linux-kernel - Re: [PATCH v6 1/3] trace: Make removal of ring buffer pages atomic

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Date:	Thu, 03 May 2012 08:57:20 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Laurent Chavey <chavey@...gle.com>,
	Justin Teravest <teravest@...gle.com>,
	David Sharp <dhsharp@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/3] trace: Make removal of ring buffer pages atomic

On Wed, 2012-05-02 at 23:40 -0700, Vaibhav Nagarnaik wrote:
> On Wed, May 2, 2012 at 6:55 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > Hmm, something in this patch breaks buffers_size_kb and friends.
> 
> I checked and I guess you are referring to the initial state of the
> buffer_size_kb, which returns:
> 0 (expanded: 1408)
> instead of:
> 7 (expanded: 1408)
> 

No I realized that. That changed with your other patch.

> I found this got in with the earlier patch which added per-cpu
> buffer_size_kb. I will send a small fix-up patch for it.

Yeah, probably should send a fix for that.

> 
> After expanding the ring buffer to various sizes, I couldn't find any
> other breakage. Is there any other behavior that you saw as odd?

The issue I see seems to trigger with trace-cmd:

[root@ixf ~]# cat /debug/tracing/buffer_size_kb 
0 (expanded: 1408)
[root@ixf ~]# trace-cmd start -e sched
/debug/tracing/events/sched/filter
/debug/tracing/events/*/sched/filter
[root@ixf ~]# cat /debug/tracing/buffer_size_kb 
0

But if I enable it via the command line it works:

[root@ixf ~]# cat /debug/tracing/buffer_size_kb 
0 (expanded: 1408)
[root@ixf ~]# echo 1 > /debug/tracing/events/sched/enable 
[root@ixf ~]# cat /debug/tracing/buffer_size_kb 
1408


Without your patch:

[root@ixf ~]# cat /debug/tracing/buffer_size_kb 
0 (expanded: 1408)
[root@ixf ~]# trace-cmd start -e sched
/debug/tracing/events/sched/filter
/debug/tracing/events/*/sched/filter
[root@ixf ~]# cat /debug/tracing/buffer_size_kb 
1408


So it seems to be trace-cmd doing something different that prevents the
expand from happening. Not sure what it is. If I get time, I'll
investigate it a little more.

Thanks,

-- Steve


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