Re: [PATCH v4 16/20] sched/core: Introduce default arch handling code for inc/dec preferred CPUs

[Yury Norov <yury.norov@gmail.com>]

On Wed, Jun 17, 2026 at 11:11:35PM +0530, Shrikanth Hegde wrote:
> Define default handlers for high/low steal time. If arch has better
> decision logic, may override the default implementation.
> 
> - If the steal time higher than threshold, reduce the number of preferred
>   CPUs by 1 core. The last core in the intersection of active and
>   preferred CPUs will be marked as non-preferred.
>   Ensure at least one core is left as preferred always.
> 
> - If the steal time lower than threshold, increase the number of preferred
>   CPUs by 1 core. First active core which is not in cpu_preferred_mask will
>   be marked as preferred.
>   If all cores are already set to preferred, bail out.

And the code below does nothing of that.
 
> Increase/Decrease may need to modify the splicing across NUMA nodes. It is
> being kept simple for now.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
> v3->v4:
> - active instead of online
> - added comment for enabling tick for nohz_full.
> 
>  include/linux/sched.h |  2 ++
>  kernel/sched/core.c   | 61 +++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 63 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 5b15353ed7ef..e435f3073ffc 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2529,5 +2529,7 @@ struct steal_monitor_t {
>  };
>  
>  extern struct steal_monitor_t steal_mon;
> +void arch_dec_preferred_cpus(struct steal_monitor_t *sm, u64 steal_ratio);
> +void arch_inc_preferred_cpus(struct steal_monitor_t *sm, u64 steal_ratio);
>  #endif
>  #endif
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f1a91021e357..c77045055604 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -11400,6 +11400,67 @@ void sched_init_steal_monitor(void)
>  	steal_mon.sampling_period_ms  = 1000;		/* once per second */
>  }
>  
> +/*
> + * Default implementation of decrementing the preferred CPUs based on steal
> + * time. This is simple logic and decrease the preferred CPUs by 1 core.
> + * It takes out the last core in the active & preferred.
> + *
> + * Ensure at least one housekeeping core is always kept as preferred
> + *
> + * Could be overwritten by arch specific handling.
> + */
> +#ifndef arch_dec_preferred_cpus
> +void arch_dec_preferred_cpus(struct steal_monitor_t *sm, u64 steal_ratio)
> +{
> +	int last_cpu, tmp_cpu;
> +	int this_cpu = raw_smp_processor_id();
> +
> +	last_cpu = cpumask_last(cpu_preferred_mask);
> +
> +	/*
> +	 * If the core belongs to the housekeeping CPUs, no action is
> +	 * taken. This leaves at least one core preferred always.
> +	 * This ensures at least some CPUs are available to run
> +	 */
> +	if (cpumask_equal(cpu_smt_mask(last_cpu), cpu_smt_mask(this_cpu)))
> +		return;
> +
> +	/*
> +	 * set tick bit for nohz_full CPU to push the task out. Once the tasks
> +	 * are pushed out, bit will be cleared
> +	 */
> +	for_each_cpu_and(tmp_cpu, cpu_smt_mask(last_cpu), cpu_active_mask) {
> +		set_cpu_preferred(tmp_cpu, false);
> +		if (tick_nohz_full_cpu(tmp_cpu))
> +			tick_nohz_dep_set_cpu(tmp_cpu, TICK_DEP_BIT_SCHED);
> +	}
> +}
> +#endif
> +
> +/*
> + * Default implementation of incrementing preferred CPUs based on steal
> + * time. This is simple logic and increases the preferred CPUs by 1 core.
> + * It adds the first core in active & !preferred
> + *
> + * Nothing to do if active == preferred
> + *
> + * Could be overwritten by arch specific handling.
> + */
> +#ifndef arch_inc_preferred_cpus
> +void arch_inc_preferred_cpus(struct steal_monitor_t *sm, u64 steal_ratio)
> +{
> +	int first_cpu, tmp_cpu;
> +
> +	first_cpu = cpumask_first_andnot(cpu_active_mask, cpu_preferred_mask);
> +	/* All CPUs are preferred. Nothing to increase further */
> +	if (first_cpu >= nr_cpu_ids)
> +		return;
> +
> +	for_each_cpu_and(tmp_cpu, cpu_smt_mask(first_cpu), cpu_active_mask)
> +		set_cpu_preferred(tmp_cpu, true);
> +}
> +#endif
> +
>  /* This is only a skeleton. Subsequent patches introduce more of it */
>  void sched_steal_detection_work(struct work_struct *work)
>  {
> -- 
> 2.47.3