Commit e2c631ba authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner
Browse files

clocksource: Revert "Remove kthread" 



I turns out that the silly spawn kthread from worker was actually needed.

clocksource_watchdog_kthread() cannot be called directly from
clocksource_watchdog_work(), because clocksource_select() calls
timekeeping_notify() which uses stop_machine(). One cannot use
stop_machine() from a workqueue() due lock inversions wrt CPU hotplug.

Revert the patch but add a comment that explain why we jump through such
apparently silly hoops.

Fixes: 7197e77a ("clocksource: Remove kthread")
Reported-by: default avatarSiegfried Metz <frame@mailbox.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarNiklas Cassel <niklas.cassel@linaro.org>
Tested-by: default avatarKevin Shanahan <kevin@shanahan.id.au>
Tested-by: default avatar <viktor_jaegerskuepper@freenet.de>
Tested-by: default avatarSiegfried Metz <frame@mailbox.org>
Cc: rafael.j.wysocki@intel.com
Cc: len.brown@intel.com
Cc: diego.viola@gmail.com
Cc: rui.zhang@intel.com
Cc: bjorn.andersson@linaro.org
Link: https://lkml.kernel.org/r/20180905084158.GR24124@hirez.programming.kicks-ass.net
parent c43c5e9f

kernel/time/clocksource.c

+30 −10
Original line number Diff line number Diff line
@@ -133,19 +133,40 @@ static void inline clocksource_watchdog_unlock(unsigned long *flags) 
	spin_unlock_irqrestore(&watchdog_lock, *flags);

}



static int clocksource_watchdog_kthread(void *data);

static void __clocksource_change_rating(struct clocksource *cs, int rating);



/*

 * Interval: 0.5sec Threshold: 0.0625s

 */

#define WATCHDOG_INTERVAL (HZ >> 1)

#define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)



static void clocksource_watchdog_work(struct work_struct *work)

{

	/*

	 * We cannot directly run clocksource_watchdog_kthread() here, because

	 * clocksource_select() calls timekeeping_notify() which uses

	 * stop_machine(). One cannot use stop_machine() from a workqueue() due

	 * lock inversions wrt CPU hotplug.

	 *

	 * Also, we only ever run this work once or twice during the lifetime

	 * of the kernel, so there is no point in creating a more permanent

	 * kthread for this.

	 *

	 * If kthread_run fails the next watchdog scan over the

	 * watchdog_list will find the unstable clock again.

	 */

	kthread_run(clocksource_watchdog_kthread, NULL, "kwatchdog");

}



static void __clocksource_unstable(struct clocksource *cs)

{

	cs->flags &= ~(CLOCK_SOURCE_VALID_FOR_HRES | CLOCK_SOURCE_WATCHDOG);

	cs->flags |= CLOCK_SOURCE_UNSTABLE;



	/*

	 * If the clocksource is registered clocksource_watchdog_work() will

	 * If the clocksource is registered clocksource_watchdog_kthread() will

	 * re-rate and re-select.

	 */

	if (list_empty(&cs->list)) {
@@ -156,7 +177,7 @@ static void __clocksource_unstable(struct clocksource *cs) 
	if (cs->mark_unstable)

		cs->mark_unstable(cs);



	/* kick clocksource_watchdog_work() */

	/* kick clocksource_watchdog_kthread() */

	if (finished_booting)

		schedule_work(&watchdog_work);

}
@@ -166,7 +187,7 @@ static void __clocksource_unstable(struct clocksource *cs) 
 * @cs:		clocksource to be marked unstable

 *

 * This function is called by the x86 TSC code to mark clocksources as unstable;

 * it defers demotion and re-selection to a work.

 * it defers demotion and re-selection to a kthread.

 */

void clocksource_mark_unstable(struct clocksource *cs)

{
@@ -391,9 +412,7 @@ static void clocksource_dequeue_watchdog(struct clocksource *cs) 
	}

}



static void __clocksource_change_rating(struct clocksource *cs, int rating);



static int __clocksource_watchdog_work(void)

static int __clocksource_watchdog_kthread(void)

{

	struct clocksource *cs, *tmp;

	unsigned long flags;
@@ -418,12 +437,13 @@ static int __clocksource_watchdog_work(void) 
	return select;

}



static void clocksource_watchdog_work(struct work_struct *work)

static int clocksource_watchdog_kthread(void *data)

{

	mutex_lock(&clocksource_mutex);

	if (__clocksource_watchdog_work())

	if (__clocksource_watchdog_kthread())

		clocksource_select();

	mutex_unlock(&clocksource_mutex);

	return 0;

}



static bool clocksource_is_watchdog(struct clocksource *cs)
@@ -442,7 +462,7 @@ static void clocksource_enqueue_watchdog(struct clocksource *cs) 
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_work(void) { return 0; }

static inline int __clocksource_watchdog_kthread(void) { return 0; }

static bool clocksource_is_watchdog(struct clocksource *cs) { return false; }

void clocksource_mark_unstable(struct clocksource *cs) { }
@@ -810,7 +830,7 @@ static int __init clocksource_done_booting(void) 
	/*

	 * Run the watchdog first to eliminate unstable clock sources

	 */

	__clocksource_watchdog_work();

	__clocksource_watchdog_kthread();

	clocksource_select();

	mutex_unlock(&clocksource_mutex);

	return 0;

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