linux-kernel - Re: [Bug #38132] [Warning] 2.6.39.x latencytop

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Message-ID: <CAOJsxLHitazDJEjiD3j_nK_4hQoiGVo_08NP_nR3FpCTqa4HwQ@mail.gmail.com>
Date:	Mon, 11 Jul 2011 10:34:05 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Florian Mickler <florian@...kler.org>,
	Andrew Watts <akwatts@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [Bug #38132] [Warning] 2.6.39.x latencytop

On Mon, Jul 11, 2011 at 12:25 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Sun, 2011-07-10 at 14:53 +0300, Pekka Enberg wrote:
>> On Sun, Jul 10, 2011 at 1:58 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>> > This message has been generated automatically as a part of a report
>> > of regressions introduced between 2.6.38 and 2.6.39.
>> >
>> > The following bug entry is on the current list of known regressions
>> > introduced between 2.6.38 and 2.6.39.  Please verify if it still should
>> > be listed and let the tracking team know (either way).
>> >
>> > Bug-Entry       : http://bugzilla.kernel.org/show_bug.cgi?id=38132
>> > Subject         : [Warning] 2.6.39.x latencytop
>> > Submitter       : Andrew Watts <akwatts@...il.com>
>> > Date            : 2011-06-14 17:07 (27 days old)
>> > Message-ID      : <80098.5633.qm@...111013.mail.gq1.yahoo.com>
>> > References      : http://marc.info/?l=linux-kernel&m=130807128506490&w=2
>>
>> This is a WARN_ON added in commit 6752ab4a ("tracing: Deprecate
>> tracing_enabled for tracing_on") that lets the user know about
>> deprecated ABI. It's somewhat unfortunate that there's no mention in
>> the changelog if everyone agreed on the deprecation or not...
>>
>> That said, assuming the deprecation is OK (hi Steven, Frederic, Ingo!)
>> I think this issue can be closed.
>
> I deprecated it because I said I would for the last year ;)
>
> The tracing_enabled never did what it was suppose to do. It was suppose
> to be a "quick" way to disable tracing without fully disabling it. It
> was suppose to try to turn things off to a low overhead but not fully
> off without any overhead. The problem is that it never really worked
> well. The only tracer that does anything different with it as suppose to
> tracing_on, is the irqsoff tracers. They stop looking at max latencies
> when you echo 0 into tracing_enabled. But that's it. Everything else,
> it's just the same as echoing 0 into tracing_on. I think the irqsoff
> tracers now even look at tracing_on if it should trace a max latency or
> not, so tracing_enabled doesn't even help with that.
>
> We can keep it around but I don't see any good reason for it besides not
> having latency top give a warning.

AFAICT, if you go ahead with the deprecation and actually remove the
file, you'll break latencytop fsync tracing.

On Mon, Jul 11, 2011 at 12:25 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> Question: Why is latency top even touching this file?

Looking at the sources, it seems to use it to enable fsync tracing. I
don't know the details so lets CC Arjan.
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