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SubjectRe: [PATCH v11 7/9] hugetlb: support file_region coalescing again
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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@google.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?

--
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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