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hrtimer: Prepare support for PREEMPT_RT

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When PREEMPT_RT is enabled, the soft interrupt thread can be preempted.  If
the soft interrupt thread is preempted in the middle of a timer callback,
then calling hrtimer_cancel() can lead to two issues:

  - If the caller is on a remote CPU then it has to spin wait for the timer
    handler to complete. This can result in unbound priority inversion.

  - If the caller originates from the task which preempted the timer
    handler on the same CPU, then spin waiting for the timer handler to
    complete is never going to end.

To avoid these issues, add a new lock to the timer base which is held
around the execution of the timer callbacks. If hrtimer_cancel() detects
that the timer callback is currently running, it blocks on the expiry
lock. When the callback is finished, the expiry lock is dropped by the
softirq thread which wakes up the waiter and the system makes progress.

This addresses both the priority inversion and the life lock issues.

The same issue can happen in virtual machines when the vCPU which runs a
timer callback is scheduled out. If a second vCPU of the same guest calls
hrtimer_cancel() it will spin wait for the other vCPU to be scheduled back
in. The expiry lock mechanism would avoid that. It'd be trivial to enable
this when paravirt spinlocks are enabled in a guest, but it's not clear
whether this is an actual problem in the wild, so for now it's an RT only
mechanism.

[ tglx: Refactored it for mainline ]

Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190726185753.737767218@linutronix.de
This commit is contained in:
Anna-Maria Gleixner 2019-07-26 20:30:59 +02:00 • committed by Thomas Gleixner
parent 1842f5a427
commit f61eff83ce
2 changed files with 105 additions and 6 deletions
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16
include/linux/hrtimer.h
View file

@ -192,6 +192,10 @@ enum hrtimer_base_type {
* @nr_retries: Total number of hrtimer interrupt retries
* @nr_hangs: Total number of hrtimer interrupt hangs
* @max_hang_time: Maximum time spent in hrtimer_interrupt
* @softirq_expiry_lock: Lock which is taken while softirq based hrtimer are
* expired
* @timer_waiters: A hrtimer_cancel() invocation waits for the timer
* callback to finish.
* @expires_next: absolute time of the next event, is required for remote
* hrtimer enqueue; it is the total first expiry time (hard
* and soft hrtimer are taken into account)
@ -218,6 +222,10 @@ struct hrtimer_cpu_base {
unsigned short nr_retries;
unsigned short nr_hangs;
unsigned int max_hang_time;
#endif
#ifdef CONFIG_PREEMPT_RT
spinlock_t softirq_expiry_lock;
atomic_t timer_waiters;
#endif
ktime_t expires_next;
struct hrtimer *next_timer;
@ -350,6 +358,14 @@ extern void hrtimers_resume(void);
DECLARE_PER_CPU(struct tick_device, tick_cpu_device);
#ifdef CONFIG_PREEMPT_RT
void hrtimer_cancel_wait_running(const struct hrtimer *timer);
#else
static inline void hrtimer_cancel_wait_running(struct hrtimer *timer)
{
cpu_relax();
}
#endif
/* Exported timer functions: */