Re: [PATCH v7 12/12] sched, virt/steal_monitor: Keep tick on for faster push on nohz_full CPU

[Shrikanth Hegde]

Hi.

On 7/10/26 3:26 AM, Shrikanth Hegde wrote:
> Enable tick on nohz full CPU when it is marked as non-preferred.
> This helps to push the task out faster and in more predictable
> manner on nohz_full CPUs.
>
> Steal time handling code will call tick_nohz_dep_set_cpu with
> TICK_DEP_BIT_SCHED. This helps to push the task out of nohz_full
> faster as push task depends on tick.
>
> If there is pinned task on non-preferred CPU, it may not stop the tick.
> That is rare case. Even then, this preferred CPU state change can
> happen only inside the guest. So even if guest stop the tick,
> it may not necessary mean power saving since host disabling the
> tick is what matters more.
>
> sched_can_stop_tick flow doesn't change if cpu_preferred. On disabling
> the feature, module ensure it restores the CPU as preferred.
>
> Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> ---

I will drop this patch 12 as it is an optimization for rare case.

The reason being,
- If there is one pinned task, it leaves the tick on.
- Can't just the clear the tick while disable, that will cause break.
- need to know if there was one pinned task in sched_can_stop_tick
  which is tricky.
- optimization is applicable only for a forever running thread on nohz_full
  CPU and non-pinned. Such use case is rare i think.

and

- Task eventually move out that nohz_full CPU, it takes a few more
  seconds. Likely it happens on natural boundary, maybe a external
  interrupt, background workqueue or kthread etc.

- One configures nohz_full CPU carefully.  If the need arises,
  lets bother about at that time.