linux-kernel - Re: [PATCH] tracing: use raw spinlocks instead of spinlocks

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Message-ID: <c62985530811040314o13394d43t201ae3fc2f1304be@mail.gmail.com>
Date:	Tue, 4 Nov 2008 12:14:33 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: use raw spinlocks instead of spinlocks

2008/11/4 Ingo Molnar <mingo@...e.hu>:
> lockdep works via a completely different principle: it instruments all
> the actual lock acquire/release calls and builds a graph of lock
> dependencies in the system, as it happens.
>
> It also guarantees that all the observed locking rules are followed
> (i.e. it proves that as long as you dont get any messages from
> lockdep, all the locking patterns are mathematically safe).
>
> So a lockdep message will most of the time occur much easier than a
> real lockup would occur - as lockdep only needs to observe
> inconsistent locking patterns to prove that a lockup _could_ occur.


Ok, so if there is such an error, I could (hopefully) see it with
early_printk....


> The NMI watchdog just observes the system and complains if it sees
> hardirqs not progressing (i.e. a hard lockup). It will detect anything
> that causes a hard lockup. (assuming that the NMI watchdog itself is
> not locked up)
>
> Regarding your lockup ... it's quite hard. Maybe you can get more
> output out of the system by using:
>
>  earlyprintk=vga,keep
>
> plus disablig regular tty output. (i.e. not passing any 'console=tty'
> line to the kernel bootup.)
>
> this way you wont get any normal printk activities (which might lock
> up), you should only get the very simple early-printk output.



Good idea. After reading your mail, I had a look on early_printk and
it shoudn't cause any tracing recursion after
your patch: the strings are directly writed to the vga. Great.


> ... but that's still not a guarantee that you'll be able to debug it
> this way :-/ Bootstrapping something this intrusive and wide is one of
> the hardest kernel development activities possible.


I've been on it for two weeks now. Yes it's very hard. But exciting!

>        Ingo
>
> ---------------->
> From 6cf87efbc7a3676e0ad7c9622ec6aec244a593bc Mon Sep 17 00:00:00 2001
> From: Ingo Molnar <mingo@...e.hu>
> Date: Tue, 4 Nov 2008 10:42:23 +0100
> Subject: [PATCH] x86 debug: mark early_printk.o as notrace
>
> Impact: do not do function-tracing in the early-printk code
>
> this is useful when earlyprintk=vga,keep is used to debug tracer
> plugins.
>
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> ---
>  arch/x86/kernel/Makefile |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index e489ff9..943fe60 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -12,6 +12,7 @@ CFLAGS_REMOVE_tsc.o = -pg
>  CFLAGS_REMOVE_rtc.o = -pg
>  CFLAGS_REMOVE_paravirt-spinlocks.o = -pg
>  CFLAGS_REMOVE_ftrace.o = -pg
> +CFLAGS_REMOVE_early_printk.o = -pg
>  endif
>
>  #
>

I will test your advices this evening and pull latest -tip to get this
patch. Thanks!
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