Re: [PATCH v5 02/11] powerpc/mm: Adds counting method to monitor lockless pgtable walks

[Christopher Lameter]

On Wed, 2 Oct 2019, Leonardo Bras wrote:

> +
> +inline unsigned long __begin_lockless_pgtbl_walk(struct mm_struct *mm,
> +                                              bool disable_irq)
> +{
> +     unsigned long irq_mask = 0;
> +
> +     if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING))
> +             atomic_inc(&mm->lockless_pgtbl_walkers);
> +

You are creating contention on a single exclusive cacheline. Doesnt this
defeat the whole purpose of the lockless page table walk? Use mmap_sem or
so should cause the same performance regression?