linux-kernel - Re: [tip:sched/locking] sched: Add p->pi_lock to task_rq

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Date:	Sun, 5 Jun 2011 11:55:55 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Arne Jansen <lists@...-jansens.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	efault@....de, npiggin@...nel.dk, akpm@...ux-foundation.org,
	frank.rowand@...sony.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/locking] sched: Add p->pi_lock to task_rq_lock()


* Arne Jansen <lists@...-jansens.de> wrote:

> >( Arne, please also double check on a working bootup that the NMI
> >   watchdog is actually ticking, by checking the NMI counts in
> >   /proc/interrupts go up slowly but surely on all CPUs. )
> 
> It does, but _very_ slowly. Some CPUs do not count up for tens of
> minutes if the machine is idle. If I generate some load like 'make
> tags', the counters go up quite quickly.
> After 4 minutes and one 'make cscope' it looks like this:
> NMI:          8         13         43          5          2
> 3        22          1   Non-maskable interrupts
> 
> But I never see a single tick on console or in dmesg, even when I
> replace the early_printk with a printk.

hm, that might be because the NMI watchdog uses halted cycles to 
tick.

That's not a problem (the kernel cannot lock up while there are no 
cycles ticking) but nevertheless could you work this around please
by starting 8 infinite shell loops:

   for ((i=0; i<8; i++)); do while : ; do : ; done & done

?

This will saturate all cores and makes sure the NMI watchdog is 
ticking everywhere.

Hopefully this wont make the bug go away :-)

This will remove one factor of uncertainty (of where the NMI watchdog 
is working or not), so it simplifies debugging.

> [   36.064321] ------------[ cut here ]------------
> [   36.064328] WARNING: at kernel/printk.c:293 do_syslog+0xbf/0x550()
> [   36.064330] Hardware name: X8SIL
> [   36.064331] Attempt to access syslog with CAP_SYS_ADMIN but no
> CAP_SYSLOG (deprecated).

Yeah, unrelated, and rather annoying looking that warning. The 
warning is borderline correct (it's messy to drop CAP_SYSLOG but keep 
CAP_SYS_ADMIN) but still, if we warned every time userspace relies on 
something the kernel provided in the past, in a somewhat messy way, 
we'd never complete bootup i guess ;-)

Thanks,

	Ingo
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