| Message ID | 20231211114420.36dde01b@gandalf.local.home (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 9e45e39dc249c970d99d2681f6bcb55736fd725c |
| Headers | show |
| Series | ring-buffer: Do not update before stamp when switching sub-buffers | expand |
On Mon, 11 Dec 2023 11:44:20 -0500 Steven Rostedt <rostedt@goodmis.org> wrote: > 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! > Looks good to me. Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> Thank you > Cc: stable@vger.kernel.org > Fixes: a389d86f7fd09 ("ring-buffer: Have nested events still record running time stamp") > 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 2596fa7b748a..02bc9986fe0d 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); > } > > -- > 2.42.0 >
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 2596fa7b748a..02bc9986fe0d 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); }