On Tue, 9 Apr 2024 15:13:09 -0400 Steven Rostedt <rost...@goodmis.org> wrote:
> From: "Steven Rostedt (Google)" <rost...@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. > Looks good to me. Acked-by: Masami Hiramatsu (Google) <mhira...@kernel.org> BTW, isn't this a real bugfix, because the page_touched can be bigger than nr_pages without this fix? Thank you, > Signed-off-by: Steven Rostedt (Google) <rost...@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); > } > } > > -- > 2.43.0 > -- Masami Hiramatsu (Google) <mhira...@kernel.org>
