Re: [PATCH v3 17/20] sched/core: Introduce default arch handling code for inc/dec preferred CPUs

[Ilya Leoshkevich <iii@linux.ibm.com>]

On 5/14/26 17:22, 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 online 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 online core which is not in cpu_preferred_mask will
>    be marked as preferred.
>    If all cores are already set to preferred, bail out.
> 
> 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>
> ---
>   include/linux/sched.h |  2 ++
>   kernel/sched/core.c   | 58 +++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 60 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 738f17d63943..2afbcd70f0ac 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2529,6 +2529,8 @@ 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 a3f65e9c7d30..195e3648b1b5 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -11368,6 +11368,64 @@ 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 online & 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();
> +
> +	cpumask_and(sm->tmp_mask, cpu_online_mask, cpu_preferred_mask);
> +	last_cpu = cpumask_last(sm->tmp_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;
> +
> +	for_each_cpu_and(tmp_cpu, cpu_smt_mask(last_cpu), cpu_online_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);

Should the opposite be done in arch_inc_preferred_cpus()?

If not, it would be good to write a comment explaining why; this
asymmetry catches the eye.

> +	}
> +}
> +#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 online & !preferred
> + *
> + * Nothing to do if online == 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_online_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_online_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)
>   {