Date:	Mon, 8 Jun 2009 19:15:01 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Young <hidave.darkstar@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] printk: add halt_delay parameter for printk delay
	in halt phase


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Mon, 8 Jun 2009 10:48:07 +0200 Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * Dave Young <hidave.darkstar@...il.com> wrote:
> > 
> > > On Mon, Jun 8, 2009 at 4:28 PM, Andrew Morton<akpm@...ux-foundation.org> wrote:
> > > > On Mon, 8 Jun 2009 10:14:39 +0200 Ingo Molnar <mingo@...e.hu> wrote:
> > > >
> > > >>
> > > >> * Dave Young <hidave.darkstar@...il.com> wrote:
> > > >>
> > > >> > Add a halt_delay module parameter in printk.c used to read the
> > > >> > printk messages in halt/poweroff/restart phase, delay each printk
> > > >> > messages by halt_delay milliseconds. It is useful for debugging if
> > > >> > there's no other way to dump kernel messages that time.
> > > >> >
> > > >> > The halt_delay max value is 65535, default value is 0, change it
> > > >> > by:
> > > >> >
> > > >> > echo xxx > /sys/module/printk/parameters/halt_delay
> > > >> >
> > > >> > Signed-off-by: Dave Young <hidave.darkstar@...il.com>
> > > >> > ---
> > > >> > Documentation/kernel-parameters.txt | __ __5 +++++
> > > >> > kernel/printk.c __ __ __ __ __ __ __ __ __ __ | __ 17 +++++++++++++++++
> > > >> > 2 files changed, 22 insertions(+)
> > > >> >
> > > >> > --- linux-2.6.orig/kernel/printk.c __2009-06-08 13:55:35.000000000 +0800
> > > >> > +++ linux-2.6/kernel/printk.c __ __ __ 2009-06-08 13:56:23.000000000 +0800
> > > >> > @@ -250,6 +250,22 @@ static inline void boot_delay_msec(void)
> > > >> > __}
> > > >> > __#endif
> > > >> >
> > > >> > +/* msecs delay after each halt/poweroff/restart phase printk,
> > > >> > + unsigned short is enough for delay in milliseconds */
> > > >> > +static unsigned short halt_delay;
> > > >> > +
> > > >> > +static inline void halt_delay_msec(void)
> > > >> > +{
> > > >> > + __ if (unlikely(halt_delay == 0 || !(system_state == SYSTEM_HALT
> > > >> > + __ __ __ __ __ __ __ __ __ __ __ __ __ || system_state == SYSTEM_POWER_OFF
> > > >> > + __ __ __ __ __ __ __ __ __ __ __ __ __ || system_state == SYSTEM_RESTART)))
> > > >> > + __ __ __ __ __ return;
> > > >>
> > > >> This is a tiny bit ugly (and goes into the vprintf path) but i can
> > > >> see no other way either - a system_state > SYSTEM_RUNNING check
> > > >> would needlessly include the suspend-to-disk state (which we dont
> > > >> want to include here).
> > > >>
> > > >> In theory we could turn system_state into a bitmask and have a
> > > >> print_delay_mask check instead of these flags ... but that is a far
> > > >> wider change and i'm not sure it's a net step forwards.
> > > >>
> > > >> I've applied your patch to tip:core/printk with small edits to the
> > > >> changelog - Linus & Andrew is Cc:ed, should they have any
> > > >> objections.
> > > >
> > > > Could we not put just a single delay in there, immediately prior to halting,
> > > > restarting or poweroffing?
> > > 
> > > But, then prink messages will still flush too fast for us to see 
> > > the detail.
> 
> Only if there are so many unlogged messages that they scroll of the
> screen.  Is that the case?

i have such an example in my logs:

[  390.206118] md: stopping all md devices.
[  391.208259] sd 0:0:0:0: [sda] Synchronizing SCSI cache
[  391.214877] igb 0000:01:00.1: PCI INT B disabled
[  391.220445] igb 0000:01:00.0: PCI INT A disabled
[  391.225897] Restarting system.
[  391.229213] machine restart
[  391.232833] ------------[ cut here ]------------
[  391.237718] WARNING: at arch/x86/kernel/smp.c:117 native_smp_send_reschedule+0x55/0x83()
[  391.246276] Hardware name: X8DTN
[  391.249762] Modules linked in:
[  391.253146] Pid: 19807, comm: reboot Not tainted 2.6.30-rc8-tip #8
[  391.259580] Call Trace:
[  391.262287]  <IRQ>  [<ffffffff81021525>] ? native_smp_send_reschedule+0x55/0x83
[  391.270136]  [<ffffffff8104a316>] warn_slowpath_common+0x77/0xa4
[  391.276395]  [<ffffffff8104a352>] warn_slowpath_null+0xf/0x11
[  391.282399]  [<ffffffff81021525>] native_smp_send_reschedule+0x55/0x83
[  391.289182]  [<ffffffff8103629f>] resched_task+0x60/0x62
[  391.294752]  [<ffffffff8103d976>] resched_cpu+0x4a/0x5e
[  391.300237]  [<ffffffff8104627f>] scheduler_tick+0x19a/0x249
[  391.306154]  [<ffffffff81054576>] update_process_times+0x4f/0x5f
[  391.312417]  [<ffffffff8106ad80>] tick_sched_timer+0x76/0x96
[  391.318331]  [<ffffffff8106ad0a>] ? tick_sched_timer+0x0/0x96
[  391.324334]  [<ffffffff81061ea2>] __run_hrtimer+0x80/0xb4
[  391.329993]  [<ffffffff81062abe>] hrtimer_interrupt+0xe7/0x145
[  391.336081]  [<ffffffff81022801>] smp_apic_timer_interrupt+0x83/0x96
[  391.342692]  [<ffffffff8100c6d3>] apic_timer_interrupt+0x13/0x20
[  391.348953]  <EOI>  [<ffffffff81022a7c>] ? default_send_IPI_mask_allbutself_phys+0x67/0x73
[  391.357752]  [<ffffffff8102532d>] ? physflat_send_IPI_allbutself+0x14/0x16
[  391.364877]  [<ffffffff8102166a>] ? native_send_call_func_ipi+0x83/0xa5
[  391.371747]  [<ffffffff81070cff>] ? smp_call_function_many+0x19b/0x1bb
[  391.378529]  [<ffffffff810128b0>] ? stop_this_cpu+0x0/0x1c
[  391.384270]  [<ffffffff81070d3f>] ? smp_call_function+0x20/0x24
[  391.390448]  [<ffffffff81021575>] ? native_smp_send_stop+0x22/0x30
[  391.396882]  [<ffffffff81020f93>] ? native_machine_shutdown+0x49/0x62
[  391.403578]  [<ffffffff81020d62>] ? native_machine_restart+0x21/0x33
[  391.410187]  [<ffffffff81020cd9>] ? machine_restart+0xa/0xc
[  391.416016]  [<ffffffff81059d67>] ? kernel_restart+0x3f/0x43
[  391.421935]  [<ffffffff81059eb8>] ? sys_reboot+0x140/0x174
[  391.427677]  [<ffffffff810628c6>] ? hrtimer_nanosleep+0x104/0x119
[  391.434025]  [<ffffffff81061e01>] ? hrtimer_wakeup+0x0/0x21
[  391.439857]  [<ffffffff8106272d>] ? hrtimer_start_range_ns+0xf/0x11
[  391.446379]  [<ffffffff8106292f>] ? sys_nanosleep+0x54/0x6c
[  391.452210]  [<ffffffff8100bbeb>] ? system_call_fastpath+0x16/0x1b
[  391.458643] ---[ end trace 4b05cad362f39345 ]---

that's 44 lines so yes it can happen.

> > Plus often there's a loop in architecture code that tries various 
> > methods of reboot. We dont know which one works - and any of them 
> > could produce warning messages. (and this happened a number of times 
> > in the past)
> > 
> > So this would mean having to find up to a hundred of 'reboot now' 
> > places in a two dozen architectures (and keeping them all maintained 
> > ongoing as well). Does not seem like a good choice to me.
> > 
> 
> hm.  If we need to actually capture all of those.
> 
> questions: is it possible for interrupts to be disabled at this 
> time? If so, can we get an NMI watchdog hit?

no, we generally turn off the nmi watchdog during shutdown, disable 
the lapic and io-apic, etc.

> Is the softlockup detector still running and if so, can it 
> trigger?

in (non-emergency) reboot, last i checked, we stopped all other CPUs 
first, and then killed the current one. There's no chance for the 
watchdog thread to run.

Anyway ... you seem to be uncomfortable about this patch - should i 
delay it for now to let it all play out? We are close to the merge 
window.

	Ingo
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