sys_info: add a default kernel sys_info mask

Which serves as a global default sys_info mask. When users want the same
system information for many error cases (panic, hung, lockup ...), they
can chose to set this global knob only once, while not setting up each
individual sys_info knobs.

This just adds a 'lazy' option, and doesn't change existing kernel
behavior as the mask is 0 by default.

Link: https://lkml.kernel.org/r/[e-mail]
Suggested-by: Andrew Morton <[e-mail]>
Signed-off-by: Feng Tang <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Lance Yang <[e-mail]>
Cc: "Paul E . McKenney" <[e-mail]>
Cc: Petr Mladek <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/admin-guide/sysctl/kernel.rst

···
 io_uring instances.
 
 
+kernel_sys_info
+===============
+A comma separated list of extra system information to be dumped when
+soft/hard lockup is detected, for example, "tasks,mem,timers,locks,...".
+Refer 'panic_sys_info' section below for more details.
+
+It serves as the default kernel control knob, which will take effect
+when a kernel module calls sys_info() with parameter==0.
+
 kexec_load_disabled
 ===================
 

lib/sys_info.c

···
 	[ilog2(SYS_INFO_BLOCKED_TASKS)]		= "blocked_tasks",
 };
 
+/*
+ * Default kernel sys_info mask.
+ * If a kernel module calls sys_info() with "parameter == 0", then
+ * this mask will be used.
+ */
+static unsigned long kernel_si_mask;
+
 /* Expecting string like "xxx_sys_info=tasks,mem,timers,locks,ftrace,..." */
 unsigned long sys_info_parse_param(char *str)
 {
···
 	else
 		return sys_info_read_handler(&table, buffer, lenp, ppos, ro_table->data);
 }
+
+static const struct ctl_table sys_info_sysctls[] = {
+	{
+		.procname	= "kernel_sys_info",
+		.data		= &kernel_si_mask,
+		.maxlen         = sizeof(kernel_si_mask),
+		.mode		= 0644,
+		.proc_handler	= sysctl_sys_info_handler,
+	},
+};
+
+static int __init sys_info_sysctl_init(void)
+{
+	register_sysctl_init("kernel", sys_info_sysctls);
+	return 0;
+}
+subsys_initcall(sys_info_sysctl_init);
 #endif
 
-void sys_info(unsigned long si_mask)
+static void __sys_info(unsigned long si_mask)
 {
 	if (si_mask & SYS_INFO_TASKS)
 		show_state();
···
 
 	if (si_mask & SYS_INFO_BLOCKED_TASKS)
 		show_state_filter(TASK_UNINTERRUPTIBLE);
+}
+
+void sys_info(unsigned long si_mask)
+{
+	__sys_info(si_mask ? : kernel_si_mask);
 }