[PATCH 4/4] perf/ring-buffer: Use regular variables for nesting

[Peter Zijlstra]

While the IRQ/NMI will nest, the nest-count will be invariant over the
actual exception, since it will decrement equal to increment.

This means we can -- carefully -- use a regular variable since the
typical LOAD-STORE race doesn't exist (similar to preempt_count).

This optimizes the ring-buffer for all LOAD-STORE architectures, since
they need to use atomic ops to implement local_t.

Suggested-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/events/internal.h    |    4 ++--
 kernel/events/ring_buffer.c |   41 ++++++++++++++++++++++++++---------------
 2 files changed, 28 insertions(+), 17 deletions(-)

--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -24,7 +24,7 @@ struct ring_buffer {
 	atomic_t			poll;		/* POLL_ for wakeups */
 
 	local_t				head;		/* write position    */
-	local_t				nest;		/* nested writers    */
+	unsigned int			nest;		/* nested writers    */
 	local_t				events;		/* event limit       */
 	local_t				wakeup;		/* wakeup stamp      */
 	local_t				lost;		/* nr records lost   */
@@ -41,7 +41,7 @@ struct ring_buffer {
 
 	/* AUX area */
 	long				aux_head;
-	local_t				aux_nest;
+	unsigned int			aux_nest;
 	long				aux_wakeup;	/* last aux_watermark boundary crossed by aux_head */
 	unsigned long			aux_pgoff;
 	int				aux_nr_pages;
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -38,7 +38,12 @@ static void perf_output_get_handle(struc
 	struct ring_buffer *rb = handle->rb;
 
 	preempt_disable();
-	local_inc(&rb->nest);
+
+	/*
+	 * Avoid an explicit LOAD/STORE such that architectures with memops
+	 * can use them.
+	 */
+	(*(volatile unsigned int *)&rb->nest)++;
 	handle->wakeup = local_read(&rb->wakeup);
 }
 
@@ -46,6 +51,17 @@ static void perf_output_put_handle(struc
 {
 	struct ring_buffer *rb = handle->rb;
 	unsigned long head;
+	unsigned int nest;
+
+	/*
+	 * If this isn't the outermost nesting, we don't have to update
+	 * @rb->user_page->data_head.
+	 */
+	nest = READ_ONCE(rb->nest);
+	if (nest > 1) {
+		WRITE_ONCE(rb->nest, nest - 1);
+		goto out;
+	}
 
 again:
 	/*
@@ -65,15 +81,6 @@ static void perf_output_put_handle(struc
 	 */
 
 	/*
-	 * If this isn't the outermost nesting, we don't have to update
-	 * @rb->user_page->data_head.
-	 */
-	if (local_read(&rb->nest) > 1) {
-		local_dec(&rb->nest);
-		goto out;
-	}
-
-	/*
 	 * Since the mmap() consumer (userspace) can run on a different CPU:
 	 *
 	 *   kernel				user
@@ -108,7 +115,7 @@ static void perf_output_put_handle(struc
 	 * write will (temporarily) publish a stale value.
 	 */
 	barrier();
-	local_set(&rb->nest, 0);
+	WRITE_ONCE(rb->nest, 0);
 
 	/*
 	 * Ensure we decrement @rb->nest before we validate the @rb->head.
@@ -116,7 +123,7 @@ static void perf_output_put_handle(struc
 	 */
 	barrier();
 	if (unlikely(head != local_read(&rb->head))) {
-		local_inc(&rb->nest);
+		WRITE_ONCE(rb->nest, 1);
 		goto again;
 	}
 
@@ -355,6 +362,7 @@ void *perf_aux_output_begin(struct perf_
 	struct perf_event *output_event = event;
 	unsigned long aux_head, aux_tail;
 	struct ring_buffer *rb;
+	unsigned int nest;
 
 	if (output_event->parent)
 		output_event = output_event->parent;
@@ -385,13 +393,16 @@ void *perf_aux_output_begin(struct perf_
 	if (!refcount_inc_not_zero(&rb->aux_refcount))
 		goto err;
 
+	nest = READ_ONCE(rb->aux_nest);
 	/*
 	 * Nesting is not supported for AUX area, make sure nested
 	 * writers are caught early
 	 */
-	if (WARN_ON_ONCE(local_xchg(&rb->aux_nest, 1)))
+	if (WARN_ON_ONCE(nest))
 		goto err_put;
 
+	WRITE_ONCE(rb->aux_nest, nest + 1);
+
 	aux_head = rb->aux_head;
 
 	handle->rb = rb;
@@ -419,7 +430,7 @@ void *perf_aux_output_begin(struct perf_
 		if (!handle->size) { /* A, matches D */
 			event->pending_disable = smp_processor_id();
 			perf_output_wakeup(handle);
-			local_set(&rb->aux_nest, 0);
+			WRITE_ONCE(rb->aux_nest, 0);
 			goto err_put;
 		}
 	}
@@ -508,7 +519,7 @@ void perf_aux_output_end(struct perf_out
 
 	handle->event = NULL;
 
-	local_set(&rb->aux_nest, 0);
+	WRITE_ONCE(rb->aux_nest, 0);
 	/* can't be last */
 	rb_free_aux(rb);
 	ring_buffer_put(rb);