Re: [PATCH v7 0/7] arm64: Add framework to turn an IPI as NMI

[Chen-Yu Tsai]

On Mon, Nov 2, 2020 at 7:46 PM Sumit Garg <sumit.garg@xxxxxxxxxx> 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:
> - NMI backtrace can leverage IPI turned as NMI to get a backtrace of CPU
>   stuck in hard lockup using magic SYSRQ.
> - kgdb relies on NMI support to round up CPUs which are stuck in hard
>   lockup state with interrupts disabled.
>
> This patch-set adds framework to turn an IPI as NMI which can be triggered
> as a pseudo NMI which in turn invokes registered NMI handlers.
>
> After this patch-set we should be able to get a backtrace for a CPU
> stuck in HARDLOCKUP. Have a look at an examples below from a hard lockup
> testcase run on Developerbox:
>
> $ echo HARDLOCKUP > /sys/kernel/debug/provoke-crash/DIRECT
>
> NMI backtrace:
> ==============
>
> # Issue Magic SysRq to dump backtrace
>
> [  376.894502] NMI backtrace for cpu 8
> [  376.894506] CPU: 8 PID: 555 Comm: bash Not tainted 5.9.0-rc3-00740-g06ff047-dirty #242
> [  376.894510] Hardware name: Socionext SynQuacer E-series DeveloperBox, BIOS build #73 Apr  6 2020
> [  376.894514] pstate: 40000005 (nZcv daif -PAN -UAO BTYPE=--)
> [  376.894517] pc : lkdtm_HARDLOCKUP+0x8/0x18
> [  376.894520] lr : lkdtm_do_action+0x24/0x30
> [  376.894524] sp : ffff800012cebd20
> [  376.894527] pmr_save: 00000060
> [  376.894530] x29: ffff800012cebd20 x28: ffff000875ae8000
> [  376.894540] x27: 0000000000000000 x26: 0000000000000000
> [  376.894550] x25: 000000000000001a x24: ffff800012cebe40
> [  376.894560] x23: 000000000000000b x22: ffff800010fc5040
> [  376.894569] x21: ffff000878b61000 x20: ffff8000113b2870
> [  376.894579] x19: 000000000000001b x18: 0000000000000010
> [  376.894588] x17: 0000000000000000 x16: 0000000000000000
> [  376.894598] x15: ffff000875ae8470 x14: 00000000000002ad
> [  376.894613] x13: 0000000000000000 x12: 0000000000000000
> [  376.894622] x11: 0000000000000007 x10: 00000000000009c0
> [  376.894631] x9 : ffff800012ceba80 x8 : ffff000875ae8a20
> [  376.894641] x7 : ffff00087f6b3280 x6 : ffff00087f6b3200
> [  376.894651] x5 : 0000000000000000 x4 : ffff00087f6a91f8
> [  376.894660] x3 : ffff00087f6b0120 x2 : 1aa310cec69eb500
> [  376.894670] x1 : 0000000000000000 x0 : 0000000000000060
> [  376.894679] Call trace:
> [  376.894683]  lkdtm_HARDLOCKUP+0x8/0x18
> [  376.894686]  direct_entry+0x124/0x1c0
> [  376.894689]  full_proxy_write+0x60/0xb0
> [  376.894693]  vfs_write+0xf0/0x230
> [  376.894696]  ksys_write+0x6c/0xf8
> [  376.894699]  __arm64_sys_write+0x1c/0x28
> [  376.894703]  el0_svc_common.constprop.0+0x74/0x1f0
> [  376.894707]  do_el0_svc+0x24/0x90
> [  376.894710]  el0_sync_handler+0x180/0x2f8
> [  376.894713]  el0_sync+0x158/0x180
>
> KGDB:
> =====
>
> # Enter kdb via Magic SysRq
>
> [6]kdb> btc
> btc: cpu status: Currently on cpu 6
> Available cpus: 0-5(I), 6, 7(I), 8, 9-23(I)
> <snip>
> Stack traceback for pid 555
> 0xffff000875ae8000      555      554  1    8   R  0xffff000875ae89c0  bash
> CPU: 8 PID: 555 Comm: bash Not tainted 5.9.0-rc3-00740-g06ff047-dirty #242
> Hardware name: Socionext SynQuacer E-series DeveloperBox, BIOS build #73 Apr  6 2020
> Call trace:
>  dump_backtrace+0x0/0x1a0
>  show_stack+0x18/0x28
>  dump_stack+0xc0/0x11c
>  kgdb_cpu_enter+0x648/0x660
>  kgdb_nmicallback+0xa0/0xa8
>  ipi_kgdb_nmicallback+0x24/0x30
>  ipi_nmi_handler+0x48/0x60
>  handle_percpu_devid_fasteoi_ipi+0x74/0x88
>  generic_handle_irq+0x30/0x48
>  handle_domain_nmi+0x48/0x80
>  gic_handle_irq+0x18c/0x34c
>  el1_irq+0xcc/0x180
>  lkdtm_HARDLOCKUP+0x8/0x18
>  direct_entry+0x124/0x1c0
>  full_proxy_write+0x60/0xb0
>  vfs_write+0xf0/0x230
>  ksys_write+0x6c/0xf8
>  __arm64_sys_write+0x1c/0x28
>  el0_svc_common.constprop.0+0x74/0x1f0
>  do_el0_svc+0x24/0x90
>  el0_sync_handler+0x180/0x2f8
>  el0_sync+0x158/0x180
> <snip>
>
> Changes in v7:
> - Add a new library function: kgdb_smp_call_nmi_hook() to expose fallback
>   mechanism to arch specific code.
> - Addressed other misc comments from Daniel.
>
> Changes in v6:
> - Two new patches: #4 and #6 which adds runtime fallback framework for
>   sysrq backtrace and kgdb roundup features.
> - Reversed order of NMI backtrace and kgdb roundup feaure patches.
> - Addressed other misc. comments from Marc.
> - I haven't picked any tags from v5 since I think there is major rework
>   involved. Masayoshi, could you please confirm if these features still
>   work for you?
>
> Changes in v5:
> - Rebased to head of upstream master.
> - Remove redundant invocation of ipi_nmi_setup().
> - Addressed misc. comments.
>
> Changes in v4:
> - Move IPI NMI framework to a separate file.
> - Get rid of hard-coded IPI_CALL_NMI_FUNC allocation.
> - Add NMI backtrace support leveraged via magic SYSRQ.
>
> Changes in v3:
> - Rebased to Marc's latest IPIs patch-set [1].
>
> [1] https://lkml.org/lkml/2020/9/1/603
>
> Changes since RFC version [1]:
> - Switch to use generic interrupt framework to turn an IPI as NMI.
> - Dependent on Marc's patch-set [2] which turns IPIs into normal
>   interrupts.
> - Addressed misc. comments from Doug on patch #4.
> - Posted kgdb NMI printk() fixup separately which has evolved since
>   to be solved using different approach via changing kgdb interception
>   of printk() in common printk() code (see patch [3]).
>
> [1] https://lkml.org/lkml/2020/4/24/328
> [2] https://lkml.org/lkml/2020/5/19/710
> [3] https://lkml.org/lkml/2020/5/20/418
>
> Sumit Garg (7):
>   arm64: Add framework to turn IPI as NMI
>   irqchip/gic-v3: Enable support for SGIs to act as NMIs
>   arm64: smp: Assign and setup an IPI as NMI
>   nmi: backtrace: Allow runtime arch specific override
>   arm64: ipi_nmi: Add support for NMI backtrace
>   kgdb: Expose default CPUs roundup fallback mechanism
>   arm64: kgdb: Roundup cpus using IPI as NMI

Tested-by: Chen-Yu Tsai <wens@xxxxxxxx>

on an ROC-RK3399-PC.