[PATCH v9 0/7] arm64: Add debug IPI for backtraces / kgdb; try to use NMI for it

[Sumit Garg]

Hi Doug,

On Fri, 2 Jun 2023 at 03:07, Douglas Anderson <dianders at chromium.org> wrote:
>
> This is an attempt to resurrect Sumit's old patch series [1] that
> allowed us to use the arm64 pseudo-NMI to get backtraces of CPUs and
> also to round up CPUs in kdb/kgdb. The last post from Sumit that I
> could find was v7, so I started my series at v8. I haven't copied all
> of his old changelongs here, but you can find them from the link.
>
> I'm really looking for a way to land this patch series. In response to
> v8, Mark Rutland indicated [2] that he was worried about the soundness
> of pseudo NMI. Those definitely need to get fixed, but IMO this patch
> series could still land in the meantime. That would at least let
> people test with it.

+1

>
> Request for anyone reading this: please help indicate your support of
> this patch series landing by replying, even if you don't have the
> background for a full review. My suspicion is that there are a lot of
> people who agree that this would be super useful to get landed.
>

You can ofcourse count me in here. This will certainly bring NMI
debugging capabilities to arm64 and I have been advocating for that
since the advent of pseudo NMIs on arm64.

> Since v8, I have cleaned up this patch series by integrating the 10th
> patch from v8 [3] into the whole series. As part of this, I renamed
> the "NMI IPI" to the "debug IPI" since it could now be backed by a
> regular IPI in the case that pseudo NMIs weren't available. With the
> fallback, this allowed me to drop some extra patches from the
> series. This feels (to me) to be pretty clean and hopefully others
> agree. Any patch I touched significantly I removed Masayoshi and
> Chen-Yu's tags from.
>
> ...also in v8, I reorderd the patches a bit in a way that seemed a
> little cleaner to me.

This cleanup looks good to me.

-Sumit

>
> Since v7, I have:
> * Addressed the small amount of feedback that was there for v7.
> * Rebased.
> * Added a new patch that prevents us from spamming the logs with idle
>   tasks.
> * Added an extra patch to gracefully fall back to regular IPIs if
>   pseudo-NMIs aren't there.
>
> It can be noted that this patch series works very well with the recent
> "hardlockup" patches that have landed through Andrew Morton's tree and
> are currently in linuxnext. It works especially well with the "buddy"
> lockup detector.
>
> [1] https://lore.kernel.org/linux-arm-kernel/1604317487-14543-1-git-send-email-sumit.garg@linaro.org/
> [2] https://lore.kernel.org/lkml/ZFvGqD%2F%2Fpm%2FlZb+p@FVFF77S0Q05N.cambridge.arm.com/
> [3] https://lore.kernel.org/r/20230419155341.v8.10.Ic3659997d6243139d0522fc3afcdfd88d7a5f030@changeid/
>
> Changes in v9:
> - Add a warning if we don't have enough IPIs for the NMI IPI
> - Added comments that we might not be using NMI always.
> - Added missing "inline"
> - Added to commit message that this doesn't catch all cases.
> - Fold in v8 patch #10 ("Fallback to a regular IPI if NMI isn't enabled")
> - Moved header file out of "include" since it didn't need to be there.
> - Remove arm64_supports_nmi()
> - Remove fallback for when debug IPI isn't available.
> - Renamed "NMI IPI" to "debug IPI" since it might not be backed by NMI.
> - Update commit description
> - arch_trigger_cpumask_backtrace() no longer returns bool
>
> Changes in v8:
> - "Provide a stub kgdb_nmicallback() if !CONFIG_KGDB" new for v8
> - "Tag the arm64 idle functions as __cpuidle" new for v8
> - Removed "#ifdef CONFIG_SMP" since arm64 is always SMP
> - debug_ipi_setup() and debug_ipi_teardown() no longer take cpu param
>
> Douglas Anderson (2):
>   arm64: idle: Tag the arm64 idle functions as __cpuidle
>   kgdb: Provide a stub kgdb_nmicallback() if !CONFIG_KGDB
>
> Sumit Garg (5):
>   irqchip/gic-v3: Enable support for SGIs to act as NMIs
>   arm64: Add framework for a debug IPI
>   arm64: smp: Assign and setup the debug IPI
>   arm64: ipi_debug: Add support for backtrace using the debug IPI
>   arm64: kgdb: Roundup cpus using the debug IPI
>
>  arch/arm64/include/asm/irq.h  |   3 +
>  arch/arm64/kernel/Makefile    |   2 +-
>  arch/arm64/kernel/idle.c      |   4 +-
>  arch/arm64/kernel/ipi_debug.c | 102 ++++++++++++++++++++++++++++++++++
>  arch/arm64/kernel/ipi_debug.h |  13 +++++
>  arch/arm64/kernel/kgdb.c      |  14 +++++
>  arch/arm64/kernel/smp.c       |  11 ++++
>  drivers/irqchip/irq-gic-v3.c  |  29 +++++++---
>  include/linux/kgdb.h          |   1 +
>  9 files changed, 168 insertions(+), 11 deletions(-)
>  create mode 100644 arch/arm64/kernel/ipi_debug.c
>  create mode 100644 arch/arm64/kernel/ipi_debug.h
>
> --
> 2.41.0.rc2.161.g9c6817b8e7-goog
>