Re: [PATCH 0/2] Add HPET NMI Watchdog support

[Alexander Graf <graf@amazon.com>]

On 02.02.26 18:43, Alexander Graf wrote:
> The current NMI watchdog relies on performance counters and consistently
> occupies one on each CPU. When running virtual machines, we want to pass
> performance counters to virtual machines so they can make use of them.
> In addition the host system wants to use performance counters to check
> the system to identify when anything looks abnormal, such as split
> locks.
>
> That makes PMCs a precious resource. So any PMC we can free up is a PMC
> we can use for something useful. That made me look at the NMI watchdog.
>
> The PMC based NMI watchdog implementation does not actually need any
> performance counting. It just needs a per-CPU NMI timer source. X86
> systems can make anything that emits an interrupt descriptor (IOAPIC,
> MSI(-X), etc) become an NMI source. So any time goes. Including the
> HPET. And while they can't really operate per-CPU, in almost all cases
> you only really want the NMI on *all* CPUs, rather than per-CPU.
>
> So I took a stab at building an HPET based NMI watchdog. In my (QEMU
> based) testing, it's fully functional and can successfully detect when
> CPUs get stuck. It even survives suspend/resume cycles.
>
> For now, its enablement is a config time option because the hardlockup
> framework does not support dynamic switching of multiple detectors.
> That's ok for our use case. But maybe something for the interested
> reader to tackle eventually :).
>
> You can enable the HPET watchdog by default by setting
>
>    CONFIG_HARDLOCKUP_DETECTOR_HPET_DEFAULT=y
>
> or passing "hpet=watchdog" to the kernel command line. When active, it
> will emit a kernel log message to indicate it works:
>
>    [    0.179176] hpet: HPET watchdog initialized on timer 0, GSI 2
>
> The HPET can only be in either watchdog or generic mode. I am a bit
> worried about IO-APIC pin allocation logic, so I opted to reuse the
> generic timer pin. And that means I'm effectively breaking the normal
> interrupt delivery path. so the easy way out was to say when watchdog is
> active, PIT and HPET are not available as timer sources. Which is ok on
> modern systems. There are way too many (unreliable) timer sources on x86
> already. Trimming a few surely won't hurt.
>
> I'm open to inputs on how to make the HPET multi-purpose though, in case
> anyone feels strongly about it.


Sorry for the resend. I caught an issue while sending out the series, 
hit ctrl-c before thinking and suddenly had a half sent series. Discard 
this one. Happy review on the real, full one :)


Alex




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