linux-kernel - Re: [git pull] tracing fixes

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Date:	Fri, 18 Jul 2008 12:35:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [git pull] tracing fixes


* Ingo Molnar <mingo@...e.hu> wrote:

> > >  CFLAGS_REMOVE_sched_clock.o = -pg
> > > +CFLAGS_REMOVE_sched.o = -mno-spe -pg
> > >  endif
> > >
> > 
> > Ingo,
> > 
> > Why not trace the scheduler functions? I found a lot of useful 
> > information from seeing what functions are being called (namely the 
> > latencies caused by the fair scheduler balancing). Not being able to 
> > trace sched.c seems to keep a lot of useful data from being accessed.
> 
> i agree in general, but it was causing lockups with:
> 
>       http://redhat.com/~mingo/misc/config-Thu_Jul_17_13_34_52_CEST_2008
> 
> note the MAXSMP in the config which sets NR_CPUS to 4096:
> 
>       CONFIG_NR_CPUS=4096
> 
> our randconfig testing stumbled on it. That is a debug helper to "tune 
> up the kernel for as large systems as possible" and can bring in 
> regressions not normally seen.

ok, figured it out today: the lockups were due to the NMI watchdog and a 
missing NMI protection in cpu_clock(). I've reactivated the topic that 
solves this problem area and it all works fine now.

the sched.o change probably made a difference just because it reduced 
the cross section between the NMI watchdog and the scheduler, making 
lockups less likely during the ftrace self-test. I'll revert it once the 
tracing/nmisafe is upstream.

	Ingo
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