Re: [PATCH v9 04/14] smp: Use task-local IPI cpumask in smp_call_function_many_cond()

[Thomas Gleixner <tglx@kernel.org>]

On Tue, Jun 30 2026 at 19:09, Chuyi Zhou wrote:
> The memory cost is explicit: one word is added to task_struct. When
> cpumask_size() fits in that word, the mask is stored inline and no
> separate allocation is needed. Larger systems allocate cpumask_size() per
> task; on x86-64 NR_CPUS=8192 this is about 1 KiB per task. For context,

It's not about. It's one kilobyte, no?

> @@ -1364,6 +1364,12 @@ struct task_struct {
>  	struct list_head		perf_event_list;
>  	struct perf_ctx_data __rcu	*perf_ctx_data;
>  #endif
> +#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPTION)
> +	union {
> +		cpumask_t			*ipi_mask_ptr;
> +		unsigned long			ipi_mask_val;
> +	};
> +#endif

Aside of this horrible ifdeffery, this construct makes me more than
nervous as it's too trivial to add code which deferences the pointer
directly. The proper thing to do is:

#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPTION)
struct task_ipi_mask {
	union {
		cpumask_t			*ipi_mask_ptr;
		unsigned long			ipi_mask_val;
	};
};
#else
struct task_ipi_mask { };
#endif

struct task_struct {
	...
        struct task_ipi_mask			__private ipi_mask;

and have the magic accessors use

         ACCESS_PRIVATE(t->ipi_mask.member);

That allows static analysis to catch any incidential direct references.

Thanks,

        tglx