tricky because we cannot rely on ctx->is_active
* to be set in case this is the nr_events 0 -> 1 transition.
+ *
+ * Instead we use task_curr(), which tells us if the task is running.
+ * However, since we use task_curr() outside of rq::lock, we can race
+ * against the actual state. This means the result can be wrong.
+ *
+ * If we get a false positive, we retry, this is harmless.
+ *
+ * If we get a false negative, things are complicated. If we are after
+ * perf_event_context_sched_in() ctx::lock will serialize us, and the
+ * value must be correct. If we're before, it doesn't matter since
+ * perf_event_context_sched_in() will program the counter.
+ *
+ * However, this hinges on the remote context switch having observed
+ * our task->perf_event_ctxp[] store, such that it will in fact take
+ * ctx::lock in perf_event_context_sched_in().
+ *
+ * We do this by task_function_call(), if the IPI fails to hit the task
+ * we know any future context switch of task must see the
+ * perf_event_ctpx[] store.
*/
-again:
+
/*
- * Cannot use task_function_call() because we need to run on the task's
- * CPU regardless of whether its current or not.
+ * This smp_mb() orders the task->perf_event_ctxp[] store with the
+ * task_cpu() load, such that if the IPI then does not find the task
+ * running, a future context switch of that task must observe the
+ * store.
*/
- if (!cpu_function_call(task_cpu(task), __perf_install_in_context, event))
+ smp_mb();
+again:
+ if (!task_function_call(task, __perf_install_in_context, event))
return;
raw_spin_lock_irq(&ctx->lock);
@@ -2348,12 +2370,16 @@ again:
raw_spin_unlock_irq(&ctx->lock);
return;
}
- raw_spin_unlock_irq(&ctx->lock);
/*
- * Since !ctx->is_active doesn't mean anything, we must IPI
- * unconditionally.
+ * If the task is not running, ctx->lock will avoid it becoming so,
+ * thus we can safely install the event.
*/
- goto again;
+ if (task_curr(task)) {
+ raw_spin_unlock_irq(&ctx->lock);
+ goto again;
+ }
+ add_event_to_ctx(event, ctx);
+ raw_spin_unlock_irq(&ctx->lock);
}
/*