On Tue, 12 Dec 2023 09:31:31 +0900 Masami Hiramatsu (Google) <mhira...@kernel.org> wrote:
> On Mon, 11 Dec 2023 11:59:49 -0500 > Steven Rostedt <rost...@goodmis.org> wrote: > > > From: "Steven Rostedt (Google)" <rost...@goodmis.org> > > > > On 32bit machines, the 64 bit timestamps are broken up into 32 bit words > > to keep from using local64_cmpxchg(), as that is very expensive on 32 bit > > architectures. > > > > On 32 bit architectures, reading these timestamps can happen in a middle > > of an update. In this case, the read returns "false", telling the caller > > that the timestamp is in the middle of an update, and it needs to assume > > it is corrupted. The code then accommodates this. > > I'm not sure but, why we don't retry reading the timestamp in this case? The timestamp that is read is a variable written by the context that this context interrupted. Reading it again will just produce the same result. > > > > > When first reserving space on the ring buffer, a "before_stamp" and > > "write_stamp" are read. If they do not match, or if either is in the > > process of being updated (false was returned from the read), an absolute > > timestamp is added and the delta is not used, as that requires reading > > theses timestamps without being corrupted. > > Ah, so here the timestamp is checked and rejected the corrupted one. > > > The one case that this does not matter is if the event is the first event > > on the sub-buffer, in which case, the event uses the sub-buffer's > > timestamp and doesn't need the other stamps for calculating them. > > > > After some work to consolidate the code, if the before or write stamps are > > in the process of updating, an absolute timestamp will be added regardless > > if the event is the first event on the sub-buffer. This is wrong as it > > should not care about the success of these reads if it is the first event > > on the sub-buffer. > > > > Fix up the parenthesis so that even if the timestamps are corrupted, if > > the event is the first event on the sub-buffer (w == 0) it still does not > > force an absolute timestamp. > > Hmm, in that case don't we remove '&& w' because either the first entry of > the sub-buffer or not, we will add an absolute timestamp if the timestamp > is in update? We do not want to add a timestamp if it's the first entry on the sub buffer, because then it's going to be using the subuffer's timestamp. > > Thank you, > > > > > Cc: sta...@vger.kernel.org > > Fixes: 58fbc3c63275c ("ring-buffer: Consolidate add_timestamp to remove > > some branches") > > Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org> > > --- > > kernel/trace/ring_buffer.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > > index 02bc9986fe0d..bc70cb9bbdb7 100644 > > --- a/kernel/trace/ring_buffer.c > > +++ b/kernel/trace/ring_buffer.c > > @@ -3584,7 +3584,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu > > *cpu_buffer, > > * absolute timestamp. > > * Don't bother if this is the start of a new page (w == 0). > > */ > > - if (unlikely(!a_ok || !b_ok || (info->before != info->after && > > w))) { > > + if (unlikely((!a_ok || !b_ok || info->before != info->after) && > > w)) { But talking with you, I think you are right that we should place the w first. if (w && unlikely(!a_ok || !b_ok || info->before != info->after)) { as the 'w' is not actually unlikely. -- Steve > > info->add_timestamp |= RB_ADD_STAMP_FORCE | > > RB_ADD_STAMP_EXTEND; > > info->length += RB_LEN_TIME_EXTEND; > > } else { > > -- > > 2.42.0 > > > >