Re: [PATCH v4 06/20] sched/core: allow only preferred CPUs in is_cpu_allowed

[K Prateek Nayak]

Hello Shrikanth,

On 6/17/2026 11:11 PM, Shrikanth Hegde wrote:
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1657,6 +1657,7 @@ struct task_struct {
>  #ifdef CONFIG_UNWIND_USER
>  	struct unwind_task_info		unwind_info;
>  #endif
> +	int				has_preferred_cpu_state;

Since this only really needs 2 bits, perhaps you can use the "u8 __pad"
in the task struct? ...

>  
>  	/* CPU-specific state of this task: */
>  	struct thread_struct		thread;

[...snip...]

> @@ -3549,6 +3568,14 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
>  	enum { cpuset, possible, fail } state = cpuset;
>  	int dest_cpu;
>  
> +	/*
> +	 * Cache value whether task's affinity spans preferred CPUs.
> +	 * This helps to avoid repeating the same for each CPU
> +	 * later in the loop. Encode call to is_cpu_allowed coming
> +	 * via select_fallback_rq.
> +	 */
> +	p->has_preferred_cpu_state = task_has_preferred_cpus(p) << 8 | 0x1;

... Or maybe it can be an s8 and this indicator can be a tri-state like:

 -1:  Cached; preferred CPUs exists
  0:  preferred CPUs not cached
  1:  Cached; preferred CPUs don't exist

and then the comparison can simply be:

    if (cached)
        return cached < 0;

ppc64le and arm64 seems to generates slightly smaller code for "< 0"
check compared to the current scheme of (& + >>) in
task_has_preferred_cpus().

-- 
Thanks and Regards,
Prateek