Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp_sync IPI only to MM CPUs

Dave Hansen
2023-06-21T17:43:41Z

On 6/20/23 07:46, Yair Podemsky wrote:
> -void tlb_remove_table_sync_one(void)
> +#ifdef CONFIG_ARCH_HAS_CPUMASK_BITS
> +#define REMOVE_TABLE_IPI_MASK mm_cpumask(mm)
> +#else
> +#define REMOVE_TABLE_IPI_MASK cpu_online_mask
> +#endif /* CONFIG_ARCH_HAS_CPUMASK_BITS */
> +
> +void tlb_remove_table_sync_one(struct mm_struct *mm)
> {
> /*
> * This isn't an RCU grace period and hence the page-tables cannot be
> @@ -200,7 +206,8 @@ void tlb_remove_table_sync_one(void)
> * It is however sufficient for software page-table walkers that rely on
> * IRQ disabling.
> */
> - smp_call_function(tlb_remove_table_smp_sync, NULL, 1);
> + on_each_cpu_mask(REMOVE_TABLE_IPI_MASK, tlb_remove_table_smp_sync,
> + NULL, true);
> }

That "REMOVE_TABLE_IPI_MASK" thing is pretty confusing. It *looks* like
a constant. It does *NOT* look at all like it consumes 'mm'. Worst
case, just create a local variable:

if (IS_ENABLED(CONFIG_ARCH_HAS_CPUMASK_BITS))
ipi_mask = mm_cpumask(mm);
else
ipi_mask = cpu_online_mask;

on_each_cpu_mask(ipi_mask, ...);

That's a billion times more clear and it'll compile down to the same thing.

I do think the CONFIG_ARCH_HAS_CPUMASK_BITS naming is also pretty
confusing, but I don't have any better suggestions. Maybe something
with "MM_CPUMASK" in it?