Re: NMI watchdog + NOHZ question

[Andi Kleen <andi@firstfloor.org>]

On Wed, Jun 24, 2009 at 03:59:14AM -0700, David Miller wrote:
> From: Andi Kleen <andi@firstfloor.org>
> Date: Wed, 24 Jun 2009 12:52:23 +0200
> 
> > On Wed, Jun 24, 2009 at 03:32:33AM -0700, David Miller wrote:
> >> From: Andi Kleen <andi@firstfloor.org>
> >> Date: Wed, 24 Jun 2009 12:23:25 +0200
> >> 
> >> >> And similarly to sparc64, if that 5+ second qla2xxx interrupt
> >> >> sequence happens after the tick_nohz_stop_sched_tick() call
> >> >> we can run into the same situation.
> >> > 
> >> > Yes it would be probably safer to do the tick disabling with
> >> > interrupts off already.
> >> 
> >> That only makes sense if you're really putting the cpu to sleep
> >> until an interrupt or similar happens.
> > 
> > That is what the idle loop is supposed to do, isn't it?
> 
> Some sparc64 cpu's don't have a yield, and therefore can't
> truly "sleep" during this loop.  That's what I'm talking
> about.

How are power saving states invoked instead? Or do they not
having any power saving idle states?

> >> > These days NMI watchdog is not used much on x86 anymore because it's 
> >> > default off, so probably people never noticed that.
> >> 
> >> I really didn't want to provide the feature that way on sparc64 which
> >> is why I made it on by default.  It would be interesting to reconsider
> >> x86's default, perhaps even only on a trial basis in -next.
> > 
> > The reason it was turned off is that there are a few systems (e.g.
> > laptops from a particular vendor) which don't handle NMIs correctly
> > in the platform. When the NMI happens while SMI is active
> > they hang. Also there were a few other strange problems
> > on other systems that went away when it was disabled.
> 
> I wonder how many of those "few other strange problems" were of
> the variety I'm diagnosing here :-)

Some likely.

But the general problem is that hardware architects do not normally
consider NMIs as owned by the OS, but rather as owned by
the platform.

> Is this realm of systems-with-NMI-issues exclusive to x86-32 
> or would it be more doable to turn it on by default for 64-bit
> x86 builds?

Some of these problems were on 64bit capable systems.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.