sysrq: do not omit current cpu when showing backtrace of all active CPUs

The backtrace of current CPU also should be printed as it is active. This
change add stack trace for current CPU and print a hint for idle CPU for
the generic workqueue based printing. (x86 already does this)

Now it looks like below:
[  279.401567] sysrq: Show backtrace of all active CPUs
[  279.407234] sysrq: CPU5:
[  279.407505] Call Trace:
[  279.408789] [<ffffffff8000606c>] dump_backtrace+0x2c/0x3a
[  279.411698] [<ffffffff800060ac>] show_stack+0x32/0x3e
[  279.411809] [<ffffffff80542258>] sysrq_handle_showallcpus+0x4c/0xc6
[  279.411929] [<ffffffff80542f16>] __handle_sysrq+0x106/0x26c
[  279.412034] [<ffffffff805436a8>] write_sysrq_trigger+0x64/0x74
[  279.412139] [<ffffffff8029cd48>] proc_reg_write+0x8e/0xe2
[  279.412252] [<ffffffff8021a8f8>] vfs_write+0x90/0x2be
[  279.412362] [<ffffffff8021acd2>] ksys_write+0xa6/0xce
[  279.412467] [<ffffffff8021ad24>] sys_write+0x2a/0x38
[  279.412689] [<ffffffff80003ff8>] ret_from_syscall+0x0/0x2
[  279.417173] sysrq: CPU6: backtrace skipped as idling
[  279.417185] sysrq: CPU4: backtrace skipped as idling
[  279.417187] sysrq: CPU0: backtrace skipped as idling
[  279.417181] sysrq: CPU7: backtrace skipped as idling
[  279.417190] sysrq: CPU1: backtrace skipped as idling
[  279.417193] sysrq: CPU3: backtrace skipped as idling
[  279.417219] sysrq: CPU2:
[  279.419179] Call Trace:
[  279.419440] [<ffffffff8000606c>] dump_backtrace+0x2c/0x3a
[  279.419782] [<ffffffff800060ac>] show_stack+0x32/0x3e
[  279.420015] [<ffffffff80542b30>] showacpu+0x5c/0x96
[  279.420317] [<ffffffff800ba71c>] flush_smp_call_function_queue+0xd6/0x218
[  279.420569] [<ffffffff800bb438>] generic_smp_call_function_single_interrupt+0x14/0x1c
[  279.420798] [<ffffffff800079ae>] handle_IPI+0xaa/0x13a
[  279.421024] [<ffffffff804dcb92>] riscv_intc_irq+0x56/0x70
[  279.421274] [<ffffffff80a05b70>] generic_handle_arch_irq+0x6a/0xfa
[  279.421518] [<ffffffff80004006>] ret_from_exception+0x0/0x10
[  279.421750] [<ffffffff80096492>] rcu_idle_enter+0x16/0x1e

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Link: https://lore.kernel.org/r/20220117154300.2808-1-changbin.du@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/sysrq.c

@@ -232,8 +232,10 @@ static void showacpu(void *dummy)
 	unsigned long flags;
 
 	/* Idle CPUs have no interesting backtrace. */
-	if (idle_cpu(smp_processor_id()))
+	if (idle_cpu(smp_processor_id())) {
+		pr_info("CPU%d: backtrace skipped as idling\n", smp_processor_id());
 		return;
+	}
 
 	raw_spin_lock_irqsave(&show_lock, flags);
 	pr_info("CPU%d:\n", smp_processor_id());
@@ -260,10 +262,13 @@ static void sysrq_handle_showallcpus(int key)
 
 		if (in_hardirq())
 			regs = get_irq_regs();
-		if (regs) {
-			pr_info("CPU%d:\n", smp_processor_id());
+
+		pr_info("CPU%d:\n", smp_processor_id());
+		if (regs)
 			show_regs(regs);
-		}
+		else
+			show_stack(NULL, NULL, KERN_INFO);
+
 		schedule_work(&sysrq_showallcpus);
 	}
 }