panic: fix panic_timeout accuracy when running on a hypervisor

I've had some complaints about panic_timeout being wildly innacurate on
shared processor PowerPC partitions (a 3 minute panic_timeout taking 30
minutes).

The problem is we loop on mdelay(1) and with a 1ms in 10ms hypervisor
timeslice each of these will take 10ms (ie 10x) longer.  I expect other
platforms with shared processor hypervisors will see the same issue.

This patch keeps the old behaviour if we have a panic_blink (only keyboard
LEDs right now) and does 1 second mdelays if we don't.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 30 insertions, 16 deletions

diff --git a/kernel/panic.c b/kernel/panic.c
index c787333282b8..13d966b4c14a 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -36,15 +36,36 @@ ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
 
 EXPORT_SYMBOL(panic_notifier_list);
 
-static long no_blink(long time)
-{
-        return 0;
-}
-
 /* Returns how long it waited in ms */
 long (*panic_blink)(long time);
 EXPORT_SYMBOL(panic_blink);
 
+static void panic_blink_one_second(void)
+{
+        static long i = 0, end;
+
+        if (panic_blink) {
+                end = i + MSEC_PER_SEC;
+
+                while (i < end) {
+                        i += panic_blink(i);
+                        mdelay(1);
+                        i++;
+                }
+        } else {
+                /*
+                 * When running under a hypervisor a small mdelay may get
+                 * rounded up to the hypervisor timeslice. For example, with
+                 * a 1ms in 10ms hypervisor timeslice we might inflate a
+                 * mdelay(1) loop by 10x.
+                 *
+                 * If we have nothing to blink, spin on 1 second calls to
+                 * mdelay to avoid this.
+                 */
+                mdelay(MSEC_PER_SEC);
+        }
+}
+
 /**
  *      panic - halt the system
  *      @fmt: The text string to print
@@ -95,9 +116,6 @@ NORET_TYPE void panic(const char * fmt, ...)
 
         bust_spinlocks(0);
 
-        if (!panic_blink)
-                panic_blink = no_blink;
-
         if (panic_timeout > 0) {
                 /*
                  * Delay timeout seconds before rebooting the machine.
@@ -105,11 +123,9 @@ NORET_TYPE void panic(const char * fmt, ...)
                  */
                 printk(KERN_EMERG "Rebooting in %d seconds..", panic_timeout);
 
-                for (i = 0; i < panic_timeout*1000; ) {
+                for (i = 0; i < panic_timeout; i++) {
                         touch_nmi_watchdog();
-                        i += panic_blink(i);
-                        mdelay(1);
-                        i++;
+                        panic_blink_one_second();
                 }
                 /*
                  * This will not be a clean reboot, with everything
@@ -135,11 +151,9 @@ NORET_TYPE void panic(const char * fmt, ...)
         }
 #endif
         local_irq_enable();
-        for (i = 0; ; ) {
+        while (1) {
                 touch_softlockup_watchdog();
-                i += panic_blink(i);
-                mdelay(1);
-                i++;
+                panic_blink_one_second();
         }
 }