Re: [PATCH 9/9] x86/UV: Add ability to disable UV NMI handler

[Mike Travis <travis@sgi.com>]

On 9/12/2013 11:35 AM, Paul E. McKenney wrote:
> On Thu, Sep 12, 2013 at 10:27:31AM -0700, Paul E. McKenney wrote:
>> On Tue, Sep 10, 2013 at 11:03:49AM +0200, Peter Zijlstra wrote:
>>> On Mon, Sep 09, 2013 at 10:07:03AM -0700, Mike Travis wrote:
>>>> On 9/9/2013 5:43 AM, Peter Zijlstra wrote:
>>>>> On Thu, Sep 05, 2013 at 05:50:41PM -0500, Mike Travis wrote:
>>>>>> For performance reasons, the NMI handler may be disabled to lessen the
>>>>>> performance impact caused by the multiple perf tools running concurently.
>>>>>> If the system nmi command is issued when the UV NMI handler is disabled,
>>>>>> the "Dazed and Confused" messages occur for all cpus.  The NMI handler is
>>>>>> disabled by setting the nmi disabled variable to '1'.  Setting it back to
>>>>>> '0' will re-enable the NMI handler.
>>>>>
>>>>> I'm not entirely sure why this is still needed now that you've moved all
>>>>> really expensive bits into the UNKNOWN handler.
>>>>>
>>>>
>>>> Yes, it could be considered optional.  My primary use was to isolate
>>>> new bugs I found to see if my NMI changes were causing them.  But it
>>>> appears that they are not since the problems occur with or without
>>>> using the NMI entry into KDB.  So it can be safely removed.
>>>
>>> OK, as a debug option it might make sense, but removing it is (of course)
>>> fine with me ;-)
>>>
>>>> (The basic problem is that if you hang out in KDB too long the machine
>>>> locks up.  
>>>
>>> Yeah, known issue. Not much you can do about it either I suspect. The
>>> system generally isn't build for things like that.
>>>
>>>> Other problems like the rcu stall detector does not have a
>>>> means to be "touched" like the nmi_watchdog_timer so it fires off a
>>>> few to many, many messages.  
>>>
>>> That however might be easily cured if you ask Paul nicely ;-)
>>
>> RCU's grace-period mechanism is supposed to be what touches it.  ;-)
>>
>> But what is it that you are looking for?  If you want to silence it
>> completely, the rcu_cpu_stall_suppress boot/sysfs parameter is what
>> you want to use.
>>
>>>> Another, any network connections will time
>>>> out if you are in KDB more than say 20 or 30 seconds.)
>>
>> Ah, you are looking for RCU to refrain from complaining about grace
>> periods that have been delayed by breakpoints in the kernel?  Is there
>> some way that RCU can learn that a breakpoint has happened?  If so,
>> this should not be hard.
> 
> But wait...  RCU relies on the jiffies counter for RCU CPU stall warnings.
> Doesn't the jiffies counter stop during breakpoints?
> 
> 							Thanx, Paul

All cpus entering the UV NMI event use local_irq_save (as does the
entry into KGDB/KDB).  So the question becomes more what happens
after all the cpus do the local_irq_restore?  The hardware clocks
are of course still running.

> 
>> If not, I must fall back on the rcu_cpu_stall_suppress that I mentioned
>> earlier.
>>
>>>> One other problem is with the perf tool.  It seems running more than
>>>> about 2 or 3 perf top instances on a medium (1k cpu threads) sized
>>>> system, they start behaving badly with a bunch of NMI stackdumps
>>>> appearing on the console.  Eventually the system become unusable.
>>>
>>> Yuck.. I haven't seen anything like that on the 'tiny' systems I have :/
>>
>> Indeed, with that definition of "medium", large must be truly impressive!
>>
>> 							Thanx, Paul
>>
>>>> On a large system (4k), the perf tools get an error message (sorry
>>>> don't have it handy at the moment) the basically implies that the
>>>> perf config option is not set.  Again, I wanted to remove the new
>>>> NMI handler to insure that it wasn't doing something weird, and
>>>> it wasn't.
>>>
>>> Cute.. 
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>