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

[Steven Rostedt]

On Wed, 27 May 2026 12:47:21 +0900
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote:

> Yeah, for the persistent ring buffer, it does not happen.
> But there seems RB_MISSED_EVENTS bit can be cleared in
> "else" path (after applying 1-8 patches)?

Note, *only* the persistent ring buffer adds RB_MISSED_EVENTS to the pages
in the write buffer. In the normal buffer, these bits are only set by this
function. That is, they would not be set from the swap of pages.

> 
> ----------
> 	if (read || (len < (commit - read)) ||
> 	    cpu_buffer->reader_page == cpu_buffer->commit_page ||
> 	    force_memcpy) { // <-- persistent ring buffer sets force_memcpy = true.
> [...]
> 	} else {
> 		/* update the entry counter */
> [...]
> 		if (!missed_events && rb_data_page_commit(dpage) & RB_MISSED_EVENTS)
> 			missed_events = -1;	
> 			//^-- we check RB_MISSED_EVENTS bit on @dpage->commit and set missed_events = -1.
> 
> 		/*
> 		 * Use the real_end for the data size,
> 		 * This gives us a chance to store the lost events
> 		 * on the page.
> 		 */
> 		if (reader->real_end)
> 			local_set(&dpage->commit, reader->real_end);
>                         // ^- only if @reader->real_end, RB_MISSED_EVENTS bit is dropped.

Because this isn't a persistent ring buffer (if it was, as you noted,
force_memcpy would be true and we wouldn't enter the else path), the
RB_MISSED_EVENTS bit in the commit would never be set here. It is *only* set
by the verifier of the persistent ring buffer logic.

> 	}
> 
> 	cpu_buffer->lost_events = 0;
> 
> 	commit = rb_data_page_commit(dpage);
> 	/*
> 	 * 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
> 		 * missed events, then record it there.
> 		 */
> 		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);
> 			commit += sizeof(missed_events);
> 		}
> 		local_add(RB_MISSED_EVENTS, &dpage->commit); // <-- @dpage->commit is updated.
> 	}

And this is the first place it would get set.

But yeah, it is very confusing and needs better comments.

Thanks,

-- Steve