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Date: Fri, 7 Feb 2020 10:44:34 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Mike Kravetz <mike.kravetz@...cle.com>
Cc: shuah <shuah@...nel.org>, David Rientjes <rientjes@...gle.com>,
Shakeel Butt <shakeelb@...gle.com>,
Greg Thelen <gthelen@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
open list <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
linux-kselftest@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v11 7/9] hugetlb: support file_region coalescing again
On Thu, Feb 6, 2020 at 4:17 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 2/3/20 3:22 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 check to see if the
> > hugetlb_cgroup uncharge info is the same as any adjacent entries. If it
> > is, instead of adding an entry we simply extend the appropriate entry.
> >
> > 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@...gle.com>
> >
> > ---
> > mm/hugetlb.c | 62 +++++++++++++++++++++++++++++++++++++++++++---------
> > 1 file changed, 52 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index ec0b55ea1506e..058dd9c8269cf 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -272,6 +272,22 @@ static void record_hugetlb_cgroup_uncharge_info(struct hugetlb_cgroup *h_cg,
> > #endif
> > }
> >
> > +static bool has_same_uncharge_info(struct file_region *rg,
> > + struct hugetlb_cgroup *h_cg,
> > + struct hstate *h)
> > +{
> > +#ifdef CONFIG_CGROUP_HUGETLB
> > + return rg &&
> > + rg->reservation_counter ==
> > + &h_cg->rsvd_hugepage[hstate_index(h)] &&
> > + rg->pages_per_hpage == pages_per_huge_page(h) &&
> > + rg->css == &h_cg->css;
> > +
> > +#else
> > + return true;
> > +#endif
> > +}
> > +
> > /* 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
> > @@ -286,7 +302,7 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,
> > long add = 0;
> > struct list_head *head = &resv->regions;
> > long last_accounted_offset = f;
> > - struct file_region *rg = NULL, *trg = NULL, *nrg = NULL;
> > + struct file_region *rg = NULL, *trg = NULL, *nrg = NULL, *prg = NULL;
> >
> > if (regions_needed)
> > *regions_needed = 0;
> > @@ -318,16 +334,34 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,
>
> I seem to be missing something. For context, here is the beginning of that
> loop:
>
> /* In this loop, we essentially handle an entry for the range
> * [last_accounted_offset, rg->from), at every iteration, with some
> * bounds checking.
> */
> list_for_each_entry_safe(rg, trg, head, link) {
> /* Skip irrelevant regions that start before our range. */
> if (rg->from < f) {
> /* If this region ends after the last accounted offset,
> * then we need to update last_accounted_offset.
> */
> if (rg->to > last_accounted_offset)
> last_accounted_offset = rg->to;
> continue;
> }
>
> /* When we find a region that starts beyond our range, we've
> * finished.
> */
> if (rg->from > t)
> break;
>
> Suppose the resv_map contains one entry [0,2) and we are going to add
> [2,4). Will we not 'continue' after the first entry and then exit loop
> without setting prg? So, there is no chance for coalescing?
>
I think you're right; prg needs to be set on all loop exits, including
the continue and break. I'm thinking with that added, the logic should
work, but I need to find a good way to test this. I thought I had good
test coverage but apparently not. I'll fix this in the next iteration.
> --
> Mike Kravetz
>
> > if (rg->from > last_accounted_offset) {
> > add += rg->from - last_accounted_offset;
> > if (!count_only) {
> > - nrg = get_file_region_entry_from_cache(
> > - resv, last_accounted_offset, rg->from);
> > - record_hugetlb_cgroup_uncharge_info(h_cg, nrg,
> > - h);
> > - list_add(&nrg->link, rg->link.prev);
> > + /* Check if the last region can be extended. */
> > + if (prg && prg->to == last_accounted_offset &&
> > + has_same_uncharge_info(prg, h_cg, h)) {
> > + prg->to = rg->from;
> > + /* Check if the next region can be extended. */
> > + } else if (has_same_uncharge_info(rg, h_cg,
> > + h)) {
> > + rg->from = last_accounted_offset;
> > + /* If neither of the regions can be extended,
> > + * add a region.
> > + */
> > + } else {
> > + nrg = get_file_region_entry_from_cache(
> > + resv, last_accounted_offset,
> > + rg->from);
> > + record_hugetlb_cgroup_uncharge_info(
> > + h_cg, nrg, h);
> > + list_add(&nrg->link, rg->link.prev);
> > + }
> > } else if (regions_needed)
> > *regions_needed += 1;
> > }
> >
> > last_accounted_offset = rg->to;
> > + /* Record rg as the 'previous file region' incase we need it
> > + * for the next iteration.
> > + */
> > + prg = rg;
> > }
> >
> > /* Handle the case where our range extends beyond
> > @@ -336,10 +370,18 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,
> > if (last_accounted_offset < t) {
> > add += t - last_accounted_offset;
> > if (!count_only) {
> > - nrg = get_file_region_entry_from_cache(
> > - resv, last_accounted_offset, t);
> > - record_hugetlb_cgroup_uncharge_info(h_cg, nrg, h);
> > - list_add(&nrg->link, rg->link.prev);
> > + /* Check if the last region can be extended. */
> > + if (prg && prg->to == last_accounted_offset &&
> > + has_same_uncharge_info(prg, h_cg, h)) {
> > + prg->to = last_accounted_offset;
> > + } else {
> > + /* If not, just create a new region. */
> > + nrg = get_file_region_entry_from_cache(
> > + resv, last_accounted_offset, t);
> > + record_hugetlb_cgroup_uncharge_info(h_cg, nrg,
> > + h);
> > + list_add(&nrg->link, rg->link.prev);
> > + }
> > } else if (regions_needed)
> > *regions_needed += 1;
> > }
> > --
> > 2.25.0.341.g760bfbb309-goog
> >
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