[PATCH v3] hrtimers: add fast path to hrtimer_get_next_event

[Stanislav Fomichev]

> Sigh. Is anybody actually reading and trying to understand what I
> write in reviews?
> 
> > Also the lockless check wants a comment why it is correct.
> 
> Is it that hard? A comment is NOT a uberlenghty explanation in the
> changelog. A comment starts with /* and ends with */ and is in the
> code.
> 
> 	/*
> 	 * Called with interrupts disabled and therefor
> 	 * protected against a switch to high resolution mode.
> 	 */
>  
> That's a comment, right?
Duh, I honestly thought you were talking about more details in the
changelog. How come you didn't like it? Piece of art :-)

Anyway, here is another respin of the patch.
Btw, maybe it also makes sense to convert _irqsave spinlocks to non-irq
ones? If we now have a comment telling we are called with interrupts
disabled, there is no way we need irq safe locks, right?

--

In hrtimer_get_next_event we unconditionally lock/unlock spinlock, even if it's
not required (hrtimer_hres_active() != 0). This patch adds fast path
when highres is active so we don't execute unnecessary operations.

Signed-off-by: Stanislav Fomichev <stfomichev@xxxxxxxxxxxxxx>
---
 kernel/hrtimer.c | 31 ++++++++++++++++++-------------
 1 file changed, 18 insertions(+), 13 deletions(-)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index ba340739c701..245c90e5626b 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -1167,23 +1167,28 @@ ktime_t hrtimer_get_next_event(void)
 	unsigned long flags;
 	int i;
 
+	/*
+	 * Called with interrupts disabled and therefore
+	 * protected against a switch to high resolution mode.
+	 */
+	if (hrtimer_hres_active())
+		return mindelta;
+
 	raw_spin_lock_irqsave(&cpu_base->lock, flags);
 
-	if (!hrtimer_hres_active()) {
-		for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++, base++) {
-			struct hrtimer *timer;
-			struct timerqueue_node *next;
+	for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++, base++) {
+		struct hrtimer *timer;
+		struct timerqueue_node *next;
 
-			next = timerqueue_getnext(&base->active);
-			if (!next)
-				continue;
+		next = timerqueue_getnext(&base->active);
+		if (!next)
+			continue;
 
-			timer = container_of(next, struct hrtimer, node);
-			delta.tv64 = hrtimer_get_expires_tv64(timer);
-			delta = ktime_sub(delta, base->get_time());
-			if (delta.tv64 < mindelta.tv64)
-				mindelta.tv64 = delta.tv64;
-		}
+		timer = container_of(next, struct hrtimer, node);
+		delta.tv64 = hrtimer_get_expires_tv64(timer);
+		delta = ktime_sub(delta, base->get_time());
+		if (delta.tv64 < mindelta.tv64)
+			mindelta.tv64 = delta.tv64;
 	}
 
 	raw_spin_unlock_irqrestore(&cpu_base->lock, flags);
-- 
1.8.3.2

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