Re: [PATCH v7 4/4] nmi_backtrace: generate one-line reports for idle cpus

Mark Rutland
2016-08-08T16:48:44Z

Hi,

[adding Lorenzo]

On Mon, Aug 08, 2016 at 12:03:38PM -0400, Chris Metcalf wrote:
> When doing an nmi backtrace of many cores, most of which are idle,
> the output is a little overwhelming and very uninformative. Suppress
> messages for cpus that are idling when they are interrupted and just
> emit one line, "NMI backtrace for N skipped: idling at pc 0xNNN".
>
> We do this by grouping all the cpuidle code together into a new
> .cpuidle.text section, and then checking the address of the
> interrupted PC to see if it lies within that section.
>
> This commit suitably tags x86, arm64, and tile idle routines,
> and only adds in the minimal framework for other architectures.

> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 659963d40bb4..fe7f93b7b11b 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -122,6 +122,7 @@ SECTIONS
> ENTRY_TEXT
> TEXT_TEXT
> SCHED_TEXT
> + CPUIDLE_TEXT
> LOCK_TEXT
> KPROBES_TEXT
> HYPERVISOR_TEXT
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index 5bb61de23201..64f088ca3192 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -48,11 +48,13 @@
> *
> * Idle the processor (wait for interrupt).
> */
> + .pushsection ".cpuidle.text","ax"
> ENTRY(cpu_do_idle)
> dsb sy // WFI may enter a low-power mode
> wfi
> ret
> ENDPROC(cpu_do_idle)
> + .popsection

>From a quick scan it looks like we only call this with interrupts
disabled, and we have no NMI. So shouldn't we be annotating
arch_cpu_idle(), which calls this and subsequently enables interrupts?

I'm also not sure what you need to do for PSCI, which is the preferred
(FW-backed) idle mechanism for arm64. The infrastrucure for that is
spread over a few files:

arch/arm64/kernel/sleep.S
arch/arm64/kernel/smccc-call.S
arch/arm64/kernel/suspend.c
drivers/cpuidle/cpuidle-arm.c
drivers/firmware/psci.c

I'm not sure where we'd be an an interruptible state, and therefore I'm
not immediately sure what we should annotate.

Thanks,
Mark.