Re: [PATCH 00/14] ring-buffer/tracing: Allow ring buffer to have bigger sub buffers

[Mathieu Desnoyers]

On 2023-12-09 22:54, Steven Rostedt wrote:
[...]
> Basically, events to the tracing subsystem are limited to just under a
> PAGE_SIZE, as the ring buffer is split into "sub buffers" of one page
> size, and an event can not be bigger than a sub buffer. This allows users
> to change the size of a sub buffer by the order:
>
>    echo 3 > /sys/kernel/tracing/buffer_subbuf_order
>
> Will make each sub buffer a size of 8 pages, allowing events to be almost
> as big as 8 pages in size (sub buffers do have meta data on them as
> well, keeping an event from reaching the same size as a sub buffer).

Specifying the "order" of subbuffer size as a power of two of
number of pages is a poor UX choice for a user-facing ABI.

I would recommend allowing the user to specify the size in bytes, and
internally bump to size to the next power of 2, with a minimum of
PAGE_SIZE.

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com