[PATCH 1/3] softlockup: add sched_clock_tick() to avoid kernel warning on kgdb resume

[Jason Wessel <jason.wessel@windriver.com>]

When CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is set, sched_clock() gets the
time from hardware such as the TSC on x86.  In this configuration kgdb
will report a softlock warning message on resuming or detaching from a
debug session.

Sequence of events in the problem case:

1) "cpu sched clock" and "hardware time" are at 100 sec prior
   to a call to kgdb_handle_exception()

2) Debugger waits in kgdb_handle_exception() for 80 sec and on exit
   the following is called ...  touch_softlockup_watchdog() -->
   __raw_get_cpu_var(touch_timestamp) = 0;

3) "cpu sched clock" = 100s (it was not updated, because the interrupt
   was disabled in kgdb) but the "hardware time" = 180 sec

4) The first timer interrupt after resuming from kgdb_handle_exception
   updates the watchdog from the "cpu sched clock"

update_process_times() { ...  run_local_timers() --> softlockup_tick()
--> check (touch_timestamp == 0) (it is "YES" here, we have set
"touch_timestamp = 0" at kgdb) --> __touch_softlockup_watchdog()
***(A)--> reset "touch_timestamp" to "get_timestamp()" (Here, the
"touch_timestamp" will still be set to 100s.)  ...

    scheduler_tick() ***(B)--> sched_clock_tick() (update "cpu sched
    clock" to "hardware time" = 180s) ...  }

5) The Second timer interrupt handler appears to have a large jump and
   trips the softlockup warning.

update_process_times() { ...  run_local_timers() --> softlockup_tick()
--> "cpu sched clock" - "touch_timestamp" = 180s-100s > 60s --> printk
"soft lockup error messages" ...  }

note: ***(A) reset "touch_timestamp" to "get_timestamp(this_cpu)"

Why is "touch_timestamp" 100 sec, instead of 180 sec?

When CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is set, the call trace of
get_timestamp() is:

get_timestamp(this_cpu) -->cpu_clock(this_cpu)
-->sched_clock_cpu(this_cpu) -->__update_sched_clock(sched_clock_data,
now)

The __update_sched_clock() function uses the GTOD tick value to create
a window to normalize the "now" values.  So if "now" value is too big
for sched_clock_data, it will be ignored.

The fix is to invoke sched_clock_tick() to update "cpu sched clock" in
order to recover from this state.  This is done by introducing the
function touch_softlockup_watchdog_sync(). This allows kgdb to request
that the sched clock is updated when the watchdog thread runs the
first time after a resume from kgdb.

[yong.zhang0@gmail.com: Use per cpu instead of an array]
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Dongdong Deng <Dongdong.Deng@windriver.com>
CC: Ingo Molnar <mingo@elte.hu>
CC: peterz@infradead.org
---
 include/linux/sched.h |    4 ++++
 kernel/kgdb.c         |    6 +++---
 kernel/softlockup.c   |   15 +++++++++++++++
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6f7bba9..8923215 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -310,6 +310,7 @@ extern void sched_show_task(struct task_struct *p);
 #ifdef CONFIG_DETECT_SOFTLOCKUP
 extern void softlockup_tick(void);
 extern void touch_softlockup_watchdog(void);
+extern void touch_softlockup_watchdog_sync(void);
 extern void touch_all_softlockup_watchdogs(void);
 extern int proc_dosoftlockup_thresh(struct ctl_table *table, int write,
 				    void __user *buffer,
@@ -323,6 +324,9 @@ static inline void softlockup_tick(void)
 static inline void touch_softlockup_watchdog(void)
 {
 }
+static inline void touch_softlockup_watchdog_sync(void)
+{
+}
 static inline void touch_all_softlockup_watchdogs(void)
 {
 }
diff --git a/kernel/kgdb.c b/kernel/kgdb.c
index 2eb517e..87f2cc5 100644
--- a/kernel/kgdb.c
+++ b/kernel/kgdb.c
@@ -596,7 +596,7 @@ static void kgdb_wait(struct pt_regs *regs)
 
 	/* Signal the primary CPU that we are done: */
 	atomic_set(&cpu_in_kgdb[cpu], 0);
-	touch_softlockup_watchdog();
+	touch_softlockup_watchdog_sync();
 	clocksource_touch_watchdog();
 	local_irq_restore(flags);
 }
@@ -1450,7 +1450,7 @@ acquirelock:
 	    (kgdb_info[cpu].task &&
 	     kgdb_info[cpu].task->pid != kgdb_sstep_pid) && --sstep_tries) {
 		atomic_set(&kgdb_active, -1);
-		touch_softlockup_watchdog();
+		touch_softlockup_watchdog_sync();
 		clocksource_touch_watchdog();
 		local_irq_restore(flags);
 
@@ -1550,7 +1550,7 @@ kgdb_restore:
 	}
 	/* Free kgdb_active */
 	atomic_set(&kgdb_active, -1);
-	touch_softlockup_watchdog();
+	touch_softlockup_watchdog_sync();
 	clocksource_touch_watchdog();
 	local_irq_restore(flags);
 
diff --git a/kernel/softlockup.c b/kernel/softlockup.c
index d225790..0d4c789 100644
--- a/kernel/softlockup.c
+++ b/kernel/softlockup.c
@@ -25,6 +25,7 @@ static DEFINE_SPINLOCK(print_lock);
 static DEFINE_PER_CPU(unsigned long, softlockup_touch_ts); /* touch timestamp */
 static DEFINE_PER_CPU(unsigned long, softlockup_print_ts); /* print timestamp */
 static DEFINE_PER_CPU(struct task_struct *, softlockup_watchdog);
+static DEFINE_PER_CPU(bool, softlock_touch_sync);
 
 static int __read_mostly did_panic;
 int __read_mostly softlockup_thresh = 60;
@@ -79,6 +80,12 @@ void touch_softlockup_watchdog(void)
 }
 EXPORT_SYMBOL(touch_softlockup_watchdog);
 
+void touch_softlockup_watchdog_sync(void)
+{
+	__raw_get_cpu_var(softlock_touch_sync) = true;
+	__raw_get_cpu_var(softlockup_touch_ts) = 0;
+}
+
 void touch_all_softlockup_watchdogs(void)
 {
 	int cpu;
@@ -118,6 +125,14 @@ void softlockup_tick(void)
 	}
 
 	if (touch_ts == 0) {
+		if (unlikely(per_cpu(softlock_touch_sync, this_cpu))) {
+			/*
+			 * If the time stamp was touched atomically
+			 * make sure the scheduler tick is up to date.
+			 */
+			per_cpu(softlock_touch_sync, this_cpu) = false;
+			sched_clock_tick();
+		}
 		__touch_softlockup_watchdog();
 		return;
 	}
-- 
1.6.4.rc1