linux-kernel - RE: [PATCH 00/12 v2] ring-buffer/tracing: Remove disabling of ring buffer while reading trace file

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Date:   Fri, 27 Mar 2020 10:07:00 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Steven Rostedt' <rostedt@...dmis.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Alexei Starovoitov" <alexei.starovoitov@...il.com>,
        Peter Wu <peter@...ensteyn.nl>,
        Jonathan Corbet <corbet@....net>,
        Tom Zanussi <zanussi@...nel.org>,
        "Shuah Khan" <shuahkhan@...il.com>, bpf <bpf@...r.kernel.org>
Subject: RE: [PATCH 00/12 v2] ring-buffer/tracing: Remove disabling of ring
 buffer while reading trace file

From: Steven Rostedt
> Sent: 27 March 2020 01:46
> On Sat, 21 Mar 2020 19:13:51 +0000
> David Laight <David.Laight@...LAB.COM> wrote:
> 
> > From: Steven Rostedt
> > > Sent: 19 March 2020 23:22
> > ...
> > >
> > > This patch series attempts to satisfy that request, by creating a
> > > temporary buffer in each of the per cpu iterators to place the
> > > read event into, such that it can be passed to users without worrying
> > > about a writer to corrupt the event while it was being written out.
> > > It also uses the fact that the ring buffer is broken up into pages,
> > > where each page has its own timestamp that gets updated when a
> > > writer crosses over to it. By copying it to the temp buffer, and
> > > doing a "before and after" test of the time stamp with memory barriers,
> > > can allow the events to be saved.
> >
> > Does this mean the you will no longer be able to look at a snapshot
> > of the trace by running 'less trace' (and typically going to the end
> > to get info for all cpus).
> 
> I changed patch 9 to be this:
> 
> It adds an option "pause-on-trace" that when set, will bring back the
> old behavior of pausing recording to the ring buffer when the trace
> file is open.
> 
> If needed, I can add a kernel command line option and a Kconfig that
> makes this set to true by default.

Maybe a different file 'trace_no_pause' ?
Along with the one that lets you read the raw trace and get EOF.

	David

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