linux-kernel - Re: [PATCH v2 -tip 3/4] tracing: make a snapshot feature available from userspace

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Date:	Fri, 07 Dec 2012 20:00:49 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Hiraku Toyooka <hiraku.toyooka.gu@...achi.com>
Cc:	yrl.pp-manager.tt@...achi.com, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Li Zefan <lizf@...fujitsu.com>
Subject: Re: [PATCH v2 -tip 3/4] tracing: make a snapshot feature available
 from userspace

On Fri, 2012-12-07 at 11:07 +0900, Hiraku Toyooka wrote:
> Hi, Steven,
> 
> (2012/11/30 23:17), Steven Rostedt wrote:
> [snip]
>  >
>  > Actually, I would have:
>  >
>  >   status\input |     0      |     1      |    else    |
>  >  --------------+------------+------------+------------+
>  >  not allocated |(do nothing)| alloc+swap |   EINVAL   |
>  >  --------------+------------+------------+------------+
>  >    allocated   |    free    |   swap     |   clear    |
>  >  --------------+------------+------------+------------+
>  >
>  > Perhaps we don't need to do the clear on swap, just let the trace
>  > continue where it left off? But in case we should swap...
>  >
> 
> I think we don't need the clear on swap too.
> I'll update my patches like this table.
> 
>  > There's a fast way to clear the tracer. Look at what the wakeup tracer
>  > does. We can make that generic. If you want, I can write that code up
>  > too. Hmm, maybe I'll do that, as it will speed things up for
>  > everyone :-)
>  >
> 
> (I looked over the wakeup tracer, but I couldn't find that code...)

Heh, sorry, you needed to look at the "update_max_tr()" in
kernel/trace/trace.c. Where we update the time_start value. Then the
output skips all timestamps before that start. This is much more
efficient than a 'reset', as we don't need to sync or anything. Just
record the timestamp of where we want to consider the buffer started,
and ignore any event before that.


> I think that seq_read() calls s_stop() even if s_start() failed.
> 
> seq_read()@fs/seq_file.c:
> 
>          p = m->op->start(m, &pos);
>          while (1) {
>                  err = PTR_ERR(p);
>                  if (!p || IS_ERR(p))
>                          break;
>                  ...
>          }
>          m->op->stop(m, p);
> 
> So, I think we need the check in s_stop(), don't we?

Crap, you're right. Hmm, why was I thinking that it didn't. I better go
and review some of my recent code to make sure that I didn't have that
wrong assumption.

-- Steve


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