Re: [PATCH 1/1] tick/nohz: Add fast-path tick stopping for idle isolated cores

[Thomas Gleixner <tglx@kernel.org>]

On Tue, Jan 06 2026 at 17:36, Ionut Nechita wrote:
> From: Ionut Nechita <ionut_n2001@yahoo.com>
>
> When a CPU is configured as nohz_full and is running the idle task with
> no tick dependencies, we can skip expensive dependency checks and

s/we can/it is possible to/

> immediately allow the tick to stop. This significantly reduces timer
> interrupts on properly isolated cores.
>
> The patch adds:

"The patch adds" is a pointless filler phrase. See

     Documentation/process/

> +	/*
> +	 * Prefetch dependency structures for better cache locality
> +	 */
> +	prefetch(&tick_dep_mask);
> +	prefetch(&ts->tick_dep_mask);
> +	prefetch(&current->tick_dep_mask);
> +	prefetch(&current->signal->tick_dep_mask);

These are really not required.

> +	/*
> +	 * Fast path for idle isolated cores: if this is an isolated CPU
> +	 * running the idle task with no dependencies, we can skip expensive
> +	 * checks and immediately allow tick to stop. This significantly
> +	 * reduces timer interrupts on properly isolated cores.
> +	 */
> +	if (tick_nohz_full_cpu(cpu) &&
> +	    is_idle_task(current) &&
> +	    !atomic_read(&tick_dep_mask) &&
> +	    !atomic_read(&ts->tick_dep_mask) &&
> +	    !atomic_read(&current->tick_dep_mask) &&
> +	    !atomic_read(&current->signal->tick_dep_mask)) {
> +		return true;

How is that different from the existing checks for the various
dependency masks, except for the added nohz_full_cpu() and
is_idle_task() conditions?

I can see that not going through the per bit checks is faster, but I
really do not see how this reduces the timer interrupts by an order of
magnitude. At least not without a proper explanation why this matters
and how this optimization is causing this improvement.

Also why is this restricted to tick_nohz_full CPUs and to the idle task?

You can avoid the per bit evaluation way simpler, which improves the
evaluation independent of context. See uncompiled patch below.

Thanks,

        tglx
---
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -344,6 +344,9 @@ static bool check_tick_dependency(atomic
 {
 	int val = atomic_read(dep);
 
+	if (likely(!tracepoint_enabled(tick_stop)))
+		return !val;
+
 	if (val & TICK_DEP_MASK_POSIX_TIMER) {
 		trace_tick_stop(0, TICK_DEP_MASK_POSIX_TIMER);
 		return true;