Messages in this thread | | | From | Sumit Garg <> | Date | Mon, 7 Aug 2023 18:16:24 +0530 | Subject | Re: [PATCH v9 0/7] arm64: Add debug IPI for backtraces / kgdb; try to use NMI for it |
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On Mon, 7 Aug 2023 at 16:11, Mark Rutland <mark.rutland@arm.com> wrote: > > Hi Doug, > > Apologies for the delay. > > On Mon, Jul 24, 2023 at 08:55:44AM -0700, Doug Anderson wrote: > > On Thu, Jun 1, 2023 at 2:37 PM Douglas Anderson <dianders@chromium.org> wrote: > > I'm looking for some ideas on what to do to move this patch series > > forward. Thanks to Daniel, the kgdb patch is now in Linus's tree which > > hopefully makes this simpler to land. I guess there is still the > > irqchip dependency that will need to be sorted out, though... > > > > Even if folks aren't in agreement about whether this is ready to be > > enabled in production, I don't think anything here is super > > objectionable or controversial, is it? Can we land it? If you feel > > like it needs extra review, would it help if I tried to drum up some > > extra people to provide review feedback? > > Ignoring the soundness issues I mentioned before (which I'm slowly chipping > away at, and you're likely lucky enough to avoid in practice)... > > Having looked over the series, I think the GICv3 bit isn't quite right, but is > easy enough to fix. I've commented on the patch with what I think we should > have there.
Thanks for catching this and I agree with your proposed fix.
> > The only major thing otherwise from my PoV is the structure of the debug IPI > framework. I'm not keen on that being a separate body of code and I think it > should live in smp.c along with the other IPIs.
That's a fair point.
> I'd also strongly prefer if we > could have separate IPI_CPU_BACKTRACE and IPI_CPU_KGDB IPIs,
With current logic of single debug IPI, it is not required for a user to enable KGDB in order to use that IPI for backtrace. The original motivation for this logic was that the IPIs are a scarce resource on arm64 as per comments from Marc. So I am fine either way to keep them separate or unified.
> and I think we can > do that either by unifying IPI_CPU_STOP && IPI_CPU_CRASH_STOP or by reclaiming > IPI_WAKEUP by reusing a different IPI for the parking protocol (e.g. > IPI_RESCHEDULE).
That sounds like a good cleanup.
> > I think it'd be nice if the series could enable NMIs for backtrace and the > CPU_{,CRASH_}STOP cases, with KGDB being the bonus atop. That way it'd be > clearly beneficial for anyone trying to debug lockups even if they're not a > KGDB user. >
It's good to see other use-cases of IPIs turned into NMIs.
-Sumit
> > Also: in case it's interesting to anyone, I've been doing benchmarks > > on sc7180-trogdor devices in preparation for enabling this. On that > > platform, I did manage to see about 4% reduction in a set of hackbench > > numbers when fully enabling pseudo-NMI. However, when I instead ran > > Speedometer 2.1 I saw no difference. See: > > > > https://issuetracker.google.com/issues/197061987 > > Thanks for the pointer! > > I know that there are a couple of things that we could do to slightly improve > local_irq_*() when using pNMIs, though I suspect that the bulk of the cost > there will come from the necessary synchronization. > > Thanks, > Mark.
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