Messages in this thread | | | Date | Mon, 7 Aug 2023 11:41:24 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v9 0/7] arm64: Add debug IPI for backtraces / kgdb; try to use NMI for it |
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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.
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. I'd also strongly prefer if we could have separate IPI_CPU_BACKTRACE and IPI_CPU_KGDB IPIs, 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).
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.
> 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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