On Sat, Aug 3, 2024 at 9:11 PM Barry Song <21cnbao@xxxxxxxxx> wrote: > > On Wed, Jul 31, 2024 at 6:49 PM <chrisl@xxxxxxxxxx> wrote: > > > > From: Kairui Song <kasong@xxxxxxxxxxx> > > > > Currently when we are freeing mTHP folios from swap cache, we free > > then one by one and put each entry into swap slot cache. Slot > > cache is designed to reduce the overhead by batching the freeing, > > but mTHP swap entries are already continuous so they can be batch > > freed without it already, it saves litle overhead, or even increase > > overhead for larger mTHP. > > > > What's more, mTHP entries could stay in swap cache for a while. > > Contiguous swap entry is an rather rare resource so releasing them > > directly can help improve mTHP allocation success rate when under > > pressure. > > > > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx> > > Acked-by: Barry Song <baohua@xxxxxxxxxx> > > I believe this is the right direction to take. Currently, entries are released > one by one, even when they are contiguous in the swap file(those nr_pages > entries are definitely in the same cluster and same si), leading to numerous > lock and unlock operations. > This approach provides batched support. > > free_swap_and_cache_nr() has the same issue, so I drafted a patch based on > your code. I wonder if you can also help test and review before I send it > officially: > > From 4bed5c08bc0f7769ee2849812acdad70c4e32ead Mon Sep 17 00:00:00 2001 > From: Barry Song <v-songbaohua@xxxxxxxx> > Date: Sat, 3 Aug 2024 20:21:14 +1200 > Subject: [PATCH RFC] mm: attempt to batch free swap entries for zap_pte_range() > > Zhiguo reported that swap release could be a serious bottleneck > during process exits[1]. With mTHP, we have the opportunity to > batch free swaps. > Thanks to the work of Chris and Kairui[2], I was able to achieve > this optimization with minimal code changes by building on their > efforts. > > [1] https://lore.kernel.org/linux-mm/20240731133318.527-1-justinjiang@xxxxxxxx/ > [2] https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-0-cb9c148b9297@xxxxxxxxxx/ > > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx> > --- > mm/swapfile.c | 43 +++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 43 insertions(+) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index ea023fc25d08..9def6dba8d26 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -156,6 +156,25 @@ static bool swap_is_has_cache(struct swap_info_struct *si, > return true; > } > > +static bool swap_is_last_map(struct swap_info_struct *si, > + unsigned long offset, int nr_pages, > + bool *any_only_cache) > +{ > + unsigned char *map = si->swap_map + offset; > + unsigned char *map_end = map + nr_pages; > + bool cached = false; > + > + do { > + if ((*map & ~SWAP_HAS_CACHE) != 1) > + return false; > + if (*map & SWAP_HAS_CACHE) > + cached = true; > + } while (++map < map_end); > + > + *any_only_cache = cached; > + return true; > +} > + > /* > * returns number of pages in the folio that backs the swap entry. If positive, > * the folio was reclaimed. If negative, the folio was not reclaimed. If 0, no > @@ -1808,6 +1827,29 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr) > if (WARN_ON(end_offset > si->max)) > goto out; > > + if (nr > 1) { > + struct swap_cluster_info *ci; > + bool batched_free; > + int i; > + > + ci = lock_cluster_or_swap_info(si, start_offset); > + if ((batched_free = swap_is_last_map(si, start_offset, nr, &any_only_cache))) { > + for (i = 0; i < nr; i++) > + WRITE_ONCE(si->swap_map[start_offset + i], SWAP_HAS_CACHE); > + } > + unlock_cluster_or_swap_info(si, ci); > + > + if (batched_free) { > + spin_lock(&si->lock); > + pr_err("%s offset:%lx nr:%lx\n", __func__,start_offset, nr); > + swap_entry_range_free(si, entry, nr); > + spin_unlock(&si->lock); > + if (any_only_cache) > + goto reclaim; > + goto out; > + } Sorry, what I actually meant was that the two gotos are reversed. if (batched_free) { if (any_only_cache) goto reclaim; spin_lock(&si->lock); swap_entry_range_free(si, entry, nr); spin_unlock(&si->lock); goto out; } > + } > + > /* > * First free all entries in the range. > */ > @@ -1828,6 +1870,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr) > if (!any_only_cache) > goto out; > > +reclaim: > /* > * Now go back over the range trying to reclaim the swap cache. This is > * more efficient for large folios because we will only try to reclaim > -- > 2.34.1 > > > > > --- > > mm/swapfile.c | 59 ++++++++++++++++++++++++++--------------------------------- > > 1 file changed, 26 insertions(+), 33 deletions(-) > > > > diff --git a/mm/swapfile.c b/mm/swapfile.c > > index 34e6ea13e8e4..9b63b2262cc2 100644 > > --- a/mm/swapfile.c > > +++ b/mm/swapfile.c > > @@ -479,20 +479,21 @@ static void inc_cluster_info_page(struct swap_info_struct *p, > > } > > > > /* > > - * The cluster ci decreases one usage. If the usage counter becomes 0, > > + * The cluster ci decreases @nr_pages usage. If the usage counter becomes 0, > > * which means no page in the cluster is in use, we can optionally discard > > * the cluster and add it to free cluster list. > > */ > > -static void dec_cluster_info_page(struct swap_info_struct *p, struct swap_cluster_info *ci) > > +static void dec_cluster_info_page(struct swap_info_struct *p, > > + struct swap_cluster_info *ci, int nr_pages) > > { > > if (!p->cluster_info) > > return; > > > > - VM_BUG_ON(ci->count == 0); > > + VM_BUG_ON(ci->count < nr_pages); > > VM_BUG_ON(cluster_is_free(ci)); > > lockdep_assert_held(&p->lock); > > lockdep_assert_held(&ci->lock); > > - ci->count--; > > + ci->count -= nr_pages; > > > > if (!ci->count) { > > free_cluster(p, ci); > > @@ -998,19 +999,6 @@ static int scan_swap_map_slots(struct swap_info_struct *si, > > return n_ret; > > } > > > > -static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx) > > -{ > > - unsigned long offset = idx * SWAPFILE_CLUSTER; > > - struct swap_cluster_info *ci; > > - > > - ci = lock_cluster(si, offset); > > - memset(si->swap_map + offset, 0, SWAPFILE_CLUSTER); > > - ci->count = 0; > > - free_cluster(si, ci); > > - unlock_cluster(ci); > > - swap_range_free(si, offset, SWAPFILE_CLUSTER); > > -} > > - > > int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order) > > { > > int order = swap_entry_order(entry_order); > > @@ -1269,21 +1257,28 @@ static unsigned char __swap_entry_free(struct swap_info_struct *p, > > return usage; > > } > > > > -static void swap_entry_free(struct swap_info_struct *p, swp_entry_t entry) > > +/* > > + * Drop the last HAS_CACHE flag of swap entries, caller have to > > + * ensure all entries belong to the same cgroup. > > + */ > > +static void swap_entry_range_free(struct swap_info_struct *p, swp_entry_t entry, > > + unsigned int nr_pages) > > { > > - struct swap_cluster_info *ci; > > unsigned long offset = swp_offset(entry); > > - unsigned char count; > > + unsigned char *map = p->swap_map + offset; > > + unsigned char *map_end = map + nr_pages; > > + struct swap_cluster_info *ci; > > > > ci = lock_cluster(p, offset); > > - count = p->swap_map[offset]; > > - VM_BUG_ON(count != SWAP_HAS_CACHE); > > - p->swap_map[offset] = 0; > > - dec_cluster_info_page(p, ci); > > + do { > > + VM_BUG_ON(*map != SWAP_HAS_CACHE); > > + *map = 0; > > + } while (++map < map_end); > > + dec_cluster_info_page(p, ci, nr_pages); > > unlock_cluster(ci); > > > > - mem_cgroup_uncharge_swap(entry, 1); > > - swap_range_free(p, offset, 1); > > + mem_cgroup_uncharge_swap(entry, nr_pages); > > + swap_range_free(p, offset, nr_pages); > > } > > > > static void cluster_swap_free_nr(struct swap_info_struct *sis, > > @@ -1343,7 +1338,6 @@ void swap_free_nr(swp_entry_t entry, int nr_pages) > > void put_swap_folio(struct folio *folio, swp_entry_t entry) > > { > > unsigned long offset = swp_offset(entry); > > - unsigned long idx = offset / SWAPFILE_CLUSTER; > > struct swap_cluster_info *ci; > > struct swap_info_struct *si; > > unsigned char *map; > > @@ -1356,19 +1350,18 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry) > > return; > > > > ci = lock_cluster_or_swap_info(si, offset); > > - if (size == SWAPFILE_CLUSTER) { > > + if (size > 1) { > > map = si->swap_map + offset; > > - for (i = 0; i < SWAPFILE_CLUSTER; i++) { > > + for (i = 0; i < size; i++) { > > val = map[i]; > > VM_BUG_ON(!(val & SWAP_HAS_CACHE)); > > if (val == SWAP_HAS_CACHE) > > free_entries++; > > } > > - if (free_entries == SWAPFILE_CLUSTER) { > > + if (free_entries == size) { > > unlock_cluster_or_swap_info(si, ci); > > spin_lock(&si->lock); > > - mem_cgroup_uncharge_swap(entry, SWAPFILE_CLUSTER); > > - swap_free_cluster(si, idx); > > + swap_entry_range_free(si, entry, size); > > spin_unlock(&si->lock); > > return; > > } > > @@ -1413,7 +1406,7 @@ void swapcache_free_entries(swp_entry_t *entries, int n) > > for (i = 0; i < n; ++i) { > > p = swap_info_get_cont(entries[i], prev); > > if (p) > > - swap_entry_free(p, entries[i]); > > + swap_entry_range_free(p, entries[i], 1); > > prev = p; > > } > > if (p) > > > > -- > > 2.46.0.rc1.232.g9752f9e123-goog > > > > Thanks > Barry >
