Re: [PATCH v21 8/9] ring-buffer: Show persistent buffer dropped events in trace file

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 22 May 2026 13:09:05 -0400
Steven Rostedt <rostedt@kernel.org> wrote:

> @@ -7187,6 +7190,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  		local_set(&reader->entries, 0);
>  		reader->read = 0;
>  		data_page->data = dpage;
> +		if (!missed_events && rb_data_page_commit(dpage) & RB_MISSED_EVENTS)
> +			missed_events = -1;
>  
>  		/*
>  		 * Use the real_end for the data size,
> @@ -7204,10 +7209,12 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  	 * Set a flag in the commit field if we lost events
>  	 */
>  	if (missed_events) {
> -		/* If there is room at the end of the page to save the
> +		/*
> +		 * If there is room at the end of the page to save the
>  		 * missed events, then record it there.
>  		 */
> -		if (buffer->subbuf_size - commit >= sizeof(missed_events)) {
> +		if (missed_events > 0 &&
> +		    buffer->subbuf_size - commit >= sizeof(missed_events)) {
>  			memcpy(&dpage->data[commit], &missed_events,
>  			       sizeof(missed_events));
>  			local_add(RB_MISSED_STORED, &dpage->commit);

After this line, we "add" RB_MISSED_EVENTS instead of set.
In this case, does it clear the RB_MISSED_EVENTS bit because
it already sets RB_MISSED_EVENTS.

			commit += sizeof(missed_events);
		}
		local_add(RB_MISSED_EVENTS, &bpage->commit);
                      ^^^ here.


Thanks,


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>