linux-kernel - [PATCH RESEND 0/2] tracing: signal tracepoints

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Date:	Mon, 19 Dec 2011 18:04:47 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Seiji Aguchi <saguchi@...hat.com>, linux-kernel@...r.kernel.org
Subject: [PATCH RESEND 0/2] tracing: signal tracepoints

Steven, sorry for delay...

On 12/02, Steven Rostedt wrote:
>
> On Tue, 2011-11-22 at 21:52 +0100, Oleg Nesterov wrote:
> >
> > > Is "result" used for anything but tracepoints? When tracing is disabled,
> > > the tracepoints should be just nops (when jump_label is enabled). Thus
> > > tracing is very light. But if we are constantly calculating "result",
> > > this is unused by those that don't use the tracing infrastructure, which
> > > is 99.99% of all users. This is what I meant.
> >
> > Ah I see. I thought you dislike OVERFLOW_FAIL/LOSE_INFO namely.
> >
> > Of course, you are right. OTOH, this patch shaves 1058 bytes from
> > .text. And without CONFIG_TRACE* gcc doesn't generate the extra code.
>
> I was just noting that when tracing is disabled (CONFIG_TRACE* is set,
> like it is on distros, but tracing is not happening), that we have extra
> code. We usually strive to have tracing configured into the kernel, but
> produces no (actually as little as possible) overhead when not actively
> tracing.

Yes, yes, I see. But I do not see any alternative. Of course, instead
of adding "int result" we could add more trace_signal_generate's into
the code, but imho this is too ugly. And in fact I am not sure this
means less overhead with CONFIG_TRACE* even if this code is nop'ed.

> That said, you know this code much more than I do. If this isn't a fast
> path, and spinning a few more CPU cycles and perhaps dirtying a few
> cache lines floats your boat. I'm OK with this change.

I simply do not know. I _think_ that the overhead is negligible, the
extra calculating just adds a couple of "mov CONSTANT, REGISTER" insns.

> > Oh. I simply do not know what can I do. Obviously, I'd like to avoid
> > the new tracepoints in __send_signal(), imho this would be ugly. But
> > the users want more info.
> >
> > OK. let me send the patch at least for review. May be someone will
> > nack it authoritatively, in this case I can relax and forward the
> > nack back to bugzilla ;)
>
> Again, if you don't think adding very slight overhead to this path is an
> issue. Go ahead and add it.

OK, thanks.

The next question is, how can I add it ;) May be Ingo or Andrew could
take these patches? Original signal tracepoints were routed via tip-tree...

Add them both to TO:, lets see who is kinder.


> > However, at least 2/2 looks very reasonable to me. In fact it looks
> > almost like the bug-fix.
>
> 2/2 looks to have the extra overhead to. Is the bug fix just with the
> trace point.
>
> Again, if you don't mind the overhead, then here:
>
> Acked-by: Steven Rostedt <rostedt@...dmis.org>

Thanks, included.

Oleg.

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