[PATCH 12/12 v2] ring-buffer: Add cached pages when freeing reader page

[Steven Rostedt <rostedt@goodmis.org>]

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From: Steven Rostedt <srostedt@redhat.com>

When the pages are removed from the ring buffer for things like
splice they are freed with ring_buffer_free_read_page().
They are also allocated with ring_buffer_alloc_read_page().

Currently the ring buffer does not take advantage of this situation.
Every time the page is freed, the ring buffer simply frees it.
When a new page is needed, it allocates it. This means that reading
several pages with splice will cause a page to be freed and allocated
several times. This is simply a waste.

This patch adds a cache of the pages freed (16 max). This allows
the pages to be reused quickly without need to go back to the memory
pool.

Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/trace/ring_buffer.c |   41 +++++++++++++++++++++++++++++++++++------
 1 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 7f6059c..40667b2 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -157,6 +157,8 @@ static unsigned long ring_buffer_flags __read_mostly = RB_BUFFERS_ON;
 
 #define BUF_PAGE_HDR_SIZE offsetof(struct buffer_data_page, data)
 
+#define RB_MAX_FREE_PAGES	16
+
 /**
  * tracing_on - enable all tracing buffers
  *
@@ -325,7 +327,10 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data);
 #define RB_MISSED_STORED	(1 << 30)
 
 struct buffer_data_page {
-	u64		 time_stamp;	/* page time stamp */
+	union {
+		struct buffer_data_page	*next;	/* for free pages */
+		u64		 time_stamp;	/* page time stamp */
+	};
 	local_t		 commit;	/* write committed index */
 	unsigned char	 data[];	/* data of buffer page */
 };
@@ -472,6 +477,9 @@ struct ring_buffer {
 	atomic_t			record_disabled;
 	cpumask_var_t			cpumask;
 
+	struct buffer_data_page		*free_pages;
+	int				nr_free_pages;
+
 	struct lock_class_key		*reader_lock_key;
 
 	struct mutex			mutex;
@@ -1184,6 +1192,7 @@ EXPORT_SYMBOL_GPL(__ring_buffer_alloc);
 void
 ring_buffer_free(struct ring_buffer *buffer)
 {
+	struct buffer_data_page *bpage;
 	int cpu;
 
 	get_online_cpus();
@@ -1200,6 +1209,11 @@ ring_buffer_free(struct ring_buffer *buffer)
 	kfree(buffer->buffers);
 	free_cpumask_var(buffer->cpumask);
 
+	while (buffer->free_pages) {
+		bpage = buffer->free_pages;
+		buffer->free_pages = bpage->next;
+		free_page((unsigned long)bpage);
+	};
 	kfree(buffer);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_free);
@@ -3717,11 +3731,17 @@ void *ring_buffer_alloc_read_page(struct ring_buffer *buffer)
 	struct buffer_data_page *bpage;
 	unsigned long addr;
 
-	addr = __get_free_page(GFP_KERNEL);
-	if (!addr)
-		return NULL;
+	if (!buffer->free_pages) {
+		addr = __get_free_page(GFP_KERNEL);
+		if (!addr)
+			return NULL;
 
-	bpage = (void *)addr;
+		bpage = (void *)addr;
+	} else {
+		bpage = buffer->free_pages;
+		buffer->free_pages = bpage->next;
+		buffer->nr_free_pages--;
+	}
 
 	rb_init_page(bpage);
 
@@ -3738,7 +3758,16 @@ EXPORT_SYMBOL_GPL(ring_buffer_alloc_read_page);
  */
 void ring_buffer_free_read_page(struct ring_buffer *buffer, void *data)
 {
-	free_page((unsigned long)data);
+	struct buffer_data_page *bpage = data;
+
+	if (buffer->nr_free_pages >= RB_MAX_FREE_PAGES) {
+		free_page((unsigned long)data);
+		return;
+	}
+
+	bpage->next = buffer->free_pages;
+	buffer->free_pages = bpage;
+	buffer->nr_free_pages++;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_free_read_page);
 
-- 
1.7.0