From: "Steven Rostedt (Google)" rostedt@goodmis.org

The "buffer_percent" logic that is used by the ring buffer splice code to only wake up the tasks when there's no data after the buffer is filled to the percentage of the "buffer_percent" file is dependent on three variables that determine the amount of data that is in the ring buffer:

1) pages_read - incremented whenever a new sub-buffer is consumed 2) pages_lost - incremented every time a writer overwrites a sub-buffer 3) pages_touched - incremented when a write goes to a new sub-buffer

The percentage is the calculation of:

(pages_touched - (pages_lost + pages_read)) / nr_pages

Basically, the amount of data is the total number of sub-bufs that have been touched, minus the number of sub-bufs lost and sub-bufs consumed. This is divided by the total count to give the buffer percentage. When the percentage is greater than the value in the "buffer_percent" file, it wakes up splice readers waiting for that amount.

It was observed that over time, the amount read from the splice was constantly decreasing the longer the trace was running. That is, if one asked for 60%, it would read over 60% when it first starts tracing, but then it would be woken up at under 60% and would slowly decrease the amount of data read after being woken up, where the amount becomes much less than the buffer percent.

This was due to an accounting of the pages_touched incrementation. This value is incremented whenever a writer transfers to a new sub-buffer. But the place where it was incremented was incorrect. If a writer overflowed the current sub-buffer it would go to the next one. If it gets preempted by an interrupt at that time, and the interrupt performs a trace, it too will end up going to the next sub-buffer. But only one should increment the counter. Unfortunately, that was not the case.

Change the cmpxchg() that does the real switch of the tail-page into a try_cmpxchg(), and on success, perform the increment of pages_touched. This will only increment the counter once for when the writer moves to a new sub-buffer, and not when there's a race and is incremented for when a writer and its preempting writer both move to the same new sub-buffer.

Link: https://lore.kernel.org/linux-trace-kernel/20240409151309.0d0e5056@gandalf.l...

Cc: stable@vger.kernel.org Cc: Mathieu Desnoyers mathieu.desnoyers@efficios.com Fixes: 2c2b0a78b3739 ("ring-buffer: Add percentage of ring buffer full to wake up reader") Acked-by: Masami Hiramatsu (Google) mhiramat@kernel.org Signed-off-by: Steven Rostedt (Google) rostedt@goodmis.org --- kernel/trace/ring_buffer.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 25476ead681b..6511dc3a00da 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -1393,7 +1393,6 @@ static void rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer, old_write = local_add_return(RB_WRITE_INTCNT, &next_page->write); old_entries = local_add_return(RB_WRITE_INTCNT, &next_page->entries);

- local_inc(&cpu_buffer->pages_touched); /* * Just make sure we have seen our old_write and synchronize * with any interrupts that come in. @@ -1430,8 +1429,9 @@ static void rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer, */ local_set(&next_page->page->commit, 0);

- /* Again, either we update tail_page or an interrupt does */ - (void)cmpxchg(&cpu_buffer->tail_page, tail_page, next_page); + /* Either we update tail_page or an interrupt does */ + if (try_cmpxchg(&cpu_buffer->tail_page, &tail_page, next_page)) + local_inc(&cpu_buffer->pages_touched); } }