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

The ring buffer timestamps are synchronized by two timestamp placeholders. One is the "before_stamp" and the other is the "write_stamp" (sometimes referred to as the "after stamp" but only in the comments. These two stamps are key to knowing how to handle nested events coming in with a lockless system.

When moving across sub-buffers, the before stamp is updated but the write stamp is not. There's an effort to put back the before stamp to something that seems logical in case there's nested events. But as the current event is about to cross sub-buffers, and so will any new nested event that happens, updating the before stamp is useless, and could even introduce new race conditions.

The first event on a sub-buffer simply uses the sub-buffer's timestamp and keeps a "delta" of zero. The "before_stamp" and "write_stamp" are not used in the algorithm in this case. There's no reason to try to fix the before_stamp when this happens.

As a bonus, it removes a cmpxchg() when crossing sub-buffers!

Link: https://lore.kernel.org/linux-trace-kernel/20231211114420.36dde01b@gandalf.l...

Cc: stable@vger.kernel.org Cc: Mark Rutland mark.rutland@arm.com Cc: Mathieu Desnoyers mathieu.desnoyers@efficios.com Fixes: a389d86f7fd09 ("ring-buffer: Have nested events still record running time stamp") Reviewed-by: Masami Hiramatsu (Google) mhiramat@kernel.org Signed-off-by: Steven Rostedt (Google) rostedt@goodmis.org --- kernel/trace/ring_buffer.c | 9 +-------- 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index dcd47895b424..c7abcc215fe2 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3607,14 +3607,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,

/* See if we shot pass the end of this buffer page */ if (unlikely(write > BUF_PAGE_SIZE)) { - /* before and after may now different, fix it up*/ - b_ok = rb_time_read(&cpu_buffer->before_stamp, &info->before); - a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after); - if (a_ok && b_ok && info->before != info->after) - (void)rb_time_cmpxchg(&cpu_buffer->before_stamp, - info->before, info->after); - if (a_ok && b_ok) - check_buffer(cpu_buffer, info, CHECK_FULL_PAGE); + check_buffer(cpu_buffer, info, CHECK_FULL_PAGE); return rb_move_tail(cpu_buffer, tail, info); }