linux-kernel - Re: [ath9k-devel] ath9k: massive unexplained latency in 2.6.27 (rc5, rc6, probably others)

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Date:	Fri, 19 Sep 2008 10:51:06 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Noonan <steven@...inklabs.net>
Cc:	"Luis R. Rodriguez" <lrodriguez@...eros.com>,
	Luis Rodriguez <Luis.Rodriguez@...eros.com>,
	"ath9k-devel@...ts.ath9k.org" <ath9k-devel@...ts.ath9k.org>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [ath9k-devel] ath9k: massive unexplained latency in 2.6.27
	(rc5, rc6, probably others)


* Steven Noonan <steven@...inklabs.net> wrote:

> On Fri, Sep 19, 2008 at 1:22 AM, Steven Noonan <steven@...inklabs.net> wrote:
> > On Fri, Sep 19, 2008 at 1:17 AM, Ingo Molnar <mingo@...e.hu> wrote:
> >> in tip/master there's an ftrace_printk() facility. You can just replace
> >> the verbose printk()s with ftrace_printk() and see the result in
> >> /debug/tracing/trace.
> >>
> >> This sort of tracing has very low overhead and can be used in an easy
> >> ad-hoc manner with no extra infrastructure. Here's a few quick-start
> >> links about how to enable the scheduler tracer:
> >>
> >>  http://redhat.com/~mingo/sched-devel.git/readme-tracer.txt
> >>  http://redhat.com/~mingo/sched-devel.git/howto-trace-latencies.txt
> >>
> >> any ftrace_printk() you add in the kernel will show up in that trace.
> >>
> >> (If the scheduling events are uninteresting and clutter the output then
> >> you might want to remove the scheduler tracing entries from kernel/*.c
> >> by removing the trace_sched_*() calls or use a less noisy tracer.)
> >>
> >> to get enough of a trace history you might want to increase the number
> >> of trace entries in /debug/tracing/trace_entries from 16K to 128K or so.
> >>
> >
> > Thanks for the suggestion, I'll do that. :)
> >
> 
> It'd be extremely convenient to have a tracer that did absolutely 
> nothing but display ftrace_printk()s.

yeah, had the same thought.

Could you please contribute one? :) I'd suggest you take the smallest 
tracer as a template: trace_sched_switch.c and turn that into 
trace_nop.c - and change all the function names and ASCII and kconfig 
glue to 'nop'/'NOP'?

That can then be used as a basis for additional ftrace plugins, or for 
ad-hoc temporary tracing.

	Ingo
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