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

[Jacob Pan <jacob.jun.pan@intel.com>]

On Tue, 18 Jun 2019 01:08:06 +0200 (CEST)
Thomas Gleixner <tglx@linutronix.de> wrote:

> Stephane,
> 
> On Mon, 17 Jun 2019, Stephane Eranian wrote:
> > On Mon, Jun 17, 2019 at 1:25 AM Thomas Gleixner
> > <tglx@linutronix.de> 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, 
I agree. modify irte might be done with cmpxchg_double() but the queued
invalidation interface for IRTE cache flush is shared with DMA and
requires holding a spinlock for enque descriptors, QI tail update etc.

Also, reserving & manipulating IRTE slot for hpet via backdoor might not
be needed if the HPET PCI BDF (found in ACPI) can be utilized. But it
might need more work to add a fake PCI device for HPET.

> 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@sgi.com>
>         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.
> 
> Thanks,
> 
> 	tglx
> 
> 
> 
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[Jacob Pan]
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