Re: [PATCH v8 7/8] hugetlb: batch TLB flushes when freeing vmemmap

Mike Kravetz
2023-10-21T19:39:12Z

On 10/21/23 11:20, Jane Chu wrote:
> Hi, Mike,
>
> On 10/18/2023 7:31 PM, Mike Kravetz wrote:
> > From: Joao Martins <joao.m.martins@oracle.com>
> >
> > Now that a list of pages is deduplicated at once, the TLB
> > flush can be batched for all vmemmap pages that got remapped.
> >
> [..]
>
> > @@ -719,19 +737,28 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_l
> > list_for_each_entry(folio, folio_list, lru) {
> > int ret = __hugetlb_vmemmap_optimize(h, &folio->page,
> > - &vmemmap_pages);
> > + &vmemmap_pages,
> > + VMEMMAP_REMAP_NO_TLB_FLUSH);
> > /*
> > * Pages to be freed may have been accumulated. If we
> > * encounter an ENOMEM, free what we have and try again.
> > + * This can occur in the case that both spliting fails
> > + * halfway and head page allocation also failed. In this
> > + * case __hugetlb_vmemmap_optimize() would free memory
> > + * allowing more vmemmap remaps to occur.
> > */
> > if (ret == -ENOMEM && !list_empty(&vmemmap_pages)) {
> > + flush_tlb_all();
> > free_vmemmap_page_list(&vmemmap_pages);
> > INIT_LIST_HEAD(&vmemmap_pages);
> > - __hugetlb_vmemmap_optimize(h, &folio->page, &vmemmap_pages);
> > + __hugetlb_vmemmap_optimize(h, &folio->page,
> > + &vmemmap_pages,
> > + VMEMMAP_REMAP_NO_TLB_FLUSH);
> > }
> > }
> > + flush_tlb_all();
>
> It seems that if folio_list is empty, we could spend a tlb flush here.
> perhaps it's worth to check against empty list up front and return ?

Good point.

hugetlb_vmemmap_optimize_folios is only called from
prep_and_add_allocated_folios and prep_and_add_bootmem_folios. I
previously thought about adding a check like the following at the
beginning of those routines.

if (list_empty(folio_list))
return;

However that seemed like over optimizing. But, such a check would avoid
the tlb flush as you point out above as well as an unnecessary
hugetlb_lock lock/unlock cycle.

We can add something like this as an optimization. I am not too concerned
about this right now because these these routines are generally called very
infrequently as the result of a user request to change the size of hugetlb
pools.
--
Mike Kravetz