[RFC PATCH 04/10] timer: Optimize _next_timer_interrupt() level iteration

Frederic Weisbecker
2020-07-01T01:10:47Z

If a level has a timer that expires before we reach the next level,
there is no need to iterate further.

The next level is reached when the 3 lower bits of the current level are
cleared. If the next event happens before/during that, the next levels
won't give us an earlier expiration.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
kernel/time/timer.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index b4838d63a016..4aa74aafdd33 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1515,6 +1515,7 @@ static unsigned long __next_timer_interrupt(struct timer_base *base)
clk = base->clk;
for (lvl = 0; lvl < LVL_DEPTH; lvl++, offset += LVL_SIZE) {
int pos = next_pending_bucket(base, offset, clk & LVL_MASK);
+ unsigned long lvl_clk = clk & LVL_CLK_MASK;

if (pos >= 0) {
unsigned long tmp = clk + (unsigned long) pos;
@@ -1522,6 +1523,13 @@ static unsigned long __next_timer_interrupt(struct timer_base *base)
tmp <<= LVL_SHIFT(lvl);
if (time_before(tmp, next))
next = tmp;
+
+ /*
+ * If the next expiration happens before we reach
+ * the next level, no need to check further.
+ */
+ if (pos <= ((LVL_CLK_DIV - lvl_clk) & LVL_CLK_MASK))
+ break;
}
/*
* Clock for the next level. If the current level clock lower
@@ -1559,7 +1567,7 @@ static unsigned long __next_timer_interrupt(struct timer_base *base)
* So the simple check whether the lower bits of the current
* level are 0 or not is sufficient for all cases.
*/
- adj = clk & LVL_CLK_MASK ? 1 : 0;
+ adj = lvl_clk ? 1 : 0;
clk >>= LVL_CLK_SHIFT;
clk += adj;
}
--
2.26.2