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Message-ID: <76797ddd-bb87-4af9-9703-1ec00a0d318c@efficios.com>
Date:   Sun, 10 Dec 2023 09:17:44 -0500
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Subject: Re: [PATCH 00/14] ring-buffer/tracing: Allow ring buffer to have
 bigger sub buffers
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To:     Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
        linux-trace-kernel@vger.kernel.org
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tzvetomir Stoyanov <tz.stoyanov@gmail.com>,
        Vincent Donnefort <vdonnefort@google.com>,
        Kent Overstreet <kent.overstreet@gmail.com>
References: <20231210035404.053677508@goodmis.org>
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
In-Reply-To: <20231210035404.053677508@goodmis.org>
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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