Re: [PATCH -tip 2/2 resend] x86, traps: Drop nmi_reason_lock untilit is really needed

[Cyrill Gorcunov]

On 03/02/2011 09:40 PM, Don Zickus wrote:
> On Wed, Mar 02, 2011 at 07:13:42PM +0300, Cyrill Gorcunov wrote:
>> On 03/02/2011 07:03 PM, Ingo Molnar wrote:
>> ...
>>>
>>> Well, the lock serializes the read-out of the 'NMI reason' port, the handling of 
>>> whatever known reason and then the reassertion of the NMI (on 32-bit). 
>>>
>>> EDAC has a callback in pci_serr_error() - and this lock serializes that. So we 
>>> cannot just remove a lock like that, if there's any chance of parallel execution on 
>>> multiple CPUs.
>>>
>>> Thanks,
>>>
>>> 	Ingo
>>
>> OK, probably we need some UV person CC'ed (not sure whom) just to explain the
>> reason for such nmi-listening model. Meanwhile -- lets drop my patch.
> 
> It's for debugging reasons.  When their huge machine deadlocks, they
> wanted an easy mechanism to dump the cpu stacks.  That mechanism was an
> nmi button.  The problem was the button would only dump the first cpu.  By
> opening up the other cpus to accept external nmis, they could dump all the
> cpus.

  Yeah, thanks Don, just noted that (actually the former commit log
/78c06176466cbd1b3f0f67709d3023c40dbebcbd/ didn't mention that x86
masks only LVT1).

> 
> Now this spinlock doesn't affect them, because they registered an nmi
> handler to catch it and dump their stack (I modified the code to use
> DIE_NMIUNKNOWN instead of DIE_NMI to avoid conflict with the
> nmi_watchdog).  But I don't know what the affect is, if that spinlock is
> not there (I sent a private email to SGI inquiring, their guy wasn't
> around this week).

  Don, do you know -- was new nmi-watchdog system tested with UV machine
somewhere?

> 
> Personally I am indifferent to this patch.  I don't have any problems with
> the code the way it is now, but can understand what you mean having stuff
> lying around as 'dead code'.  I had thought Intel would have pushed more
> patches upstream to remove the BSP lock-in by now.
> 
> Cheers,
> Don
> 

-- 
    Cyrill
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