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clocksource: Revert "Remove kthread"

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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: 7197e77abc ("clocksource: Remove kthread")
Reported-by: Siegfried Metz <frame@mailbox.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Niklas Cassel <niklas.cassel@linaro.org>
Tested-by: Kevin Shanahan <kevin@shanahan.id.au>
Tested-by: viktor_jaegerskuepper@freenet.de
Tested-by: Siegfried 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
This commit is contained in:
Peter Zijlstra 2018-09-05 10:41:58 +02:00 committed by Thomas Gleixner
parent c43c5e9f52
commit e2c631ba75
1 changed files with 30 additions and 10 deletions
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40
kernel/time/clocksource.c
View file

@ -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;