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

[Leonardo Bras]

On Tue, 2019-10-08 at 15:11 +0000, Christopher Lameter wrote:
> 
> 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?

Sorry, I did not understand that question.
I mean, this is just a refcount and never causes a lock.  


FYI: This function was updated as following, and will be in v6:

#ifdef CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING
	atomic_inc(&mm->lockless_pgtbl_walkers);
#endif
	smp_mb();

IS_ENABLED doesnt work fine if CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING
is not defined, causing an error: the mm member lockless_pgtbl_walkers
doesn't exist.
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