Messages in this thread | | | Date | Thu, 12 Sep 2013 11:35:41 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 9/9] x86/UV: Add ability to disable UV NMI handler |
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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.. > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > >
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