From 14c62da21b2b865f4fc0c49edd74ed7299927d35 Mon Sep 17 00:00:00 2001 From: Ryan Roberts Date: Mon, 8 Apr 2024 19:39:42 +0100 Subject: [PATCH] mm: swap: simplify struct percpu_cluster struct percpu_cluster stores the index of cpu's current cluster and the offset of the next entry that will be allocated for the cpu. These two pieces of information are redundant because the cluster index is just (offset / SWAPFILE_CLUSTER). The only reason for explicitly keeping the cluster index is because the structure used for it also has a flag to indicate "no cluster". However this data structure also contains a spin lock, which is never used in this context, as a side effect the code copies the spinlock_t structure, which is questionable coding practice in my view. So let's clean this up and store only the next offset, and use a sentinal value (SWAP_NEXT_INVALID) to indicate "no cluster". SWAP_NEXT_INVALID is chosen to be 0, because 0 will never be seen legitimately; The first page in the swap file is the swap header, which is always marked bad to prevent it from being allocated as an entry. This also prevents the cluster to which it belongs being marked free, so it will never appear on the free list. This change saves 16 bytes per cpu. And given we are shortly going to extend this mechanism to be per-cpu-AND-per-order, we will end up saving 16 * 9 = 144 bytes per cpu, which adds up if you have 256 cpus in the system. Link: https://lkml.kernel.org/r/20240408183946.2991168-4-ryan.roberts@arm.com Signed-off-by: Ryan Roberts Reviewed-by: "Huang, Ying" Cc: Barry Song <21cnbao@gmail.com> Cc: Barry Song Cc: Chris Li Cc: David Hildenbrand Cc: Gao Xiang Cc: Kefeng Wang Cc: Lance Yang Cc: Matthew Wilcox (Oracle) Cc: Michal Hocko Cc: Yang Shi Cc: Yu Zhao Signed-off-by: Andrew Morton --- include/linux/swap.h | 9 ++++++++- mm/swapfile.c | 22 +++++++++++----------- 2 files changed, 19 insertions(+), 12 deletions(-) diff --git a/include/linux/swap.h b/include/linux/swap.h index 2d8f2b950ddfa..ce35fd4abdf0f 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -260,13 +260,20 @@ struct swap_cluster_info { #define CLUSTER_FLAG_FREE 1 /* This cluster is free */ #define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */ +/* + * The first page in the swap file is the swap header, which is always marked + * bad to prevent it from being allocated as an entry. This also prevents the + * cluster to which it belongs being marked free. Therefore 0 is safe to use as + * a sentinel to indicate next is not valid in percpu_cluster. + */ +#define SWAP_NEXT_INVALID 0 + /* * We assign a cluster to each CPU, so each CPU can allocate swap entry from * its own cluster and swapout sequentially. The purpose is to optimize swapout * throughput. */ struct percpu_cluster { - struct swap_cluster_info index; /* Current cluster index */ unsigned int next; /* Likely next allocation offset */ }; diff --git a/mm/swapfile.c b/mm/swapfile.c index 20c45757f2b25..e3f8554752788 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -609,7 +609,7 @@ scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si, return false; percpu_cluster = this_cpu_ptr(si->percpu_cluster); - cluster_set_null(&percpu_cluster->index); + percpu_cluster->next = SWAP_NEXT_INVALID; return true; } @@ -622,14 +622,14 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si, { struct percpu_cluster *cluster; struct swap_cluster_info *ci; - unsigned long tmp, max; + unsigned int tmp, max; new_cluster: cluster = this_cpu_ptr(si->percpu_cluster); - if (cluster_is_null(&cluster->index)) { + tmp = cluster->next; + if (tmp == SWAP_NEXT_INVALID) { if (!cluster_list_empty(&si->free_clusters)) { - cluster->index = si->free_clusters.head; - cluster->next = cluster_next(&cluster->index) * + tmp = cluster_next(&si->free_clusters.head) * SWAPFILE_CLUSTER; } else if (!cluster_list_empty(&si->discard_clusters)) { /* @@ -649,9 +649,7 @@ new_cluster: * Other CPUs can use our cluster if they can't find a free cluster, * check if there is still free entry in the cluster */ - tmp = cluster->next; - max = min_t(unsigned long, si->max, - (cluster_next(&cluster->index) + 1) * SWAPFILE_CLUSTER); + max = min_t(unsigned long, si->max, ALIGN(tmp + 1, SWAPFILE_CLUSTER)); if (tmp < max) { ci = lock_cluster(si, tmp); while (tmp < max) { @@ -662,12 +660,13 @@ new_cluster: unlock_cluster(ci); } if (tmp >= max) { - cluster_set_null(&cluster->index); + cluster->next = SWAP_NEXT_INVALID; goto new_cluster; } - cluster->next = tmp + 1; *offset = tmp; *scan_base = tmp; + tmp += 1; + cluster->next = tmp < max ? tmp : SWAP_NEXT_INVALID; return true; } @@ -3163,8 +3162,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags) } for_each_possible_cpu(cpu) { struct percpu_cluster *cluster; + cluster = per_cpu_ptr(p->percpu_cluster, cpu); - cluster_set_null(&cluster->index); + cluster->next = SWAP_NEXT_INVALID; } } else { atomic_inc(&nr_rotate_swap); -- 2.30.2