Re: [RFC PATCH v4 20/21] iommu/vt-d: hpet: Reserve an interrupt remampping table entry for watchdog

[Ricardo Neri]

On Tue, Jun 18, 2019 at 01:08:06AM +0200, Thomas Gleixner wrote:
> Stephane,
> 
> On Mon, 17 Jun 2019, Stephane Eranian wrote:
> > On Mon, Jun 17, 2019 at 1:25 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > > Great that there is no trace of any mail from Andi or Stephane about this
> > > on LKML. There is no problem with talking offlist about this stuff, but
> > > then you should at least provide a rationale for those who were not part of
> > > the private conversation.
> > >
> > Let me add some context to this whole patch series. The pressure on the
> > core PMU counters is increasing as more people want to use them to
> > measure always more events. When the PMU is overcommitted, i.e., more
> > events than counters for them, there is multiplexing. It comes with an
> > overhead that is too high for certain applications. One way to avoid this
> > is to lower the multiplexing frequency, which is by default 1ms, but that
> > comes with loss of accuracy. Another approach is to measure only a small
> > number of events at a time and use multiple runs, but then you lose
> > consistent event view. Another approach is to push for increasing the
> > number of counters. But getting new hardware counters takes time. Short
> > term, we can investigate what it would take to free one cycle-capable
> > counter which is commandeered by the hard lockup detector on all X86
> > processors today. The functionality of the watchdog, being able to get a
> > crash dump on kernel deadlocks, is important and we cannot simply disable
> > it. At scale, many bugs are exposed and thus machines
> > deadlock. Therefore, we want to investigate what it would take to move
> > the detector to another NMI-capable source, such as the HPET because the
> > detector does not need high low granularity timer and interrupts only
> > every 2s.
> 
> I'm well aware about the reasons for this.
> 
> > Furthermore, recent Intel erratum, e.g., the TSX issue forcing the TFA
> > code in perf_events, have increased the pressure even more with only 3
> > generic counters left. Thus, it is time to look at alternative ways of
> > getting a hard lockup detector (NMI watchdog) from another NMI source
> > than the PMU. To that extent, I have been discussing about alternatives.
> >
> > Intel suggested using the HPET and Ricardo has been working on
> > producing this patch series. It is clear from your review
> > that the patches have issues, but I am hoping that they can be
> > resolved with constructive feedback knowing what the end goal is.
> 
> Well, I gave constructive feedback from the very first version on. But
> essential parts of that feedback have been ignored for whatever reasons.
> 
> > As for the round-robin changes, yes, we discussed this as an alternative
> > to avoid overloading CPU0 with handling all of the work to broadcasting
> > IPI to 100+ other CPUs.
> 
> I can understand the reason why you don't want to do that, but again, I
> said way before this was tried that changing affinity from NMI context with
> the IOMMU cannot work by just calling into the iommu code and it needs some
> deep investigation with the IOMMU wizards whether a preallocated entry can
> be used lockless (including the subsequently required flush).
> 
> The outcome is that the change was implemented by simply calling into
> functions which I told that they cannot be called from NMI context.
> 
> Unless this problem is not solved and I doubt it can be solved after
> talking to IOMMU people and studying manuals, the round robin mechanics in
> the current form are not going to happen. We'd need a SMI based lockup
> detector to debug the resulting livelock wreckage.
> 
> There are two possible options:
> 
>   1) Back to the IPI approach
> 
>      The probem with broadcast is that it sends IPIs one by one to each
>      online CPU, which sums up with a large number of CPUs.
> 
>      The interesting question is why the kernel does not utilize the all
>      excluding self destination shorthand for this. The SDM is not giving
>      any information.
> 
>      But there is a historic commit which is related and gives a hint:
> 
>         commit e77deacb7b078156fcadf27b838a4ce1a65eda04
>         Author: Keith Owens <kaos@xxxxxxx>
>         Date:   Mon Jun 26 13:59:56 2006 +0200
> 
>         [PATCH] x86_64: Avoid broadcasting NMI IPIs
>     
>         On some i386/x86_64 systems, sending an NMI IPI as a broadcast will
>     	reset the system.  This seems to be a BIOS bug which affects
>     	machines where one or more cpus are not under OS control.  It
>     	occurs on HT systems with a version of the OS that is not compiled
>     	without HT support.  It also occurs when a system is booted with
>     	max_cpus=n where 2 <= n < cpus known to the BIOS.  The fix is to
>     	always send NMI IPI as a mask instead of as a broadcast.
> 
>     I can see the issue with max_cpus and that'd be trivial to solve by
>     disabling the HPET watchdog when maxcpus < num_present_cpus is on the
>     command line (That's broken anyway with respect to MCEs. See the stupid
>     dance we need to do for 'nosmt').
> 
>     Though the HT part of the changelog is unparseable garbage but might be
>     a cryptic hint to the 'nosmt' mess. Though back then we did not have
>     a way to disable the siblings (or did we?). Weird...
> 
>     It definitely would be worthwhile to experiment with that. if we could
>     use shorthands (also for regular IPIs) that would be a great
>     improvement in general and would nicely solve that NMI issue. Beware of
>     the dragons though.
> 
>   2) Delegate round robin to irq_work
> 
>     Use the same mechanism as perf for stuff which needs to be done outside
>     of NMI context.
> 
>     That can solve the issue, but the drawback is that in case the NMI hits
>     a locked up interrupt disabled region the affinity stays on the same
>     CPU as the regular IPI which kicks the irq work is not going to be
>     handled.  Might not be a big issue as we could detect the situation
>     that the IPI comes back to the same CPU. Not pretty and lots of nasty
>     corner case and race condition handling.
> 
>     There is another issue with that round robin scheme as I pointed out to
>     Ricardo:
> 
>       With a small watchdog threshold and tons of CPUs the time to switch
>       the affinity becomes short. That brings the HPET reprogramming (in
>       case of oneshot) into the SMI endagered zone and aside of that it
>       will eat performance as well because with lets say 1 second threshold
>       and 1000 CPUs we are going to flush the interrupt remapping
>       table/entry once per millisecond. No idea how big the penalty is, but
>       it's certainly not free.
> 
>     One possible way out would be to use a combined approach of building
>     CPU groups (lets say 8) where one of the CPUs gets the NMI and IPIs the
>     other 7 and then round robins to the next group. Whether that's any
>     better, I can't tell.
> 
> Sorry that I can't come up with the magic cure and just can provide more
> questions than answers.

I am sorry it took this long to reply to this thread. I'd like to kindly
ask for your feedback on my proposed changes for the following iteration.

 * Jacob and Ashok, who work on IOMMU stuff, agreed that updating the
   interrupt remapping table in NMI context is not possible as we would
   always fall into locking issues. Hence, as you suggest, I will defer
   this to an irq_work (and add checks in case the interrupt affinity did
   not change).

 * You have said in the past that you would not like to have a
   request_nmi() interface for x86. For !IRQ_REMAP this is not a problem
   as the MSI message can be programmed directly in the hardlockup
   detector. However, for IRQ_REMAP, I would need to use request_irq()
   and possibly have an interrupt handler that does nothing. The NMI
   handler is still needed. This looks a little awkward but it does
   allow to use the existing IRQ subsystem and hierarchy.

 * Since in x86 there is not a IRQF_NMI flag, the interrupt remapping
   driver needs a way to identify the special case in which the entity
   requesting the interrupt is the HPET hardlockup detector. This can be
   done in the interrupt remapping driver .alloc() function. It would
   look at the irq_alloc_info to determine if it is of type
   IRQ_ALLOC_TYPE_HPET. If this is the case, the data of the associated
   HPET channel is available. Such channel had been previously reserved
   to be used by the HPET initalization code and added to the
   IR-HPET-MSI domain. Once identified, the interrupt remapping driver
   changes the delivery mode to NMI before creating the interrupt
   remapping table entry. Furthermore, this will not break the AMD
   driver as all it has to do is implement the same one-line change.

 * In order to avoid HPET NMI interrupts that are too close together, as
   you described earlier, a mixed mechanism can be used in which groups
   of N CPUs receive IPIs (using your updated shorthands). The affinity of
   the HPET interrupt would only target every (N+1)'th CPU.

I have a quick-and-dirty implementation of this but wanted to check with
you first if this sounds reasonable.

Thanks and BR,
Ricardo