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Commit 5322ea58 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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perf/ring-buffer: Use regular variables for nesting 



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: default avatarAlexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: acme@kernel.org
Cc: mark.rutland@arm.com
Cc: namhyung@kernel.org
Cc: yabinc@google.com
Link: http://lkml.kernel.org/r/20190517115418.481392777@infradead.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 4d839dd9

kernel/events/internal.h

+2 −2
Original line number Diff line number Diff line
@@ -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;

kernel/events/ring_buffer.c

+26 −15
Original line number Diff line number Diff line
@@ -38,7 +38,12 @@ static void perf_output_get_handle(struct perf_output_handle *handle) 
	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(struct perf_output_handle *handle) 
{

	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:

	/*
@@ -64,15 +80,6 @@ static void perf_output_put_handle(struct perf_output_handle *handle) 
	 * load above to be stale.

	 */



	/*

	 * 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:

	 *
@@ -108,7 +115,7 @@ static void perf_output_put_handle(struct perf_output_handle *handle) 
	 * 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(struct perf_output_handle *handle) 
	 */

	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_output_handle *handle, 
	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_output_handle *handle, 
	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_output_handle *handle, 
		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_output_handle *handle, unsigned long size) 


	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);