Re: [PATCH v2 2/2] hugetlb: optimize update_and_free_pages_bulk to avoid lock cycles

[Mike Kravetz]

On 07/18/23 09:31, James Houghton wrote:
> On Mon, Jul 17, 2023 at 5:50 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> >
> > update_and_free_pages_bulk is designed to free a list of hugetlb pages
> > back to their associated lower level allocators.  This may require
> > allocating vmemmmap pages associated with each hugetlb page.  The
> > hugetlb page destructor must be changed before pages are freed to lower
> > level allocators.  However, the destructor must be changed under the
> > hugetlb lock.  This means there is potentially one lock cycle per page.
> >
> > Minimize the number of lock cycles in update_and_free_pages_bulk by:
> > 1) allocating necessary vmemmap for all hugetlb pages on the list
> > 2) take hugetlb lock and clear destructor for all pages on the list
> > 3) free all pages on list back to low level allocators
> >
> > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > ---
> >  mm/hugetlb.c | 38 ++++++++++++++++++++++++++++++++++----
> >  1 file changed, 34 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 4a910121a647..e6b780291539 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1856,13 +1856,43 @@ static void update_and_free_hugetlb_folio(struct hstate *h, struct folio *folio,
> >  static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
> >  {
> >         struct page *page, *t_page;
> > -       struct folio *folio;
> > +       bool clear_dtor = false;
> >
> > +       /*
> > +        * First allocate required vmemmmap for all pages on list.  If vmemmap
> > +        * can not be allocated, we can not free page to lower level allocator,
> > +        * so add back as hugetlb surplus page.
> > +        */
> >         list_for_each_entry_safe(page, t_page, list, lru) {
> > -               folio = page_folio(page);
> > -               update_and_free_hugetlb_folio(h, folio, false);
> > -               cond_resched();
> > +               if (HPageVmemmapOptimized(page)) {
> > +                       if (hugetlb_vmemmap_restore(h, page)) {
> > +                               spin_lock_irq(&hugetlb_lock);
> > +                               add_hugetlb_folio(h, page_folio(page), true);
> > +                               spin_unlock_irq(&hugetlb_lock);
> > +                       } else
> > +                               clear_dtor = true;
> > +                       cond_resched();
> > +               }
> > +       }
> > +
> > +       /*
> > +        * If vmemmmap allocation performed above, then take lock to clear
> 
> s/vmemmmap/vmemmap. Also is a little hard to understand, something
> like "If vmemmap allocation was performed above for any folios,
> then..." seems clearer to me.
> 

Typo :(
Yes, that would be more clear ... see below.

> > +        * destructor of all pages on list.
> > +        */
> > +       if (clear_dtor) {
> > +               spin_lock_irq(&hugetlb_lock);
> > +               list_for_each_entry(page, list, lru)
> > +                       __clear_hugetlb_destructor(h, page_folio(page));
> > +               spin_unlock_irq(&hugetlb_lock);
> >         }
> 
> I'm not too familiar with this code, but the above block seems weird
> to me. If we successfully allocated the vmemmap for *any* folio, we
> clear the hugetlb destructor for all the folios? I feel like we should
> only be clearing the hugetlb destructor for all folios if the vmemmap
> allocation succeeded for *all* folios. If the code is functionally
> correct as is, I'm a little bit confused why we need `clear_dtor`; it
> seems like this function doesn't really need it. (I could have some
> huge misunderstanding here.)
> 

Yes, it is a bit strange.

I was thinking this has to also handle the case where hugetlb vmemmap
optimization is off system wide.  In that case, clear_dtor would never
be set and there is no sense in ever walking the list and calling
__clear_hugetlb_destructor() would would be a NOOP in this case.  Think
of the case where there are TBs of hugetlb pages.

That is one of the reasons I made __clear_hugetlb_destructor() check
for the need to modify the destructor.  The other reason is in the
dissolve_free_huge_page() code path where we allocate vmemmap.  I
suppose, there could be an explicit call to __clear_hugetlb_destructor()
in dissolve_free_huge_page.  But, I thought it might be better if
we just handled both cases here.

My thinking is that the clear_dtor boolean would tell us if vmemmap was
restored for ANY hugetlb page.  I am aware that just because vmemmap was
allocated for one page, does not mean that it was allocated for others.
However, in the common case where hugetlb vmemmap optimization is on
system wide, we would have allocated vmemmap for all pages on the list
and would need to clear the destructor for them all.

So, clear_dtor is really just an optimization for the
hugetlb_free_vmemmap=off case.  Perhaps that is just over thinking and
not a useful miro-optimization.

Thanks for taking a look!
-- 
Mike Kravetz

> > +
> > +       /*
> > +        * Free pages back to low level allocators.  vmemmap and destructors
> > +        * were taken care of above, so update_and_free_hugetlb_folio will
> > +        * not need to take hugetlb lock.
> > +        */
> > +       list_for_each_entry_safe(page, t_page, list, lru)
> > +               update_and_free_hugetlb_folio(h, page_folio(page), false);
> >  }
> >
> >  struct hstate *size_to_hstate(unsigned long size)
> > --
> > 2.41.0
> >