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Commit 2667de81 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

perf_counter: Allow for a wakeup watermark 



Currently we wake the mmap() consumer once every PAGE_SIZE of data
and/or once event wakeup_events when specified.

For high speed sampling this results in too many wakeups wrt. the
buffer size, hence change this.

We move the default wakeup limit to 1/4-th the buffer size, and
provide for means to manually specify this limit.

Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 850bc73f

include/linux/perf_counter.h

+8 −2
Original line number Diff line number Diff line
@@ -199,10 +199,14 @@ struct perf_counter_attr { 
				inherit_stat   :  1, /* per task counts       */

				enable_on_exec :  1, /* next exec enables     */

				task           :  1, /* trace fork/exit       */

				watermark      :  1, /* wakeup_watermark      */



				__reserved_1   : 50;

				__reserved_1   : 49;



	union {

		__u32		wakeup_events;	  /* wakeup every n events */

		__u32		wakeup_watermark; /* bytes before wakeup   */

	};

	__u32			__reserved_2;



	__u64			__reserved_3;
@@ -521,6 +525,8 @@ struct perf_mmap_data { 
	atomic_t			wakeup;		/* needs a wakeup    */

	atomic_t			lost;		/* nr records lost   */



	long				watermark;	/* wakeup watermark  */



	struct perf_counter_mmap_page   *user_page;

	void				*data_pages[0];

};

kernel/perf_counter.c

+19 −13
Original line number Diff line number Diff line
@@ -2176,6 +2176,13 @@ static int perf_mmap_data_alloc(struct perf_counter *counter, int nr_pages) 
	data->nr_pages = nr_pages;

	atomic_set(&data->lock, -1);



	if (counter->attr.watermark) {

		data->watermark = min_t(long, PAGE_SIZE * nr_pages,

				      counter->attr.wakeup_watermark);

	}

	if (!data->watermark)

		data->watermark = max(PAGE_SIZE, PAGE_SIZE * nr_pages / 4);



	rcu_assign_pointer(counter->data, data);



	return 0;
@@ -2517,23 +2524,15 @@ struct perf_output_handle { 
	unsigned long		flags;

};



static bool perf_output_space(struct perf_mmap_data *data,

			      unsigned int offset, unsigned int head)

static bool perf_output_space(struct perf_mmap_data *data, unsigned long tail,

			      unsigned long offset, unsigned long head)

{

	unsigned long tail;

	unsigned long mask;



	if (!data->writable)

		return true;



	mask = (data->nr_pages << PAGE_SHIFT) - 1;

	/*

	 * Userspace could choose to issue a mb() before updating the tail

	 * pointer. So that all reads will be completed before the write is

	 * issued.

	 */

	tail = ACCESS_ONCE(data->user_page->data_tail);

	smp_rmb();



	offset = (offset - tail) & mask;

	head   = (head   - tail) & mask;
@@ -2679,7 +2678,7 @@ static int perf_output_begin(struct perf_output_handle *handle, 
{

	struct perf_counter *output_counter;

	struct perf_mmap_data *data;

	unsigned int offset, head;

	unsigned long tail, offset, head;

	int have_lost;

	struct {

		struct perf_event_header header;
@@ -2717,16 +2716,23 @@ static int perf_output_begin(struct perf_output_handle *handle, 
	perf_output_lock(handle);



	do {

		/*

		 * Userspace could choose to issue a mb() before updating the

		 * tail pointer. So that all reads will be completed before the

		 * write is issued.

		 */

		tail = ACCESS_ONCE(data->user_page->data_tail);

		smp_rmb();

		offset = head = atomic_long_read(&data->head);

		head += size;

		if (unlikely(!perf_output_space(data, offset, head)))

		if (unlikely(!perf_output_space(data, tail, offset, head)))

			goto fail;

	} while (atomic_long_cmpxchg(&data->head, offset, head) != offset);



	handle->offset	= offset;

	handle->head	= head;



	if ((offset >> PAGE_SHIFT) != (head >> PAGE_SHIFT))

	if (head - tail > data->watermark)

		atomic_set(&data->wakeup, 1);



	if (have_lost) {