Re: [PATCH v12 7/9] hugetlb: support file_region coalescing again

[Mina Almasry <almasrymina@google.com>]

On Tue, Feb 18, 2020 at 7:31 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 2/11/20 1:31 PM, Mina Almasry wrote:
> > An earlier patch in this series disabled file_region coalescing in order
> > to hang the hugetlb_cgroup uncharge info on the file_region entries.
> >
> > This patch re-adds support for coalescing of file_region entries.
> > Essentially everytime we add an entry, we call a recursive function that
> > tries to coalesce the added region with the regions next to it. The
> > worst case call depth for this function is 3: one to coalesce with the
> > region next to it, one to coalesce to the region prev, and one to reach
> > the base case.
> >
> > This is an important performance optimization as private mappings add
> > their entries page by page, and we could incur big performance costs for
> > large mappings with lots of file_region entries in their resv_map.
> >
> > Signed-off-by: Mina Almasry <almasrymina@google.com>
> >
> > ---
> >
> > Changes in v12:
> > - Changed logic for coalescing. Instead of checking inline to coalesce
> > with only the region on next or prev, we now have a recursive function
> > that takes care of coalescing in both directions.
> > - For testing this code I added a bunch of debug code that checks that
> > the entries in the resv_map are coalesced appropriately. This passes
> > with libhugetlbfs tests.
> >
> > ---
> >  mm/hugetlb.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 85 insertions(+)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 2d62dd35399db..45219cb58ac71 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -276,6 +276,86 @@ static void record_hugetlb_cgroup_uncharge_info(struct hugetlb_cgroup *h_cg,
> >  #endif
> >  }
> >
> > +static bool has_same_uncharge_info(struct file_region *rg,
> > +                                struct file_region *org)
> > +{
> > +#ifdef CONFIG_CGROUP_HUGETLB
> > +     return rg && org &&
> > +            rg->reservation_counter == org->reservation_counter &&
> > +            rg->css == org->css;
> > +
> > +#else
> > +     return true;
> > +#endif
> > +}
> > +
> > +#ifdef CONFIG_DEBUG_VM
> > +static void dump_resv_map(struct resv_map *resv)
> > +{
> > +     struct list_head *head = &resv->regions;
> > +     struct file_region *rg = NULL;
> > +
> > +     pr_err("--------- start print resv_map ---------\n");
> > +     list_for_each_entry(rg, head, link) {
> > +             pr_err("rg->from=%ld, rg->to=%ld, rg->reservation_counter=%px, rg->css=%px\n",
> > +                    rg->from, rg->to, rg->reservation_counter, rg->css);
> > +     }
> > +     pr_err("--------- end print resv_map ---------\n");
> > +}
> > +
> > +/* Debug function to loop over the resv_map and make sure that coalescing is
> > + * working.
> > + */
> > +static void check_coalesce_bug(struct resv_map *resv)
> > +{
> > +     struct list_head *head = &resv->regions;
> > +     struct file_region *rg = NULL, *nrg = NULL;
> > +
> > +     list_for_each_entry(rg, head, link) {
> > +             nrg = list_next_entry(rg, link);
> > +
> > +             if (&nrg->link == head)
> > +                     break;
> > +
> > +             if (nrg->reservation_counter && nrg->from == rg->to &&
> > +                 nrg->reservation_counter == rg->reservation_counter &&
> > +                 nrg->css == rg->css) {
> > +                     dump_resv_map(resv);
> > +                     VM_BUG_ON(true);
> > +             }
> > +     }
> > +}
> > +#endif
> > +
> > +static void coalesce_file_region(struct resv_map *resv, struct file_region *rg)
> > +{
> > +     struct file_region *nrg = NULL, *prg = NULL;
> > +
> > +     prg = list_prev_entry(rg, link);
> > +     if (&prg->link != &resv->regions && prg->to == rg->from &&
> > +         has_same_uncharge_info(prg, rg)) {
> > +             prg->to = rg->to;
> > +
> > +             list_del(&rg->link);
> > +             kfree(rg);
> > +
> > +             coalesce_file_region(resv, prg);
> > +             return;
> > +     }
> > +
> > +     nrg = list_next_entry(rg, link);
> > +     if (&nrg->link != &resv->regions && nrg->from == rg->to &&
> > +         has_same_uncharge_info(nrg, rg)) {
> > +             nrg->from = rg->from;
> > +
> > +             list_del(&rg->link);
> > +             kfree(rg);
> > +
> > +             coalesce_file_region(resv, nrg);
> > +             return;
> > +     }
> > +}
> > +
> >  /* Must be called with resv->lock held. Calling this with count_only == true
> >   * will count the number of pages to be added but will not modify the linked
> >   * list. If regions_needed != NULL and count_only == true, then regions_needed
> > @@ -327,6 +407,7 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,
> >                               record_hugetlb_cgroup_uncharge_info(h_cg, h,
> >                                                                   resv, nrg);
> >                               list_add(&nrg->link, rg->link.prev);
> > +                             coalesce_file_region(resv, nrg);
> >                       } else if (regions_needed)
> >                               *regions_needed += 1;
> >               }
> > @@ -344,11 +425,15 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,
> >                               resv, last_accounted_offset, t);
> >                       record_hugetlb_cgroup_uncharge_info(h_cg, h, resv, nrg);
> >                       list_add(&nrg->link, rg->link.prev);
> > +                     coalesce_file_region(resv, nrg);
> >               } else if (regions_needed)
> >                       *regions_needed += 1;
> >       }
> >
> >       VM_BUG_ON(add < 0);
> > +#ifdef CONFIG_DEBUG_VM
> > +     check_coalesce_bug(resv);
> > +#endif
>
> Some distros have CONFIG_DEBUG_VM on in their default kernels.  Fedora comes
> to mind.  Yes, this means 'VM_BUG_ON()' become 'BUG_ON()'.  That is somewhat
> accepted.  I don't think we want this debug code behind CONFIG_DEBUG_VM and
> called each time a file region is added.  It may be overkill to make it a
> debug option via the config system.  Perhaps, just make it something like
> CONFIG_DEBUG_HUGETLB_RSV_REGION and let hugetlb developers turn it on if
> they would like?
>

Ah, I feel like adding a whole config to accommodate this function is
overkill and will end up being dead code anyway as no-one remembers to
turn it on. If it's the same to you I'll take it out of the patch and
leave it as a local change for me to test with.

> Other than that, the code looks good.
> --
> Mike Kravetz
>
> >       return add;
> >  }
> >
> > --
> > 2.25.0.225.g125e21ebc7-goog
> >