Messages in this thread | | | Date | Tue, 22 Mar 2011 12:11:18 -0500 | From | Jack Steiner <> | Subject | Re: [PATCH] x86, UV: Fix NMI handler for UV platforms |
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On Mon, Mar 21, 2011 at 03:37:40PM -0400, Don Zickus wrote: > On Mon, Mar 21, 2011 at 01:22:35PM -0500, Jack Steiner wrote: > > On Mon, Mar 21, 2011 at 01:51:10PM -0400, Don Zickus wrote: > > > On Mon, Mar 21, 2011 at 07:26:51PM +0300, Cyrill Gorcunov wrote: > > > > On 03/21/2011 07:14 PM, Ingo Molnar wrote: > > > > > > > > > > * Jack Steiner <steiner@sgi.com> wrote: > > > > > > > > > >> This fixes a problem seen on UV systems handling NMIs from the node controller. > > > > >> The original code used the DIE notifier as the hook to get to the UV NMI > > > > >> handler. This does not work if performance counters are active - the hw_perf > > > > >> code consumes the NMI and the UV handler is not called. > > > > > > Well that is a bug in the perf code. We have been dealing with 'perf' > > > swallowing NMIs for a couple of releases now. I think we got rid of most > > > of the cases (p4 and acme's core2 quad are the only cases I know that are > > > still an issue). > > > > > > I would much prefer to investigate the reason why this is happening > > > because the perf nmi handler is supposed to check the global interrupt bit > > > to determine if the perf counters caused the nmi or not otherwise fall > > > through to other handler like SGI's nmi button in this case. > > > > The patch that I posted is based on a RHEL6.1 patch that I'm running internally. > > Unless something has very recently changed in the RH sources, the perf > > NMI handler unconditionally returns NOTIFY_STOP if it handles an NMI. > > If no NMI was handled, it returns NOTIFY_DONE. This sometimes works > > and allows the platform generated NMI to be processed but if both NMI > > sources trigger at about he same time, the lower priority event > > will be lost. > > Not necessarily, if both are triggered, you should still get _two_ NMIs. > It may get processed in the wrong order but it should still get correctly > processed.
How certain are you that multiple NMIs triggered at about the same time will deliver discrete NMI events? I updated the patch so that I'm running with:
- no special code in traps.c (I removed the traps.c code that was in the patch I posted) - used die_notifier for calling the UV nmi handler - UV priority is higher than the hw_perf priority
Both hw_perf (perf top) & UV NMIs work correctly under light loads. However, if I run for 10 - 15 minutes injecting UV NMIs at a rate of about 30/min, "perf top" stops generating output. Strace shows that it continues to poll() but no data is received.
While "perf top" is hung, if I inject an NMI into the system in a way that will NOT be consumed by the UV nmi handler, "perf top" resumes output but will stop again after a few minutes.
AFAICT, the UV nmi handler is not consuming extra NMI interrupts. I can't rule out that I'm missing something but I don't see it.
Do you have any ideas or clues???
> > > > > The root cause of the problem is that architecturally, x86 does not > > have a way to identifies the source(s) that cause an NMI. If multiple > > events occur at about the same time, there is no way that I can see that the > > OS can detect it. > > There are registers we can check to see who owns trigger the NMI (at least > for the perf code, the SGI code maybe not, which is why I set it to a > lower priority to be a catch-all). > > I'm not aware of the x86 architecture dropping NMIs, so they should all > get processed. It is just a matter of which subsystems get determine if > they are the source of the NMI or not. > > > > > > > > > My first impression is the skip nmi logic in the perf handler is probably > > > accidentally thinking the SGI external nmi is the perf's 'extra' nmi it is > > > supposed to skip and thus swallows it. At least that is the impression I > > > > Agree > > > > > > > get from the RedHat bugzilla which says SGI is running 'perf top', getting > > > a hang, then pressing their nmi button to see the stack traces. > > > > > > Jack, > > > > > > I worked through a number of these issues upstream and I already talked to > > > George and Russ over here at RedHat about working through the issue over > > > here with them. They can help me get access to your box to help debug. > > > > Russ is right down the hall. > > Great! > > Cheers, > Don
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