hrtimer: Use NOHZ information for locality

The decision to keep a timer which is associated to the local CPU on that
CPU does not take NOHZ information into account. As a result there are a
lot of hrtimer base switch invocations which end up not switching the base
and stay on the local CPU. That's just work for nothing and can be further
improved.

If the local CPU is part of the NOISE housekeeping mask, then check:

  1) Whether the local CPU has the tick running, which means it is
     either not idle or already expecting a timer soon.

  2) Whether the tick is stopped and need_resched() is set, which
     means the CPU is about to exit idle.

This reduces the amount of hrtimer base switch attempts, which end up on
the local CPU anyway, significantly and prepares for further optimizations.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260224163430.673473029@kernel.org

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index b87995f3ae2a8807d3953707b2b7d8f808778a91..4caf2dfff7209cacc5130a498f23aa2df3364369 100644 (file)
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1231,7 +1231,18 @@ static __always_inline bool hrtimer_prefer_local(bool is_local, bool is_first, b
                 */
                if (!is_local)
                        return false;
-               return is_first || is_pinned;
+               if (is_first || is_pinned)
+                       return true;
+
+               /* Honour the NOHZ full restrictions */
+               if (!housekeeping_cpu(smp_processor_id(), HK_TYPE_KERNEL_NOISE))
+                       return false;
+
+               /*
+                * If the tick is not stopped or need_resched() is set, then
+                * there is no point in moving the timer somewhere else.
+                */
+               return !tick_nohz_tick_stopped() || need_resched();
        }
        return is_local;
 }