ACK w/cmt: [PATCH Xenial SRU] clocksource: Allow unregistering the watchdog

[Tim Gardner]

Nope, I'm gonna let the MS dudes verify it since they have the HV setup
(and I don't).

rtg

On 06/06/2016 07:14 AM, Chris J Arges wrote:
> Clean cherry-pick, already in 4.6.
> 
> Were you able to crashdump a Xenial hyper-v guest without this patch? It worked
> for me last time I tried a few weeks ago.
> 
> --chris
> 
> On Fri, Jun 03, 2016 at 08:38:35PM -0600, Tim Gardner wrote:
>> From: Vitaly Kuznetsov <vkuznets at redhat.com>
>>
>> BugLink: http://bugs.launchpad.net/bugs/1588965
>>
>> Hyper-V vmbus module registers TSC page clocksource when loaded. This is
>> the clocksource with the highest rating and thus it becomes the watchdog
>> making unloading of the vmbus module impossible.
>> Separate clocksource_select_watchdog() from clocksource_enqueue_watchdog()
>> and use it on clocksource register/rating change/unregister.
>>
>> After all, lobotomized monkeys may need some love too.
>>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets at redhat.com>
>> Cc: John Stultz <john.stultz at linaro.org>
>> Cc: Dexuan Cui <decui at microsoft.com>
>> Cc: K. Y. Srinivasan <kys at microsoft.com>
>> Link: http://lkml.kernel.org/r/1453483913-25672-1-git-send-email-vkuznets@redhat.com
>> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
>> (cherry picked from commit bbf66d897adf2bb0c310db96c97e8db6369f39e1)
>> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
>> ---
>>  kernel/time/clocksource.c | 52 ++++++++++++++++++++++++++++++++++++++---------
>>  1 file changed, 42 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
>> index 1347882..b98810d 100644
>> --- a/kernel/time/clocksource.c
>> +++ b/kernel/time/clocksource.c
>> @@ -323,13 +323,42 @@ static void clocksource_enqueue_watchdog(struct clocksource *cs)
>>  		/* cs is a watchdog. */
>>  		if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS)
>>  			cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;
>> +	}
>> +	spin_unlock_irqrestore(&watchdog_lock, flags);
>> +}
>> +
>> +static void clocksource_select_watchdog(bool fallback)
>> +{
>> +	struct clocksource *cs, *old_wd;
>> +	unsigned long flags;
>> +
>> +	spin_lock_irqsave(&watchdog_lock, flags);
>> +	/* save current watchdog */
>> +	old_wd = watchdog;
>> +	if (fallback)
>> +		watchdog = NULL;
>> +
>> +	list_for_each_entry(cs, &clocksource_list, list) {
>> +		/* cs is a clocksource to be watched. */
>> +		if (cs->flags & CLOCK_SOURCE_MUST_VERIFY)
>> +			continue;
>> +
>> +		/* Skip current if we were requested for a fallback. */
>> +		if (fallback && cs == old_wd)
>> +			continue;
>> +
>>  		/* Pick the best watchdog. */
>> -		if (!watchdog || cs->rating > watchdog->rating) {
>> +		if (!watchdog || cs->rating > watchdog->rating)
>>  			watchdog = cs;
>> -			/* Reset watchdog cycles */
>> -			clocksource_reset_watchdog();
>> -		}
>>  	}
>> +	/* If we failed to find a fallback restore the old one. */
>> +	if (!watchdog)
>> +		watchdog = old_wd;
>> +
>> +	/* If we changed the watchdog we need to reset cycles. */
>> +	if (watchdog != old_wd)
>> +		clocksource_reset_watchdog();
>> +
>>  	/* Check if the watchdog timer needs to be started. */
>>  	clocksource_start_watchdog();
>>  	spin_unlock_irqrestore(&watchdog_lock, flags);
>> @@ -404,6 +433,7 @@ static void clocksource_enqueue_watchdog(struct clocksource *cs)
>>  		cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;
>>  }
>>  
>> +static void clocksource_select_watchdog(bool fallback) { }
>>  static inline void clocksource_dequeue_watchdog(struct clocksource *cs) { }
>>  static inline void clocksource_resume_watchdog(void) { }
>>  static inline int __clocksource_watchdog_kthread(void) { return 0; }
>> @@ -736,6 +766,7 @@ int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
>>  	clocksource_enqueue(cs);
>>  	clocksource_enqueue_watchdog(cs);
>>  	clocksource_select();
>> +	clocksource_select_watchdog(false);
>>  	mutex_unlock(&clocksource_mutex);
>>  	return 0;
>>  }
>> @@ -758,6 +789,7 @@ void clocksource_change_rating(struct clocksource *cs, int rating)
>>  	mutex_lock(&clocksource_mutex);
>>  	__clocksource_change_rating(cs, rating);
>>  	clocksource_select();
>> +	clocksource_select_watchdog(false);
>>  	mutex_unlock(&clocksource_mutex);
>>  }
>>  EXPORT_SYMBOL(clocksource_change_rating);
>> @@ -767,12 +799,12 @@ EXPORT_SYMBOL(clocksource_change_rating);
>>   */
>>  static int clocksource_unbind(struct clocksource *cs)
>>  {
>> -	/*
>> -	 * I really can't convince myself to support this on hardware
>> -	 * designed by lobotomized monkeys.
>> -	 */
>> -	if (clocksource_is_watchdog(cs))
>> -		return -EBUSY;
>> +	if (clocksource_is_watchdog(cs)) {
>> +		/* Select and try to install a replacement watchdog. */
>> +		clocksource_select_watchdog(true);
>> +		if (clocksource_is_watchdog(cs))
>> +			return -EBUSY;
>> +	}
>>  
>>  	if (cs == curr_clocksource) {
>>  		/* Select and try to install a replacement clock source */
>> -- 
>> 1.9.1
>>
>>
>> -- 
>> kernel-team mailing list
>> kernel-team at lists.ubuntu.com
>> https://lists.ubuntu.com/mailman/listinfo/kernel-team


-- 
Tim Gardner tim.gardner at canonical.com