linux-kernel - [PATCH v7 04/28] ring-buffer: Add non-consuming read for ring-buffer remotes

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Message-ID: <20251003133825.2068970-5-vdonnefort@google.com>
Date: Fri,  3 Oct 2025 14:38:01 +0100
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, 
	aneesh.kumar@...nel.org, kernel-team@...roid.com, 
	linux-kernel@...r.kernel.org, Vincent Donnefort <vdonnefort@...gle.com>
Subject: [PATCH v7 04/28] ring-buffer: Add non-consuming read for ring-buffer remotes

Hopefully, the remote will only swap pages on the kernel instruction (via
the swap_reader_page() callback). This means we know at what point the
ring-buffer geometry has changed. It is therefore possible to rearrange
the kernel view of that ring-buffer to allow non-consuming read.

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

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 54f3f47b392b..eb556e691b5e 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -5347,10 +5347,51 @@ static bool rb_read_remote_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
 	return rb_num_of_entries(cpu_buffer);
 }
 
+static void rb_update_remote_head(struct ring_buffer_per_cpu *cpu_buffer)
+{
+	struct buffer_page *next, *orig;
+	int retry = 3;
+
+	orig = next = cpu_buffer->head_page;
+	rb_inc_page(&next);
+
+	/* Run after the writer */
+	while (cpu_buffer->head_page->page->time_stamp > next->page->time_stamp) {
+		rb_inc_page(&next);
+
+		rb_list_head_clear(cpu_buffer->head_page->list.prev);
+		rb_inc_page(&cpu_buffer->head_page);
+		rb_set_list_to_head(cpu_buffer->head_page->list.prev);
+
+		if (cpu_buffer->head_page == orig) {
+			if (WARN_ON_ONCE(!(--retry)))
+				return;
+		}
+	}
+
+	orig = cpu_buffer->commit_page = cpu_buffer->head_page;
+	retry = 3;
+
+	while (cpu_buffer->commit_page->page->time_stamp < next->page->time_stamp) {
+		rb_inc_page(&next);
+		rb_inc_page(&cpu_buffer->commit_page);
+
+		if (cpu_buffer->commit_page == orig) {
+			if (WARN_ON_ONCE(!(--retry)))
+				return;
+		}
+	}
+}
+
 static void rb_iter_reset(struct ring_buffer_iter *iter)
 {
 	struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
 
+	if (cpu_buffer->remote) {
+		rb_read_remote_meta_page(cpu_buffer);
+		rb_update_remote_head(cpu_buffer);
+	}
+
 	/* Iterator usage is expected to have record disabled */
 	iter->head_page = cpu_buffer->reader_page;
 	iter->head = cpu_buffer->reader_page->read;
@@ -5503,7 +5544,7 @@ rb_update_iter_read_stamp(struct ring_buffer_iter *iter,
 static struct buffer_page *
 __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
 {
-	struct buffer_page *new_reader, *prev_reader;
+	struct buffer_page *new_reader, *prev_reader, *prev_head, *new_head, *last;
 
 	if (!rb_read_remote_meta_page(cpu_buffer))
 		return NULL;
@@ -5527,10 +5568,32 @@ __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
 
 	WARN_ON_ONCE(prev_reader == new_reader);
 
-	cpu_buffer->reader_page->page = new_reader->page;
-	cpu_buffer->reader_page->id = new_reader->id;
-	cpu_buffer->reader_page->read = 0;
-	cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp;
+	prev_head = new_reader;  /* New reader was also the previous head */
+	new_head = prev_head;
+	rb_inc_page(&new_head);
+	last = prev_head;
+	rb_dec_page(&last);
+
+	/* Clear the old HEAD flag */
+	rb_list_head_clear(cpu_buffer->head_page->list.prev);
+
+	prev_reader->list.next = prev_head->list.next;
+	prev_reader->list.prev = prev_head->list.prev;
+
+	/* Swap prev_reader with new_reader */
+	last->list.next = &prev_reader->list;
+	new_head->list.prev = &prev_reader->list;
+
+	new_reader->list.prev = &new_reader->list;
+	new_reader->list.next = &new_head->list;
+
+	/* Reactivate the HEAD flag */
+	rb_set_list_to_head(&last->list);
+
+	cpu_buffer->head_page = new_head;
+	cpu_buffer->reader_page = new_reader;
+	cpu_buffer->pages = &new_head->list;
+	cpu_buffer->read_stamp = new_reader->page->time_stamp;
 	cpu_buffer->lost_events = cpu_buffer->meta_page->reader.lost_events;
 
 	return rb_page_size(cpu_buffer->reader_page) ? cpu_buffer->reader_page : NULL;
@@ -6114,7 +6177,7 @@ ring_buffer_read_start(struct trace_buffer *buffer, int cpu, gfp_t flags)
 	struct ring_buffer_per_cpu *cpu_buffer;
 	struct ring_buffer_iter *iter;
 
-	if (!cpumask_test_cpu(cpu, buffer->cpumask) || buffer->remote)
+	if (!cpumask_test_cpu(cpu, buffer->cpumask))
 		return NULL;
 
 	iter = kzalloc(sizeof(*iter), flags);
-- 
2.51.0.618.g983fd99d29-goog