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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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

> 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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