[PATCH v14 04/30] ring-buffer: Add non-consuming read for ring-buffer remotes

Vincent Donnefort
2026-03-09T16:26:05Z

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.

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 88218377fa29..fb39dd43e00b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -5388,10 +5388,51 @@ 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;
@@ -5544,7 +5585,7 @@ 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;
@@ -5568,10 +5609,32 @@ __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;
@@ -6155,7 +6218,7 @@ 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_obj(*iter, flags);
--
2.53.0.473.g4a7958ca14-goog