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

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Message-ID: <20200322140756.7257b867@gandalf.local.home>
Date:   Sun, 22 Mar 2020 14:07:56 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     David Laight <David.Laight@...LAB.COM>
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

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).

If there's a use case for this, it will be trivial to add an option to
bring back the old behavior. If you want that, I can do that, and even add
a config that makes it the default.

> 
> A lot of the time trace is being written far too fast for it to make
> any sense to try to read it continuously.
> 
> Also, if BPF start using ftrace, no one will be able to use it for
> 'normal debugging' on such systems.

I believe its used for debugging bpf, not for normal tracing. BPF only
uses this when it has their trace_printk() using it. Which gives that nasty
"THIS IS A DEBUG KERNEL" message ;-)   Thus, I don't think you need to
worry about bpf having this in production.

-- Steve