linux-kernel - Re: [PATCH v10 0/2] Introducing trace buffer mapping by user-space

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Date: Tue, 9 Jan 2024 13:47:48 +0000
From: Vincent Donnefort <vdonnefort@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	mathieu.desnoyers@...icios.com, kernel-team@...roid.com
Subject: Re: [PATCH v10 0/2] Introducing trace buffer mapping by user-space

On Tue, Jan 09, 2024 at 08:20:57AM -0500, Steven Rostedt wrote:
> 
> Hi Masami, thanks for looking at this.
> 
> On Tue, 9 Jan 2024 22:04:45 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> 
> > > The tracing ring-buffers can be stored on disk or sent to network
> > > without any copy via splice. However the later doesn't allow real time
> > > processing of the traces. A solution is to give userspace direct access
> > > to the ring-buffer pages via a mapping. An application can now become a
> > > consumer of the ring-buffer, in a similar fashion to what trace_pipe
> > > offers.  
> > 
> > I think this is very nice feature. But this series seems just a feature,
> > no document and no example code. Can you add 2 patches to add those?
> > I know libtracefs already provide a support code, but I think it is
> > better to have a test code under tools/testing/selftests/ring_buffer.
> 
> Yeah, we should have sample code and a test.

Ack. I will recycle what I had in the cover letter in a ring_buffer selftest.

> 
> > 
> > I also wonder what happen if other operation (e.g. taking snapshot) happens
> > while mmaping the ring buffer.
> 
> Hmm, good point. We should disable snapshots when mapped, and also prevent
> mapping with latency tracer if we are not already doing that.

ring_buffer_swap_cpu() is already disabled when mapped as well as
resize_disabled set. Is something else necessary?

> 
> -- Steve