Re: [PATCH v3] ring-buffer: Simplify reservation with try_cmpxchg() loop

Mathieu Desnoyers
2024-02-20T14:50:14Z

On 2024-02-20 09:19, Steven Rostedt wrote:
> On Mon, 19 Feb 2024 18:20:32 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
>> Instead of using local_add_return() to reserve the ring buffer data,
>> Mathieu Desnoyers suggested using local_cmpxchg(). This would simplify the
>> reservation with the time keeping code.
>>
>> Although, it does not get rid of the double time stamps (before_stamp and
>> write_stamp), using cmpxchg() does get rid of the more complex case when
>> an interrupting event occurs between getting the timestamps and reserving
>> the data, as when that happens, it just tries again instead of dealing
>> with it.
>>
>> Before we had:
>>
>> w = local_read(&tail_page->write);
>> /* get time stamps */
>> write = local_add_return(length, &tail_page->write);
>> if (write - length == w) {
>> /* do simple case */
>> } else {
>> /* do complex case */
>> }
>>
>> By switching the local_add_return() to a local_try_cmpxchg() it can now be:
>>
>> w = local_read(&tail_page->write);
>> again:
>> /* get time stamps */
>> if (!local_try_cmpxchg(&tail_page->write, &w, w + length))
>> goto again;
>>
>> /* do simple case */
>
> Something about this logic is causing __rb_next_reserve() to sometimes
> always return -EAGAIN and triggering the:
>
> RB_WARN_ON(cpu_buffer, ++nr_loops > 1000)
>
> Which disables the ring buffer.
>
> I'm not sure what it is, but until I do, I'm removing the patch from my
> queue.

Try resetting the info->add_timestamp flags to add_ts_default on goto again
within __rb_reserve_next().

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.