Subject: [PATCH 11/15] arm64: kdump: implement machine_crash_shutdown()

From: [email-masked] (AKASHI Takahiro)
Date: Tue, 10 Nov 2015 10:23:56 +0900

On 11/07/2015 04:14 AM, Geoff Levand wrote:
> From: AKASHI Takahiro <[email-masked]>
>
> kdump calls machine_crash_shutdown() to shut down non-boot cpus and
> save registers' status in per-cpu ELF notes before starting the crash
> dump kernel. See kernel_kexec().
>
> ipi_cpu_stop() is a bit modified and used to support this behavior.

I've got some concerns of using ipi_cpu_stop().

> Signed-off-by: AKASHI Takahiro <[email-masked]>
> ---
>   arch/arm64/include/asm/kexec.h    | 34 +++++++++++++++++++++++++++++++++-
>   arch/arm64/kernel/machine_kexec.c | 31 +++++++++++++++++++++++++++++--
>   arch/arm64/kernel/smp.c           | 16 ++++++++++++++--
>   3 files changed, 76 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kexec.h b/arch/arm64/include/asm/kexec.h
> index 46d63cd..555a955 100644
> --- a/arch/arm64/include/asm/kexec.h
> +++ b/arch/arm64/include/asm/kexec.h
> @@ -30,6 +30,8 @@
>
>   #if !defined(__ASSEMBLY__)
>
> +extern bool in_crash_kexec;
> +
>   /**
>    * crash_setup_regs() - save registers for the panic kernel
>    *
> @@ -40,7 +42,37 @@
>   static inline void crash_setup_regs(struct pt_regs *newregs,
>   				    struct pt_regs *oldregs)
>   {
> -	/* Empty routine needed to avoid build errors. */
> +	if (oldregs) {
> +		memcpy(newregs, oldregs, sizeof(*newregs));
> +	} else {
> +		__asm__ __volatile__ (
> +			"stp	 x0,   x1, [%3, #16 *  0]\n"
> +			"stp	 x2,   x3, [%3, #16 *  1]\n"
> +			"stp	 x4,   x5, [%3, #16 *  2]\n"
> +			"stp	 x6,   x7, [%3, #16 *  3]\n"
> +			"stp	 x8,   x9, [%3, #16 *  4]\n"
> +			"stp	x10,  x11, [%3, #16 *  5]\n"
> +			"stp	x12,  x13, [%3, #16 *  6]\n"
> +			"stp	x14,  x15, [%3, #16 *  7]\n"
> +			"stp	x16,  x17, [%3, #16 *  8]\n"
> +			"stp	x18,  x19, [%3, #16 *  9]\n"
> +			"stp	x20,  x21, [%3, #16 * 10]\n"
> +			"stp	x22,  x23, [%3, #16 * 11]\n"
> +			"stp	x24,  x25, [%3, #16 * 12]\n"
> +			"stp	x26,  x27, [%3, #16 * 13]\n"
> +			"stp	x28,  x29, [%3, #16 * 14]\n"
> +			"str	x30,	   [%3, #16 * 15]\n"
> +			"mov	%0, sp\n"
> +			"adr	%1, 1f\n"
> +			"mrs	%2, spsr_el1\n"
> +		"1:"
> +			: "=r" (newregs->sp),
> +			  "=r" (newregs->pc),
> +			  "=r" (newregs->pstate)
> +			: "r"  (&newregs->regs)
> +			: "memory"
> +		);
> +	}
>   }
>
>   #endif /* !defined(__ASSEMBLY__) */
> diff --git a/arch/arm64/kernel/machine_kexec.c b/arch/arm64/kernel/machine_kexec.c
> index da28a26..d2d7e90 100644
> --- a/arch/arm64/kernel/machine_kexec.c
> +++ b/arch/arm64/kernel/machine_kexec.c
> @@ -9,6 +9,7 @@
>    * published by the Free Software Foundation.
>    */
>
> +#include <linux/kernel.h>
>   #include <linux/kexec.h>
>   #include <linux/of_fdt.h>
>   #include <linux/slab.h>
> @@ -23,6 +24,7 @@
>   extern const unsigned char arm64_relocate_new_kernel[];
>   extern const unsigned long arm64_relocate_new_kernel_size;
>
> +bool in_crash_kexec;
>   static unsigned long kimage_start;
>
>   /**
> @@ -203,13 +205,38 @@ void machine_kexec(struct kimage *kimage)
>   	 */
>
>   	cpu_soft_restart(virt_to_phys(cpu_reset),
> -		is_hyp_mode_available(),
> +		in_crash_kexec ? 0 : is_hyp_mode_available(),
>   		reboot_code_buffer_phys, kimage->head, kimage_start);
>
>   	BUG(); /* Should never get here. */
>   }
>
> +/**
> + * machine_crash_shutdown - shutdown non-boot cpus and save registers
> + */
>   void machine_crash_shutdown(struct pt_regs *regs)
>   {
> -	/* Empty routine needed to avoid build errors. */
> +	struct pt_regs dummy_regs;
> +	int cpu;
> +
> +	local_irq_disable();
> +
> +	in_crash_kexec = true;
> +
> +	/*
> +	 * clear and initialize the per-cpu info. This is necessary
> +	 * because, otherwise, slots for offline cpus would never be
> +	 * filled up. See smp_send_stop().
> +	 */
> +	memset(&dummy_regs, 0, sizeof(dummy_regs));
> +	for_each_possible_cpu(cpu)
> +		crash_save_cpu(&dummy_regs, cpu);
> +
> +	/* shutdown non-boot cpus */
> +	smp_send_stop();
> +
> +	/* for boot cpu */
> +	crash_save_cpu(regs, smp_processor_id());
> +
> +	pr_info("Starting crashdump kernel...\n");
>   }
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index dbdaacd..88aec66 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -37,6 +37,7 @@
>   #include <linux/completion.h>
>   #include <linux/of.h>
>   #include <linux/irq_work.h>
> +#include <linux/kexec.h>
>
>   #include <asm/alternative.h>
>   #include <asm/atomic.h>
> @@ -54,6 +55,8 @@
>   #include <asm/ptrace.h>
>   #include <asm/virt.h>
>
> +#include "cpu-reset.h"
> +
>   #define CREATE_TRACE_POINTS
>   #include <trace/events/ipi.h>
>
> @@ -679,8 +682,12 @@ static DEFINE_RAW_SPINLOCK(stop_lock);
>   /*
>    * ipi_cpu_stop - handle IPI from smp_send_stop()
>    */
> -static void ipi_cpu_stop(unsigned int cpu)
> +static void ipi_cpu_stop(unsigned int cpu, struct pt_regs *regs)
>   {
> +#ifdef CONFIG_KEXEC
> +	/* printing messages may slow down the shutdown. */
> +	if (!in_crash_kexec)
> +#endif
>   	if (system_state == SYSTEM_BOOTING ||
>   	    system_state == SYSTEM_RUNNING) {
>   		raw_spin_lock(&stop_lock);
> @@ -693,6 +700,11 @@ static void ipi_cpu_stop(unsigned int cpu)
>
>   	local_irq_disable();
>
> +#ifdef CONFIG_KEXEC
> +	if (in_crash_kexec)
> +		crash_save_cpu(regs, cpu);
> +#endif /* CONFIG_KEXEC */
> +
>   	while (1)
>   		cpu_relax();
>   }

cpu_relax() is defined as asm("yield"), and this puts all but boot cpu into
a infinite loop of nop (actually, whether nop or other depends on hw implementation).
Thus all the secondary cpus are still running busy loop even after crash dump kernel
has started up, and the chip can potentially get overheated.
I ran into this situation when I tested the code on Hikey, and the system was
forced to be shut down by thermal driver.

So I'd like to modify the code a bit like:
if (in_crash_kernel {
     crash_save_cpu(regs, cpu);
     while (1)
         asm("wfi"); /* irq is disabled here. */
}

Does this make sense?

-Takahiro AKASHI

> @@ -723,7 +735,7 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
>
>   	case IPI_CPU_STOP:
>   		irq_enter();
> -		ipi_cpu_stop(cpu);
> +		ipi_cpu_stop(cpu, regs);
>   		irq_exit();
>   		break;
>
>