From 062eacf57ad91b5c272f89dc964fd6dd9715ea7d Mon Sep 17 00:00:00 2001 From: "Uladzislau Rezki (Sony)" Date: Thu, 30 Mar 2023 21:06:38 +0200 Subject: [PATCH] mm: vmalloc: remove a global vmap_blocks xarray A global vmap_blocks-xarray array can be contented under heavy usage of the vm_map_ram()/vm_unmap_ram() APIs. The lock_stat shows that a "vmap_blocks.xa_lock" lock is a second in a top-list when it comes to contentions: ---------------------------------------- class name con-bounces contentions ... ---------------------------------------- vmap_area_lock: 2554079 2554276 ... -------------- vmap_area_lock 1297948 [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910 vmap_area_lock 1256330 [<000000009d927bf3>] free_vmap_block+0x4a/0xe0 vmap_area_lock 1 [<00000000c95c05a7>] find_vm_area+0x16/0x70 -------------- vmap_area_lock 1738590 [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910 vmap_area_lock 815688 [<000000009d927bf3>] free_vmap_block+0x4a/0xe0 vmap_area_lock 1 [<00000000c1d619d7>] __get_vm_area_node+0xd2/0x170 vmap_blocks.xa_lock: 862689 862698 ... ------------------- vmap_blocks.xa_lock 378418 [<00000000625a5626>] vm_map_ram+0x359/0x4a0 vmap_blocks.xa_lock 484280 [<00000000caa2ef03>] xa_erase+0xe/0x30 ------------------- vmap_blocks.xa_lock 576226 [<00000000caa2ef03>] xa_erase+0xe/0x30 vmap_blocks.xa_lock 286472 [<00000000625a5626>] vm_map_ram+0x359/0x4a0 ... that is a result of running vm_map_ram()/vm_unmap_ram() in a loop. The test creates 64(on 64 CPUs system) threads and each one maps/unmaps 1 page. After this change the "xa_lock" can be considered as a noise in the same test condition: ... &xa->xa_lock#1: 10333 10394 ... -------------- &xa->xa_lock#1 5349 [<00000000bbbc9751>] xa_erase+0xe/0x30 &xa->xa_lock#1 5045 [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0 -------------- &xa->xa_lock#1 7326 [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0 &xa->xa_lock#1 3068 [<00000000bbbc9751>] xa_erase+0xe/0x30 ... Running the test_vmalloc.sh run_test_mask=1024 nr_threads=64 nr_pages=5 shows around ~8 percent of throughput improvement of vm_map_ram() and vm_unmap_ram() APIs. This patch does not fix vmap_area_lock/free_vmap_area_lock and purge_vmap_area_lock bottle-necks, it is rather a separate rework. Link: https://lkml.kernel.org/r/20230330190639.431589-1-urezki@gmail.com Signed-off-by: Uladzislau Rezki (Sony) Reviewed-by: Lorenzo Stoakes Reviewed-by: Baoquan He Cc: Christoph Hellwig Cc: Dave Chinner Cc: Matthew Wilcox (Oracle) Cc: Oleksiy Avramchenko Signed-off-by: Andrew Morton --- mm/vmalloc.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 63 insertions(+), 8 deletions(-) diff --git a/mm/vmalloc.c b/mm/vmalloc.c index 63a22557dc55..add33286a43b 100644 --- a/mm/vmalloc.c +++ b/mm/vmalloc.c @@ -1915,6 +1915,13 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr) struct vmap_block_queue { spinlock_t lock; struct list_head free; + + /* + * An xarray requires an extra memory dynamically to + * be allocated. If it is an issue, we can use rb-tree + * instead. + */ + struct xarray vmap_blocks; }; struct vmap_block { @@ -1932,11 +1939,48 @@ struct vmap_block { static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue); /* - * XArray of vmap blocks, indexed by address, to quickly find a vmap block - * in the free path. Could get rid of this if we change the API to return a - * "cookie" from alloc, to be passed to free. But no big deal yet. + * In order to fast access to any "vmap_block" associated with a + * specific address, we use a hash. + * + * A per-cpu vmap_block_queue is used in both ways, to serialize + * an access to free block chains among CPUs(alloc path) and it + * also acts as a vmap_block hash(alloc/free paths). It means we + * overload it, since we already have the per-cpu array which is + * used as a hash table. When used as a hash a 'cpu' passed to + * per_cpu() is not actually a CPU but rather a hash index. + * + * A hash function is addr_to_vb_xarray() which hashes any address + * to a specific index(in a hash) it belongs to. This then uses a + * per_cpu() macro to access an array with generated index. + * + * An example: + * + * CPU_1 CPU_2 CPU_0 + * | | | + * V V V + * 0 10 20 30 40 50 60 + * |------|------|------|------|------|------|... + * CPU0 CPU1 CPU2 CPU0 CPU1 CPU2 + * + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus + * it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock; + * + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus + * it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock; + * + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus + * it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock. + * + * This technique almost always avoids lock contention on insert/remove, + * however xarray spinlocks protect against any contention that remains. */ -static DEFINE_XARRAY(vmap_blocks); +static struct xarray * +addr_to_vb_xarray(unsigned long addr) +{ + int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus(); + + return &per_cpu(vmap_block_queue, index).vmap_blocks; +} /* * We should probably have a fallback mechanism to allocate virtual memory @@ -1974,6 +2018,7 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask) struct vmap_block_queue *vbq; struct vmap_block *vb; struct vmap_area *va; + struct xarray *xa; unsigned long vb_idx; int node, err; void *vaddr; @@ -2007,8 +2052,9 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask) bitmap_set(vb->used_map, 0, (1UL << order)); INIT_LIST_HEAD(&vb->free_list); + xa = addr_to_vb_xarray(va->va_start); vb_idx = addr_to_vb_idx(va->va_start); - err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask); + err = xa_insert(xa, vb_idx, vb, gfp_mask); if (err) { kfree(vb); free_vmap_area(va); @@ -2026,8 +2072,10 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask) static void free_vmap_block(struct vmap_block *vb) { struct vmap_block *tmp; + struct xarray *xa; - tmp = xa_erase(&vmap_blocks, addr_to_vb_idx(vb->va->va_start)); + xa = addr_to_vb_xarray(vb->va->va_start); + tmp = xa_erase(xa, addr_to_vb_idx(vb->va->va_start)); BUG_ON(tmp != vb); spin_lock(&vmap_area_lock); @@ -2139,6 +2187,7 @@ static void vb_free(unsigned long addr, unsigned long size) unsigned long offset; unsigned int order; struct vmap_block *vb; + struct xarray *xa; BUG_ON(offset_in_page(size)); BUG_ON(size > PAGE_SIZE*VMAP_MAX_ALLOC); @@ -2147,7 +2196,10 @@ static void vb_free(unsigned long addr, unsigned long size) order = get_order(size); offset = (addr & (VMAP_BLOCK_SIZE - 1)) >> PAGE_SHIFT; - vb = xa_load(&vmap_blocks, addr_to_vb_idx(addr)); + + xa = addr_to_vb_xarray(addr); + vb = xa_load(xa, addr_to_vb_idx(addr)); + spin_lock(&vb->lock); bitmap_clear(vb->used_map, offset, (1UL << order)); spin_unlock(&vb->lock); @@ -3525,6 +3577,7 @@ static size_t vmap_ram_vread_iter(struct iov_iter *iter, const char *addr, { char *start; struct vmap_block *vb; + struct xarray *xa; unsigned long offset; unsigned int rs, re; size_t remains, n; @@ -3543,7 +3596,8 @@ static size_t vmap_ram_vread_iter(struct iov_iter *iter, const char *addr, * Area is split into regions and tracked with vmap_block, read out * each region and zero fill the hole between regions. */ - vb = xa_load(&vmap_blocks, addr_to_vb_idx((unsigned long)addr)); + xa = addr_to_vb_xarray((unsigned long) addr); + vb = xa_load(xa, addr_to_vb_idx((unsigned long)addr)); if (!vb) goto finished_zero; @@ -4337,6 +4391,7 @@ void __init vmalloc_init(void) p = &per_cpu(vfree_deferred, i); init_llist_head(&p->list); INIT_WORK(&p->wq, delayed_vfree_work); + xa_init(&vbq->vmap_blocks); } /* Import existing vmlist entries. */