Re: [RFC Patch v1 0/4] arm64: Introduce new IPI as IPI_CALL_NMI_FUNC

[Sumit Garg <sumit.garg@linaro.org>]

On Sat, 25 Apr 2020 at 02:20, Doug Anderson <dianders@chromium.org> wrote:
>
> Hi,
>
> On Fri, Apr 24, 2020 at 4:11 AM Sumit Garg <sumit.garg@linaro.org> wrote:
> >
> > With pseudo NMIs support available its possible to configure SGIs to be
> > triggered as pseudo NMIs running in NMI context. And kernel features
> > such as kgdb relies on NMI support to round up CPUs which are stuck in
> > hard lockup state with interrupts disabled.
> >
> > This patch-set adds support for IPI_CALL_NMI_FUNC which can be triggered
> > as a pseudo NMI which in turn is leveraged via kgdb to round up CPUs.
> >
> > After this patch-set we should be able to get a backtrace for a CPU
> > stuck in HARDLOCKUP. Have a look at an example below from a testcase run
> > on Developerbox:
> >
> > $ echo HARDLOCKUP > /sys/kernel/debug/provoke-crash/DIRECT
> >
> > # Enter kdb via Magic SysRq
> >
> > [11]kdb> btc
> > btc: cpu status: Currently on cpu 11
> > Available cpus: 0-10(I), 11, 12(I), 13, 14-23(I)
> > <snip>
> > Stack traceback for pid 623
> > 0xffff00086a644600      623      622  1   13   R  0xffff00086a644fc0  bash
> > CPU: 13 PID: 623 Comm: bash Not tainted 5.7.0-rc2 #27
> > Hardware name: Socionext SynQuacer E-series DeveloperBox, BIOS build #73 Apr  6 2020
> > Call trace:
> >  dump_backtrace+0x0/0x198
> >  show_stack+0x18/0x28
> >  dump_stack+0xb8/0x100
> >  kgdb_cpu_enter+0x5c0/0x5f8
> >  kgdb_nmicallback+0xa0/0xa8
> >  handle_IPI+0x190/0x200
> >  gic_handle_irq+0x2b8/0x2d8
> >  el1_irq+0xcc/0x180
> >  lkdtm_HARDLOCKUP+0x8/0x18
> >  direct_entry+0x124/0x1c0
> >  full_proxy_write+0x60/0xb0
> >  __vfs_write+0x1c/0x48
> >  vfs_write+0xe4/0x1d0
> >  ksys_write+0x6c/0xf8
> >  __arm64_sys_write+0x1c/0x28
> >  el0_svc_common.constprop.0+0x74/0x1f0
> >  do_el0_svc+0x24/0x90
> >  el0_sync_handler+0x178/0x2b8
> >  el0_sync+0x158/0x180
> > <snip>
> >
> > Looking forward to your comments/feedback.
> >
> > Sumit Garg (4):
> >   arm64: smp: Introduce a new IPI as IPI_CALL_NMI_FUNC
> >   irqchip/gic-v3: Add support to handle SGI as pseudo NMI
> >   irqchip/gic-v3: Enable arch specific IPI as pseudo NMI
> >   arm64: kgdb: Round up cpus using IPI_CALL_NMI_FUNC
> >
> >  arch/arm64/include/asm/hardirq.h |  2 +-
> >  arch/arm64/include/asm/smp.h     |  1 +
> >  arch/arm64/kernel/kgdb.c         | 15 +++++++++++++++
> >  arch/arm64/kernel/smp.c          | 36 +++++++++++++++++++++++++++++++++++-
> >  drivers/irqchip/irq-gic-v3.c     | 36 +++++++++++++++++++++++++++++++-----
> >  5 files changed, 83 insertions(+), 7 deletions(-)
>
> This is amazing!
>
> * picked your patches back to my current 5.4 tree
> * turned on "CONFIG_ARM64_PSEUDO_NMI"
> * set the "irqchip.gicv3_pseudo_nmi=1" command line
>
> ...and bam I can trace on the locked up CPU instead of being left in the dark.
>
> I'm not sure I'm going to be too much use in actually doing the review
> of the code since I'm not really an expert at how SGIs work (it took
> me a while to realize that it must stand for software generated
> interrupts) nor the bowels of the GIC.  I tried to do what little
> review I could.
>
> In any case, I'll keep this in my local patch stack for now and keep
> testing it to make sure I don't notice any weird problems.

Thanks for your review and testing.

-Sumit

>
> -Doug