Re: [PATCH] perf/x86/amd: Change NMI latency mitigation to use a timestamp

[Thomas Gleixner]

On Thu, 1 Aug 2019, Lendacky, Thomas wrote:
> On 8/1/19 4:16 PM, Peter Zijlstra wrote:
> > On Thu, Aug 01, 2019 at 06:57:41PM +0000, Lendacky, Thomas wrote:
> >> From: Tom Lendacky <thomas.lendacky@xxxxxxx>
> >>
> >> It turns out that the NMI latency workaround from commit 6d3edaae16c6
> >> ("x86/perf/amd: Resolve NMI latency issues for active PMCs") ends up
> >> being too conservative and results in the perf NMI handler claiming NMIs
> >> to easily on AMD hardware when the NMI watchdog is active.
> >>
> >> This has an impact, for example, on the hpwdt (HPE watchdog timer) module.
> >> This module can produce an NMI that is used to reset the system. It
> >> registers an NMI handler for the NMI_UNKNOWN type and relies on the fact
> >> that nothing has claimed an NMI so that its handler will be invoked when
> >> the watchdog device produces an NMI. After the referenced commit, the
> >> hpwdt module is unable to process its generated NMI if the NMI watchdog is
> >> active, because the current NMI latency mitigation results in the NMI
> >> being claimed by the perf NMI handler.
> >>
> >> Update the AMD perf NMI latency mitigation workaround to, instead, use a
> >> window of time. Whenever a PMC is handled in the perf NMI handler, set a
> >> timestamp which will act as a perf NMI window. Any NMIs arriving within
> >> that window will be claimed by perf. Anything outside that window will
> >> not be claimed by perf. The value for the NMI window is set to 100 msecs.
> >> This is a conservative value that easily covers any NMI latency in the
> >> hardware. While this still results in a window in which the hpwdt module
> >> will not receive its NMI, the window is now much, much smaller.
> > 
> > Blergh, I so hate all this. The proposed patch is basically duct tape.
> 
> Yeah, I'm not a fan either.
> 
> > 
> > The horribly retarded x86 NMI infrastructure strikes again :/
> > 
> > Tom; do you have any idea how expensive it is to twiddle CR8 and play
> > games with interrupt priorities instead of piling world + dog on this
> > one NMI line? (as compared to CLI/STI)
> 
> I can check on that.  What are you thinking?

Avoid the whole NMI mess, make the PMC interrupt a proper vector in the
highest prio bucket and instead of using CLI/STI use CR8. That would have
the additional advantage that we could prevent perf "NMI" then occsionally :)

Thanks,

	tglx