[PATCH v33 00/14] add kdump support

[David Woodhouse]

On Fri, 2017-03-17 at 15:33 +0000, Mark Rutland wrote:
> 
> We can certainly log a better message, e.g.
>         
>         bool kdump = (image == kexec_crash_image);
>         bool stuck_cpus = cpus_are_stuck_in_kernel() ||
>                           num_online_cpus() > 1;
> 
>         BUG_ON(stuck_cpus && !kdump);
>         WARN(stuck_cpus, "Unable to offline CPUs, kdump will be unreliable.\n");

No, in this case the CPUs *were* offlined correctly, or at least "as
designed", by smp_send_crash_stop(). And if that hadn't worked, as
verified by *its* synchronisation method based on the atomic_t
waiting_for_crash_ipi, then *it* would have complained for itself:

	if (atomic_read(&waiting_for_crash_ipi) > 0)
		pr_warning("SMP: failed to stop secondary CPUs %*pbl\n",
			   cpumask_pr_args(cpu_online_mask));

It's just that smp_send_crash_stop() (or more specifically
ipi_cpu_crash_stop()) doesn't touch the online cpu mask. Unlike the
ARM32 equivalent function machien_crash_nonpanic_core(), which does.

It wasn't clear if that was *intentional*, to allow the original
contents of the online mask before the crash to be seen in the
resulting vmcore... or purely an accident. 

FWIW if I trigger a crash on CPU 1 my kdump (still 4.9.8+v32) doesn't work.
I end up booting the kdump kernel on CPU#1 and then it gets distinctly unhappy...

[    0.000000] Booting Linux on physical CPU 0x1
...
[    0.017125] Detected PIPT I-cache on CPU1
[    0.017138] GICv3: CPU1: found redistributor 0 region 0:0x00000000f0280000
[    0.017147] CPU1: Booted secondary processor [411fd073]
[    0.017339] Detected PIPT I-cache on CPU2
[    0.017347] GICv3: CPU2: found redistributor 2 region 0:0x00000000f02c0000
[    0.017354] CPU2: Booted secondary processor [411fd073]
[    0.017537] Detected PIPT I-cache on CPU3
[    0.017545] GICv3: CPU3: found redistributor 3 region 0:0x00000000f02e0000
[    0.017551] CPU3: Booted secondary processor [411fd073]
[    0.017576] Brought up 4 CPUs
[    0.017587] SMP: Total of 4 processors activated.
...
[   31.745809] INFO: rcu_sched detected stalls on CPUs/tasks:
[   31.751299] 	1-...: (30 GPs behind) idle=c90/0/0 softirq=0/0 fqs=0 
[   31.757557] 	2-...: (30 GPs behind) idle=608/0/0 softirq=0/0 fqs=0 
[   31.763814] 	3-...: (30 GPs behind) idle=604/0/0 softirq=0/0 fqs=0 
[   31.770069] 	(detected by 0, t=5252 jiffies, g=-270, c=-271, q=0)
[   31.776161] Task dump for CPU 1:
[   31.779381] swapper/1       R  running task        0     0      1 0x00000080
[   31.786446] Task dump for CPU 2:
[   31.789666] swapper/2       R  running task        0     0      1 0x00000080
[   31.796725] Task dump for CPU 3:
[   31.799945] swapper/3       R  running task        0     0      1 0x00000080

Is some of that platform-specific?

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 701c085..41d238e 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -129,7 +129,7 @@ static struct sysrq_key_op sysrq_unraw_op = {
 #define sysrq_unraw_op (*(struct sysrq_key_op *)NULL)
 #endif /* CONFIG_VT */
 
-static void sysrq_handle_crash(int key)
+static void do_sysrq_handle_crash(int key)
 {
 	char *killer = NULL;
 
@@ -143,6 +143,12 @@ static void sysrq_handle_crash(int key)
 	wmb();
 	*killer = 1;
 }
+
+static void sysrq_handle_crash(int key)
+{
+	smp_call_on_cpu(1, (void *)do_sysrq_handle_crash, 0, 1);
+}
+
 static struct sysrq_key_op sysrq_crash_op = {
 	.handler	= sysrq_handle_crash,
 	.help_msg	= "crash(c)",
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