[PATCH v5 1/4] ring-buffer: add ring_buffer_memory_size()

[Xiang Gao]

Add a helper to report the page memory consumed by a ring buffer for a
given CPU. It accounts for the data sub-buffers, the reader page, the
cached read page, and the mmap metadata page. Unlike
ring_buffer_capacity(), which reports the usable data capacity, this
reports the full pages allocated to the buffer.

The function is generic and returns a size for any buffer type, so
callers that need the memory footprint of remote or persistent buffers
can use it as well.

Signed-off-by: Xiang Gao <gaoxiang17@xxxxxxxxxx>
---
 include/linux/ring_buffer.h |  1 +
 kernel/trace/ring_buffer.c  | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 649b2276b557..e83e94623e76 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -174,6 +174,7 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter);
 bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter);
 
 unsigned long ring_buffer_capacity(struct trace_buffer *buffer, int cpu);
+unsigned long ring_buffer_memory_size(struct trace_buffer *buffer, int cpu);
 unsigned long ring_buffer_max_event_size(struct trace_buffer *buffer);
 
 void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu);
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 1fd28a72e6ee..0396ec671998 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -6566,6 +6566,43 @@ unsigned long ring_buffer_capacity(struct trace_buffer *buffer, int cpu)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_capacity);
 
+/**
+ * ring_buffer_memory_size - return the memory used by the buffer (in bytes)
+ * @buffer: The ring buffer.
+ * @cpu: The CPU to get ring buffer memory from.
+ *
+ * Returns the page memory consumed by @cpu, including the data sub-buffers,
+ * the reader page, the cached read page, and the mmap metadata page. Unlike
+ * ring_buffer_capacity(), which reports the usable data capacity, this
+ * accounts for the full pages allocated to the buffer.
+ */
+unsigned long ring_buffer_memory_size(struct trace_buffer *buffer, int cpu)
+{
+	struct ring_buffer_per_cpu *cpu_buffer;
+	unsigned long subbuf_size;
+	unsigned long size;
+
+	if (!cpumask_test_cpu(cpu, buffer->cpumask))
+		return 0;
+
+	cpu_buffer = buffer->buffers[cpu];
+	subbuf_size = PAGE_SIZE << READ_ONCE(buffer->subbuf_order);
+
+	/* Data sub-buffers plus the reader page. */
+	size = (READ_ONCE(cpu_buffer->nr_pages) + 1) * subbuf_size;
+
+	/* The cached read page, if present, is a full sub-buffer page. */
+	if (READ_ONCE(cpu_buffer->free_page.data))
+		size += subbuf_size;
+
+	/* The mmap metadata page is a single system page. */
+	if (READ_ONCE(cpu_buffer->meta_page))
+		size += PAGE_SIZE;
+
+	return size;
+}
+EXPORT_SYMBOL_GPL(ring_buffer_memory_size);
+
 /**
  * ring_buffer_max_event_size - return the max data size of an event
  * @buffer: The ring buffer.
-- 
2.34.1