linux-kernel - Re: NMI watchdog setup_lapic_nmi

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Message-Id: <20081007132158.009669d2.akpm@linux-foundation.org>
Date:	Tue, 7 Oct 2008 13:21:58 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	eranian@...il.com
Cc:	eranian@...glemail.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, andi@...stfloor.org, tglx@...utronix.de,
	Shaohua Li <shaohua.li@...el.com>
Subject: Re: NMI watchdog setup_lapic_nmi_watchdog() problem

On Tue, 7 Oct 2008 17:45:32 +0200
"stephane eranian" <eranian@...glemail.com> wrote:

> Hello,
> 
> I was doing some more testing with perfmon when I ran into
> a problem with the NMI watchdog code in 2.6.27-rc8.
> 
> Since 2.6.20, it is possible to enable/disable the NMI watchdog
> on-the-fly via /proc/sys/kernel/nmi_watchdog. This is a nice option
> which avoids having to reboot the kernel.
> 
> Enabling/disabling the NMI watchdog uses two internal functions
> enable_lapic_nmi_watchdog() and disable_lapic_nmi_watchdog().
> 
> Enable_lapic_nmi_watchdog() uses  a IPI handler to setup the
> APIC on each CPU. However, it turns out that this handler, namely,
> setup_apic_nmi_watchdog() relies on some explicit ordering constraint
> due to suspend/resume constraints as explained in the comment
> below:
> 
> void setup_apic_nmi_watchdog(void *unused)
> {
>         if (__get_cpu_var(wd_enabled))
>                 return;
> 
>         /* cheap hack to support suspend/resume */
>         /* if cpu0 is not active neither should the other cpus */
>         if (smp_processor_id() != 0 && atomic_read(&nmi_active) <= 0)
>                 return;
> 
>         switch (nmi_watchdog) {
> [snip]
> }
> 
> Supposing watchdog was disabled via /proc, nmi_active = 0. Then if you
> re-enable, and if CPU0 is not the first to execute the IPI handler, then none
> of the other CPUS will re-enable their NMI watchdog timer.  On a quad-core
> system, I have seen, for instance, 2 out of 4 with NMI watchdogs re-enabled.
> 
> I am not an expert at suspend/resume. I am assuming there was a race condition
> there and that's why this code was added early on. The problem is that it now
> conflicts with the /proc option.
> 
> It is not clear to me how this works during boot. Obviously the order
> is respected
> and all CPUs have their NMI watchdog enabled.
> 
> Until I understand the suspend/resume issue, it is hard to provide a
> fix for this.
> 
> Any comments?

The "cheap hack" was added in September 2006.

2.6.20 was released in Feb 2007.

So presumably this problem has always been there, since
/proc/sys/kernel/nmi_watchdog was first added, only nobody has hit it
before.

Have you only recently started to use /proc/sys/kernel/nmi_watchdog, or
did it work OK on any earlier kernel?

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