[PATCH 6/9] timer: Expand clk forward logic beyond nohz

[Frederic Weisbecker]

As for next_expiry, we want to expand base->clk catch up logic beyond
nohz in order to avoid triggering useless softirqs.

If we want to fire softirqs only to expire pending timers, periodic
base->clk increments must be skippable for random amounts of time.
Therefore we must prepare to catch-up with missing updates whenever we
need an up-to-date base clock.

Signed-off-by: Frederic Weisbecker <frede...@kernel.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Anna-Maria Gleixner <anna-ma...@linutronix.de>
Cc: Juri Lelli <juri.le...@redhat.com>
---
 kernel/time/timer.c | 26 ++++----------------------
 1 file changed, 4 insertions(+), 22 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 501d36ef0b75..ffa2c956d968 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -885,19 +885,12 @@ get_target_base(struct timer_base *base, unsigned tflags)
 
 static inline void forward_timer_base(struct timer_base *base)
 {
-#ifdef CONFIG_NO_HZ_COMMON
        unsigned long jnow;
 
-       /*
-        * We only forward the base when we are idle or have just come out of
-        * idle (must_forward_clk logic), and have a delta between base clock
-        * and jiffies. In the common case, run_timers will take care of it.
-        */
-       if (likely(!base->must_forward_clk))
+       if (!base->must_forward_clk)
                return;
 
        jnow = READ_ONCE(jiffies);
-       base->must_forward_clk = base->is_idle;
        if ((long)(jnow - base->clk) < 2)
                return;
 
@@ -912,7 +905,6 @@ static inline void forward_timer_base(struct timer_base 
*base)
                        return;
                base->clk = base->next_expiry;
        }
-#endif
 }
 
 
@@ -1666,10 +1658,8 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 
basem)
                 * logic is only maintained for the BASE_STD base, deferrable
                 * timers may still see large granularity skew (by design).
                 */
-               if ((expires - basem) > TICK_NSEC) {
-                       base->must_forward_clk = true;
+               if ((expires - basem) > TICK_NSEC)
                        base->is_idle = true;
-               }
        }
        raw_spin_unlock(&base->lock);
 
@@ -1768,16 +1758,7 @@ static inline void __run_timers(struct timer_base *base)
        /*
         * timer_base::must_forward_clk must be cleared before running
         * timers so that any timer functions that call mod_timer() will
-        * not try to forward the base. Idle tracking / clock forwarding
-        * logic is only used with BASE_STD timers.
-        *
-        * The must_forward_clk flag is cleared unconditionally also for
-        * the deferrable base. The deferrable base is not affected by idle
-        * tracking and never forwarded, so clearing the flag is a NOOP.
-        *
-        * The fact that the deferrable base is never forwarded can cause
-        * large variations in granularity for deferrable timers, but they
-        * can be deferred for long periods due to idle anyway.
+        * not try to forward the base.
         */
        base->must_forward_clk = false;
 
@@ -1790,6 +1771,7 @@ static inline void __run_timers(struct timer_base *base)
                while (levels--)
                        expire_timers(base, heads + levels);
        }
+       base->must_forward_clk = true;
        raw_spin_unlock_irq(&base->lock);
        timer_base_unlock_expiry(base);
 }
-- 
2.26.2