timers: Optimize _next_timer_interrupt() level iteration

If a level has a timer that expires before reaching 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 provide an earlier expiration.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200717140551.29076-7-frederic@kernel.org

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index af1c08b0b168cad7a624aa903299753e9a06c306..9abc41715fd2abb9e660b34f533e7b7e3e3bde81 100644 (file)
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1526,6 +1526,7 @@ 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;
@@ -1533,6 +1534,13 @@ 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
@@ -1570,7 +1578,7 @@ 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;
        }