[patch V2 4/5] posix-cpu-timers: Expiry timers directly when in task work context

Thomas Gleixner
2020-07-16T20:22:20Z

If the expiry happens in task context, there is no point in collecting the
expired timers on a list first. Just expire them directly.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
kernel/time/posix-cpu-timers.c | 92 +++++++++++++++++++++++++++++++++++------
1 file changed, 79 insertions(+), 13 deletions(-)

--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -753,13 +753,71 @@ static void posix_cpu_timer_get(struct k

#define MAX_COLLECTED 20

-static u64 collect_timerqueue(struct timerqueue_head *head,
+#ifdef CONFIG_POSIX_CPU_TIMERS_TASK_WORK
+/*
+ * Expiry in task work context. Access to sighand->siglock is safe here.
+ */
+static void handle_expired_timer(struct cpu_timer *ctmr,
+ struct list_head *firing)
+{
+ struct k_itimer *timer;
+ int cpu_firing;
+
+ /*
+ * Unlock sighand lock so the timer can be locked. Keep interrupts
+ * disabled accross the lock switch.
+ */
+ spin_unlock(&current->sighand->siglock);
+ timer = container_of(ctmr, struct k_itimer, it.cpu);
+ spin_lock(&timer->it_lock);
+ cpu_firing = timer->it.cpu.firing;
+ timer->it.cpu.firing = 0;
+ /*
+ * The firing flag is -1 if this raced with a reset of the timer,
+ * which already reported this almost-firing as an overrun. So
+ * don't generate an event.
+ */
+ if (likely(cpu_firing >= 0))
+ cpu_timer_fire(timer);
+ /*
+ * Drop timer lock again and reacquire sighand lock. Allow
+ * interrupts to come in between so this wont block interrupts
+ * accross the delivery of a gazillion of timers.
+ */
+ spin_unlock_irq(&timer->it_lock);
+ spin_lock_irq(&current->sighand->siglock);
+}
+#else
+/*
+ * Expiry in interupt context. Just move them to the firing list.
+ */
+static void handle_expired_timer(struct cpu_timer *ctmr,
+ struct list_head *firing)
+{
+ list_add_tail(&ctmr->elist, firing);
+}
+#endif
+
+static u64 collect_timerqueue(struct posix_cputimer_base *base,
struct list_head *firing, u64 now)
{
struct timerqueue_node *next;
int i = 0;

- while ((next = timerqueue_getnext(head))) {
+ /*
+ * Reset the expiry cache first when expiry context is task work.
+ * This is required because when sighand lock is dropped new timers
+ * can be enqueued. That's not a problem for regular posix timers
+ * as the expiry time would be correct when expire_timerqueue()
+ * returns, but the expiry cache is also used by itimers which do
+ * not have a corresponding posix timer and therefore a simple
+ * update after expire_timerqueue() might overwrite their newly
+ * written expiry time.
+ */
+ if (IS_ENABLED(CONFIG_POSIX_CPU_TIMERS_TASK_WORK))
+ base->nextevt = U64_MAX;
+
+ while ((next = timerqueue_getnext(&base->tqhead))) {
struct cpu_timer *ctmr;
u64 expires;

@@ -771,7 +829,7 @@ static u64 collect_timerqueue(struct tim

ctmr->firing = 1;
cpu_timer_dequeue(ctmr);
- list_add_tail(&ctmr->elist, firing);
+ handle_expired_timer(ctmr, firing);
}

return U64_MAX;
@@ -783,10 +841,8 @@ static void collect_posix_cputimers(stru
struct posix_cputimer_base *base = pct->bases;
int i;

- for (i = 0; i < CPUCLOCK_MAX; i++, base++) {
- base->nextevt = collect_timerqueue(&base->tqhead, firing,
- samples[i]);
- }
+ for (i = 0; i < CPUCLOCK_MAX; i++, base++)
+ base->nextevt = collect_timerqueue(base, firing, samples[i]);
}

static inline void check_dl_overrun(struct task_struct *tsk)
@@ -812,9 +868,10 @@ static bool check_rlimit(u64 time, u64 l
}

/*
- * Check for any per-thread CPU timers that have fired and move them off
- * the tsk->cpu_timers[N] list onto the firing list. Here we update the
- * tsk->it_*_expires values to reflect the remaining thread CPU timers.
+ * Check for any per-thread CPU timers that have fired and depending on the
+ * context (task work or interrupt) move them off the tsk->cpu_timers[N]
+ * list onto the firing list or expire them directly. Update the expiry
+ * cache as well to reflect the remaining thread CPU timers.
*/
static void check_thread_timers(struct task_struct *tsk,
struct list_head *firing)
@@ -889,9 +946,11 @@ static void check_cpu_itimer(struct task
}

/*
- * Check for any per-thread CPU timers that have fired and move them
- * off the tsk->*_timers list onto the firing list. Per-thread timers
- * have already been taken off.
+ * Check for any per-process CPU timers that have fired and depending on
+ * the context (task work or interrupt) move them off the tsk->signal timer
+ * list onto the firing list or expire them directly. Update the expiry
+ * cache to reflect the resulting state. Per-thread timers have already
+ * been handled.
*/
static void check_process_timers(struct task_struct *tsk,
struct list_head *firing)
@@ -1115,6 +1174,12 @@ static void handle_posix_cpu_timers(stru
unlock_task_sighand(tsk, &flags);

/*
+ * If task work delivery is enabled, the timers are already
+ * expired.
+ */
+ if (IS_ENABLED(CONFIG_POSIX_CPU_TIMERS_TASK_WORK))
+ goto out;
+ /*
* Now that all the timers on our list have the firing flag,
* no one will touch their list entries but us. We'll take
* each timer's lock before clearing its firing flag, so no
@@ -1136,6 +1201,7 @@ static void handle_posix_cpu_timers(stru
cpu_timer_fire(timer);
spin_unlock(&timer->it_lock);
}
+out:
lockdep_posixtimer_exit();
}