Re: [External] Re: [PATCH v4 4/6] mm: hugetlb: retry dissolve page when hitting race

[Muchun Song]

On Wed, Jan 13, 2021 at 6:38 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 13-01-21 18:14:55, Muchun Song wrote:
> > On Wed, Jan 13, 2021 at 5:33 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Wed 13-01-21 13:22:07, Muchun Song wrote:
> > > > There is a race between dissolve_free_huge_page() and put_page().
> > > > Theoretically, we should return -EBUSY when we encounter this race.
> > > > In fact, we have a chance to successfully dissolve the page if we
> > > > do a retry. Because the race window is quite small. If we seize
> > > > this opportunity, it is an optimization for increasing the success
> > > > rate of dissolving page.
> > > >
> > > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > > > ---
> > > >  mm/hugetlb.c | 20 ++++++++++++++++++--
> > > >  1 file changed, 18 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > > index 4a9011e12175..898e4ea43e13 100644
> > > > --- a/mm/hugetlb.c
> > > > +++ b/mm/hugetlb.c
> > > > @@ -1772,6 +1772,7 @@ int dissolve_free_huge_page(struct page *page)
> > > >  {
> > > >       int rc = -EBUSY;
> > > >
> > > > +retry:
> > > >       /* Not to disrupt normal path by vainly holding hugetlb_lock */
> > > >       if (!PageHuge(page))
> > > >               return 0;
> > > > @@ -1793,8 +1794,23 @@ int dissolve_free_huge_page(struct page *page)
> > > >                * We should make sure that the page is already on the free list
> > > >                * when it is dissolved.
> > > >                */
> > > > -             if (unlikely(!PageHugeFreed(head)))
> > > > -                     goto out;
> > > > +             if (unlikely(!PageHugeFreed(head))) {
> > > > +                     spin_unlock(&hugetlb_lock);
> > > > +
> > > > +                     /*
> > > > +                      * Theoretically, we should return -EBUSY when we
> > > > +                      * encounter this race. In fact, we have a chance
> > > > +                      * to successfully dissolve the page if we do a
> > > > +                      * retry. Because the race window is quite small.
> > > > +                      * If we seize this opportunity, it is an optimization
> > > > +                      * for increasing the success rate of dissolving page.
> > > > +                      */
> > > > +                     while (PageHeadHuge(head) && !PageHugeFreed(head)) {
> > > > +                             cond_resched();
> > > > +                             cpu_relax();
> > > > +                     }
> > > > +                     goto retry;
> > >
> > > OK, so you have done the retry here. Please fold it into the previous
> > > patch. Also do we need cpu_relax on top of cond_resched as well?
> >
> > Because the previous patch is a bugfix and should be backprt to the other
> > stable tree, right?
>
> Yes, it is a bugfix but it arguably opens another issue so the follow up
> patch should better be applied along with it.

OK. I will fold this one into the previous one. Thanks.

>
> > I just want the fix patch to be small enough.
> > So I do the retry in this patch. If you do not agree with this. I
> > will fold this into the previous patch.
> >
> > Do you mean this?
> >
> > cpu_relax();
> > cond_resched();
> > cpu_relax();
>
> No, I am questiong the use of cpu_relax. What is the point?

If there is no task to be scheduled. Here is just a while loop.
The cpu_relax is a good thing to insert into busy-wait loops,
right?


>
> --
> Michal Hocko
> SUSE Labs