linux-kernel - [PATCH v2 01/12] ring-buffer: Check for empty ring-buffer with rb_num_of

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Message-ID: <20250108114536.627715-2-vdonnefort@google.com>
Date: Wed,  8 Jan 2025 11:45:25 +0000
From: Vincent Donnefort <vdonnefort@...gle.com>
To: rostedt@...dmis.org, mhiramat@...nel.org, mathieu.desnoyers@...icios.com, 
	linux-trace-kernel@...r.kernel.org, maz@...nel.org, oliver.upton@...ux.dev, 
	joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com
Cc: kvmarm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, 
	jstultz@...gle.com, qperret@...gle.com, will@...nel.org, 
	kernel-team@...roid.com, linux-kernel@...r.kernel.org, 
	Vincent Donnefort <vdonnefort@...gle.com>
Subject: [PATCH v2 01/12] ring-buffer: Check for empty ring-buffer with rb_num_of_entries()

Currently there are two ways of identifying an empty ring-buffer. One
relying on the current status of the commit / reader page
(rb_per_cpu_empty()) and the other on the write and read counters
(rb_num_of_entries() used in rb_get_reader_page()).

with rb_num_of_entries(). This intends to ease later
introduction of ring-buffer writers which are out of the kernel control
and with whom, the only information available is through the meta-page
counters.

Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 60210fb5b211..479bbbcbf61b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4682,40 +4682,22 @@ int ring_buffer_write(struct trace_buffer *buffer,
 }
 EXPORT_SYMBOL_GPL(ring_buffer_write);
 
-static bool rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer)
+/*
+ * The total entries in the ring buffer is the running counter
+ * of entries entered into the ring buffer, minus the sum of
+ * the entries read from the ring buffer and the number of
+ * entries that were overwritten.
+ */
+static inline unsigned long
+rb_num_of_entries(struct ring_buffer_per_cpu *cpu_buffer)
 {
-	struct buffer_page *reader = cpu_buffer->reader_page;
-	struct buffer_page *head = rb_set_head_page(cpu_buffer);
-	struct buffer_page *commit = cpu_buffer->commit_page;
-
-	/* In case of error, head will be NULL */
-	if (unlikely(!head))
-		return true;
-
-	/* Reader should exhaust content in reader page */
-	if (reader->read != rb_page_size(reader))
-		return false;
-
-	/*
-	 * If writers are committing on the reader page, knowing all
-	 * committed content has been read, the ring buffer is empty.
-	 */
-	if (commit == reader)
-		return true;
-
-	/*
-	 * If writers are committing on a page other than reader page
-	 * and head page, there should always be content to read.
-	 */
-	if (commit != head)
-		return false;
+	return local_read(&cpu_buffer->entries) -
+		(local_read(&cpu_buffer->overrun) + cpu_buffer->read);
+}
 
-	/*
-	 * Writers are committing on the head page, we just need
-	 * to care about there're committed data, and the reader will
-	 * swap reader page with head page when it is to read data.
-	 */
-	return rb_page_commit(commit) == 0;
+static bool rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer)
+{
+	return !rb_num_of_entries(cpu_buffer);
 }
 
 /**
@@ -4861,19 +4843,6 @@ void ring_buffer_record_enable_cpu(struct trace_buffer *buffer, int cpu)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_record_enable_cpu);
 
-/*
- * The total entries in the ring buffer is the running counter
- * of entries entered into the ring buffer, minus the sum of
- * the entries read from the ring buffer and the number of
- * entries that were overwritten.
- */
-static inline unsigned long
-rb_num_of_entries(struct ring_buffer_per_cpu *cpu_buffer)
-{
-	return local_read(&cpu_buffer->entries) -
-		(local_read(&cpu_buffer->overrun) + cpu_buffer->read);
-}
-
 /**
  * ring_buffer_oldest_event_ts - get the oldest event timestamp from the buffer
  * @buffer: The ring buffer
-- 
2.47.1.613.gc27f4b7a9f-goog