Re: [Patch -v3 4/4] Make reboot_cpuid a kernel parameter.

[Robin Holt <holt@sgi.com>]

On Mon, Apr 15, 2013 at 10:47:49AM -0700, H. Peter Anvin wrote:
> We usually don't break working user setups as a matter if policy.  Only if it can't be avoided.
> 
> Robin Holt <holt@sgi.com> wrote:
> 
> >On Mon, Apr 15, 2013 at 10:18:31AM -0700, H. Peter Anvin wrote:
> >> Um... doesn't this break anyone who currently depends on this?
> >
> >Yes.  That is also why I kept this separate and not tagged for
> >stable. They would need to start using the more general kernel
> >parameter
> >to get the same functionality.  Should I have done it differently?

How about:
Author: Robin Holt <holt@sgi.com>
Date:   Mon Apr 15 11:39:33 2013 -0500

    Make reboot_cpuid a kernel parameter.
    
    Moving the reboot=s<##> parameter for x86 to a kernel parameter
    proper.  I did not find any other arch that was specifying the
    reboot cpu.
    
    I left a compatibility mode in there.  The new parameter always
    takes precedence.  I also fixed up the current code to support
    up to cpuid's up to the current max of 4096.
    
    Signed-off-by: Robin Holt <holt@sgi.com>
    To: Ingo Molnar <mingo@redhat.com>
    To: Russ Anderson <rja@sgi.com>
    Cc: Shawn Guo <shawn.guo@linaro.org>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Robin Holt <holt@sgi.com>
    Cc: "rusty@rustcorp.com.au" <rusty@rustcorp.com.au>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: the arch/x86 maintainers <x86@kernel.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4609e81..11ebb48 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2597,6 +2597,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Format: <reboot_mode>[,<reboot_mode2>[,...]]
 			See arch/*/kernel/reboot.c or arch/*/kernel/process.c
 
+	reboot_cpuid=	[KNL] cpuid to use for reboot/halt, etc operations.
+
 	relax_domain_level=
 			[KNL, SMP] Set scheduler's default relax_domain_level.
 			See Documentation/cgroups/cpusets.txt.
diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 76fa1e9..cdcf28f 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -49,10 +49,6 @@ int reboot_force;
  */
 static int reboot_default = 1;
 
-#ifdef CONFIG_SMP
-static int reboot_cpu = -1;
-#endif
-
 /*
  * This is set if we need to go through the 'emergency' path.
  * When machine_emergency_restart() is called, we may be on
@@ -63,6 +59,8 @@ static int reboot_emergency;
 /* This is set by the PCI code if either type 1 or type 2 PCI is detected */
 bool port_cf9_safe = false;
 
+extern int reboot_cpuid;
+
 /*
  * reboot=b[ios] | s[mp] | t[riple] | k[bd] | e[fi] [, [w]arm | [c]old] | p[ci]
  * warm   Don't set the cold reboot flag
@@ -97,10 +95,14 @@ static int __init reboot_setup(char *str)
 
 #ifdef CONFIG_SMP
 		case 's':
-			if (isdigit(*(str+1))) {
-				reboot_cpu = (int) (*(str+1) - '0');
-				if (isdigit(*(str+2)))
-					reboot_cpu = reboot_cpu*10 + (int)(*(str+2) - '0');
+			if (reboot_cpuid < 0) {
+				int coffset;
+
+				reboot_cpuid = 0;
+				for (coffset = 1; isdigit(*(str + coffset)); coffset++) {
+					reboot_cpuid *= 10;
+					reboot_cpuid += (int)(*(str+2) - '0');
+				}
 			}
 			/*
 			 * We will leave sorting out the final value
@@ -615,25 +617,10 @@ void native_machine_shutdown(void)
 	/* Stop the cpus and apics */
 #ifdef CONFIG_SMP
 
-	/* The boot cpu is always logical cpu 0 */
-	int reboot_cpu_id = 0;
-
-	/* See if there has been given a command line override */
-	if ((reboot_cpu != -1) && (reboot_cpu < nr_cpu_ids) &&
-		cpu_online(reboot_cpu))
-		reboot_cpu_id = reboot_cpu;
-
-	/* Make certain the cpu I'm about to reboot on is online */
-	if (!cpu_online(reboot_cpu_id))
-		reboot_cpu_id = smp_processor_id();
-
-	/* Make certain I only run on the appropriate processor */
-	set_cpus_allowed_ptr(current, cpumask_of(reboot_cpu_id));
-
 	/*
-	 * O.K Now that I'm on the appropriate processor, stop all of the
-	 * others. Also disable the local irq to not receive the per-cpu
-	 * timer interrupt which may trigger scheduler's load balance.
+	 * Stop all of the others. Also disable the local irq to
+	 * not receive the per-cpu timer interrupt which may trigger
+	 * scheduler's load balance.
 	 */
 	local_irq_disable();
 	stop_other_cpus();
diff --git a/kernel/reboot.c b/kernel/reboot.c
index 187a0d8..8d7b7d4 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -8,6 +8,7 @@
 #include <linux/kexec.h>
 #include <linux/kmod.h>
 #include <linux/kmsg_dump.h>
+#include <linux/moduleparam.h>
 #include <linux/reboot.h>
 #include <linux/syscalls.h>
 #include <linux/syscore_ops.h>
@@ -66,13 +67,18 @@ int unregister_reboot_notifier(struct notifier_block *nb)
 }
 EXPORT_SYMBOL(unregister_reboot_notifier);
 
+int reboot_cpuid = -1;
+core_param(reboot_cpuid, reboot_cpuid, int, 0644);
+
 void migrate_to_reboot_cpu(void)
 {
 	/* The boot cpu is always logical cpu 0 */
-	int reboot_cpu_id = 0;
+	int reboot_cpu_id = reboot_cpuid;
 
 	/* Make certain the cpu I'm about to reboot on is online */
 	if (!cpu_online(reboot_cpu_id))
+		reboot_cpu_id = 0;
+	if (!cpu_online(reboot_cpu_id))
 		reboot_cpu_id = smp_processor_id();
 
 	/* Prevent races with other tasks migrating this task. */