mm, swap: remove contention workaround for swap cache

Swap cluster setup will try to shuffle the clusters on initialization.  It
was helpful to avoid contention for the swap cache space.  The cluster
size (2M) was much smaller than each swap cache space (64M), so shuffling
the cluster means the allocator will try to allocate swap slots that are
in different swap cache spaces for each CPU, reducing the chance of two
CPUs using the same swap cache space, and hence reducing the contention.

Now, swap cache is managed by swap clusters, this shuffle is pointless.
Just remove it, and clean up related macros.

This also improves the HDD swap performance as shuffling IO is a bad idea
for HDD, and now the shuffling is gone.  Test have shown a ~40%
performance gain for HDD [1]:

Doing sequential swap in of 8G data using 8 processes with usemem, average
of 3 test runs:

Before: 1270.91 KB/s per process
After:  1849.54 KB/s per process

Link: https://lore.kernel.org/linux-mm/CAMgjq7AdauQ8=X0zeih2r21QoV=-WWj1hyBxLWRzq74n-C=-Ng@mail.gmail.com/
Link: https://lkml.kernel.org/r/20250910160833.3464-14-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202504241621.f27743ec-lkp@intel.com
Acked-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/swap.h b/mm/swap.h
index adcd85fa85381fb41a284cb77b6581400bdca40e..fe5c209220828059d91fbf6a337092459550d14e 100644 (file)
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -198,10 +198,6 @@ int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug);
 void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug);
 
 /* linux/mm/swap_state.c */
-/* One swap address space for each 64M swap space */
-#define SWAP_ADDRESS_SPACE_SHIFT       14
-#define SWAP_ADDRESS_SPACE_PAGES       (1 << SWAP_ADDRESS_SPACE_SHIFT)
-#define SWAP_ADDRESS_SPACE_MASK                (SWAP_ADDRESS_SPACE_PAGES - 1)
 extern struct address_space swap_space __ro_after_init;
 static inline struct address_space *swap_address_space(swp_entry_t entry)
 {

diff --git a/mm/swapfile.c b/mm/swapfile.c
index a81ada4a361de9432322d603b17557dddf6e7df0..89659928465e728e6d26cdc39530e14a0a27219d 100644 (file)
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3203,21 +3203,14 @@ static int setup_swap_map(struct swap_info_struct *si,
        return 0;
 }
 
-#define SWAP_CLUSTER_INFO_COLS                                         \
-       DIV_ROUND_UP(L1_CACHE_BYTES, sizeof(struct swap_cluster_info))
-#define SWAP_CLUSTER_SPACE_COLS                                                \
-       DIV_ROUND_UP(SWAP_ADDRESS_SPACE_PAGES, SWAPFILE_CLUSTER)
-#define SWAP_CLUSTER_COLS                                              \
-       max_t(unsigned int, SWAP_CLUSTER_INFO_COLS, SWAP_CLUSTER_SPACE_COLS)
-
 static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
                                                union swap_header *swap_header,
                                                unsigned long maxpages)
 {
        unsigned long nr_clusters = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
        struct swap_cluster_info *cluster_info;
-       unsigned long i, j, idx;
        int err = -ENOMEM;
+       unsigned long i;
 
        cluster_info = kvcalloc(nr_clusters, sizeof(*cluster_info), GFP_KERNEL);
        if (!cluster_info)
@@ -3266,22 +3259,13 @@ static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
                INIT_LIST_HEAD(&si->frag_clusters[i]);
        }
 
-       /*
-        * Reduce false cache line sharing between cluster_info and
-        * sharing same address space.
-        */
-       for (j = 0; j < SWAP_CLUSTER_COLS; j++) {
-               for (i = 0; i < DIV_ROUND_UP(nr_clusters, SWAP_CLUSTER_COLS); i++) {
-                       struct swap_cluster_info *ci;
-                       idx = i * SWAP_CLUSTER_COLS + j;
-                       ci = cluster_info + idx;
-                       if (idx >= nr_clusters)
-                               continue;
-                       if (ci->count) {
-                               ci->flags = CLUSTER_FLAG_NONFULL;
-                               list_add_tail(&ci->list, &si->nonfull_clusters[0]);
-                               continue;
-                       }
+       for (i = 0; i < nr_clusters; i++) {
+               struct swap_cluster_info *ci = &cluster_info[i];
+
+               if (ci->count) {
+                       ci->flags = CLUSTER_FLAG_NONFULL;
+                       list_add_tail(&ci->list, &si->nonfull_clusters[0]);
+               } else {
                        ci->flags = CLUSTER_FLAG_FREE;
                        list_add_tail(&ci->list, &si->free_clusters);
                }

diff --git a/mm/zswap.c b/mm/zswap.c
index 3dda4310099eb02e35f85521847c54271bad9e21..cba7077fda4047f741cd68e9998fdd9c7286b0a7 100644 (file)
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -225,10 +225,13 @@ static bool zswap_has_pool;
 * helpers and fwd declarations
 **********************************/
 
+/* One swap address space for each 64M swap space */
+#define ZSWAP_ADDRESS_SPACE_SHIFT 14
+#define ZSWAP_ADDRESS_SPACE_PAGES (1 << ZSWAP_ADDRESS_SPACE_SHIFT)
 static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
 {
        return &zswap_trees[swp_type(swp)][swp_offset(swp)
-               >> SWAP_ADDRESS_SPACE_SHIFT];
+               >> ZSWAP_ADDRESS_SPACE_SHIFT];
 }
 
 #define zswap_pool_debug(msg, p)                       \
@@ -1674,7 +1677,7 @@ int zswap_swapon(int type, unsigned long nr_pages)
        struct xarray *trees, *tree;
        unsigned int nr, i;
 
-       nr = DIV_ROUND_UP(nr_pages, SWAP_ADDRESS_SPACE_PAGES);
+       nr = DIV_ROUND_UP(nr_pages, ZSWAP_ADDRESS_SPACE_PAGES);
        trees = kvcalloc(nr, sizeof(*tree), GFP_KERNEL);
        if (!trees) {
                pr_err("alloc failed, zswap disabled for swap type %d\n", type);