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Message-ID: <4A6F139C.6070806@windriver.com>
Date: Tue, 28 Jul 2009 10:05:00 -0500
From: Jason Wessel <jason.wessel@...driver.com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
"Deng, Dongdong" <Dongdong.Deng@...driver.com>
Subject: Re: [PATCH] softlockup: fix problem with long kernel pauses from
kgdb
Peter Zijlstra wrote:
> On Mon, 2009-07-27 at 15:03 -0500, Jason Wessel wrote:
>> The fix is to simply invoke sched_clock_tick() to update "cpu sched
>> clock" on exit from kgdb_handle_exception.
>
> Is that a regular IRQ context, or is that NMI context?
>
>> Signed-off-by: Dongdong Deng <Dongdong.Deng@...driver.com>
>> Signed-off-by: Jason Wessel <jason.wessel@...driver.com>
>> Cc: Ingo Molnar <mingo@...e.hu>
>> Cc: peterz@...radead.org
>> ---
>> kernel/softlockup.c | 3 +++
>> 1 file changed, 3 insertions(+)
>>
>> --- a/kernel/softlockup.c
>> +++ b/kernel/softlockup.c
>> @@ -118,6 +118,9 @@ void softlockup_tick(void)
>> }
>>
>> if (touch_timestamp == 0) {
>> + /* If the time stamp was touched externally make sure the
>> + * scheduler tick is up to date as well */
>> + sched_clock_tick();
>> __touch_softlockup_watchdog();
>> return;
>> }
>>
>
> Aside from the funny comment style (please fix) the fix does look
> sensible.
It turns out that further testing of this patch shows a regression in
the ability to detect certain lockups. It is a direct result of the
way the scheduling code makes use of the touch_softlockup_watchdog().
With the above proposed patch the tick was getting updated after a
resume, but was also getting updated with the run_timers(), and if
that happened before the softlockup tick, no softlockup would get
reported (note that I was using some test code to induce softlockups).
The patch below is a bit more invasive but solves the problem by
allowing kgdb to request that the sched cpu clock is updated only when
returning from a state where we know we need to force the update.
Would this change be an acceptable solution to allow kgdb to
peacefully exist with the softlockup code?
Thanks,
Jason.
-----
From: Jason Wessel <jason.wessel@...driver.com>
Subject: [PATCH] softlockup: add sched_clock_tick() to avoid kernel warning on kgdb resume
When CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is set sched_clock() gets the
time from hardware, such as from TSC. In this configuration kgdb will
report a softlock warning messages 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 "touch_timestamp" is 100 sec, instead of 180 sec?
With the CONFIG_HAVE_UNSTABLE_SCHED_CLOCK" 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" values 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(), which allows kgdb to
request that the sched clock is updated when the watchdog thread runs
the first time after a resume from kgdb.
Signed-off-by: Jason Wessel <jason.wessel@...driver.com>
Signed-off-by: Dongdong Deng <Dongdong.Deng@...driver.com>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: peterz@...radead.org
---
include/linux/sched.h | 4 ++++
kernel/kgdb.c | 6 +++---
kernel/softlockup.c | 16 ++++++++++++++++
3 files changed, 23 insertions(+), 3 deletions(-)
--- a/kernel/softlockup.c
+++ b/kernel/softlockup.c
@@ -79,6 +79,14 @@ void touch_softlockup_watchdog(void)
}
EXPORT_SYMBOL(touch_softlockup_watchdog);
+static int softlock_touch_sync[NR_CPUS];
+
+void touch_softlockup_watchdog_sync(void)
+{
+ softlock_touch_sync[raw_smp_processor_id()] = 1;
+ __raw_get_cpu_var(touch_timestamp) = 0;
+}
+
void touch_all_softlockup_watchdogs(void)
{
int cpu;
@@ -118,6 +126,14 @@ void softlockup_tick(void)
}
if (touch_timestamp == 0) {
+ if (unlikely(softlock_touch_sync[this_cpu])) {
+ /*
+ * If the time stamp was touched atomically
+ * make sure the scheduler tick is up to date.
+ */
+ softlock_touch_sync[this_cpu] = 0;
+ sched_clock_tick();
+ }
__touch_softlockup_watchdog();
return;
}
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -300,6 +300,7 @@ extern void sched_show_task(struct task_
#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,
struct file *filp, void __user *buffer,
@@ -313,6 +314,9 @@ 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)
{
}
--- a/kernel/kgdb.c
+++ b/kernel/kgdb.c
@@ -590,7 +590,7 @@ static void kgdb_wait(struct pt_regs *re
/* 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);
}
@@ -1433,7 +1433,7 @@ acquirelock:
atomic_read(&kgdb_cpu_doing_single_step) != cpu) {
atomic_set(&kgdb_active, -1);
- touch_softlockup_watchdog();
+ touch_softlockup_watchdog_sync();
clocksource_touch_watchdog();
local_irq_restore(flags);
@@ -1526,7 +1526,7 @@ acquirelock:
kgdb_restore:
/* Free kgdb_active */
atomic_set(&kgdb_active, -1);
- touch_softlockup_watchdog();
+ touch_softlockup_watchdog_sync();
clocksource_touch_watchdog();
local_irq_restore(flags);
--
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