Messages in this thread | | | Date | Tue, 30 May 2017 18:28:38 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter |
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On Tue, May 30, 2017 at 06:51:28AM -0700, Andi Kleen wrote: > On Tue, May 30, 2017 at 11:25:23AM +0200, Peter Zijlstra wrote: > > On Sun, May 28, 2017 at 01:31:09PM -0700, Stephane Eranian wrote: > > > Ultimately, I would like to see the watchdog move out of the PMU. That > > > is the only sensible solution. > > > You just need a resource able to interrupt on NMI or you handle > > > interrupt masking in software as has > > > been proposed on LKML. > > > > So even if we do the soft masking, we still need to deal with regions > > where the interrupts are disabled. Once an interrupt hits the soft mask > > we still hardware mask. > > > > So to get full and reliable coverage we still need an NMI source. > > You would only need a single one per system however, not one per CPU. > RCU already tracks all the CPUs, all we need is a single NMI watchdog > that makes sure RCU itself does not get stuck. > > So we just have to find a single watchdog somewhere that can trigger > NMI.
But then you have to IPI broadcast the NMI, which is less than ideal.
RCU doesn't have that problem because the quiescent state is a global thing. CPU progress, which is what the NMI watchdog tests, is very much per logical CPU though.
> > I agree that it would be lovely to free up the one counter though. > > One option is to use the TCO watchdog in the chipset instead. > Unfortunatley it's not an universal solution because some BIOS lock > the TCO watchdog for their own use. But if you have a BIOS that > doesn't do that it should work.
I suppose you could also route the HPET to the NMI vector and other similar things. Still, you're then stuck with IPI broadcasts, which suck.
> > One other approach is running the watchdog off of _any_ PMI, then all we > > need to ensure is that PMIs happen semi regularly. There are two cases > > where this becomes 'interesting': > > Seems fairly complex.
Yes.. :/
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