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[86.156.84.164]) by smtp.gmail.com with ESMTPSA id q10-20020adff94a000000b002ca864b807csm25975862wrr.0.2023.03.27.13.09.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 27 Mar 2023 13:09:32 -0700 (PDT) Date: Mon, 27 Mar 2023 21:09:32 +0100 From: Lorenzo Stoakes To: "Uladzislau Rezki (Sony)" Cc: Andrew Morton , linux-mm@kvack.org, LKML , Baoquan He , Christoph Hellwig , Matthew Wilcox , Dave Chinner , Oleksiy Avramchenko Subject: Re: [PATCH v3 1/2] mm: vmalloc: Remove a global vmap_blocks xarray Message-ID: <132e2d5c-0c1f-4fff-850c-b3fb084455bb@lucifer.local> References: <20230327170126.406044-1-urezki@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230327170126.406044-1-urezki@gmail.com> X-Rspam-User: X-Rspamd-Queue-Id: 8ABD8C000A X-Rspamd-Server: rspam01 X-Stat-Signature: foz7buffs55n9if8sksonssw5feesf7g X-HE-Tag: 1679947775-907099 X-HE-Meta: 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 MoPOabOL 75olFSyF0CwlISZlYKHWRl6aNg8RhV0LfmNsfq7li8wF2Ab/0yOSLTp6FQ6H6o7o2taMg9NxyOo1IR1j17gC54vWpdzxzSVfCdvIoYSE91FDNuLdAnjdmW6Te7OtHy5ZugWetlTzZ/JlKG43azlXfZ5ecQpkl8E507+gufWAUl51/P6l+ksgOXx/Az0BvPFP15Z492tLY3JWno9Qws03Fyl+FGEfTLLcGxHQqU0g4FzHtIAVxMTOJq9W+DJKtmNLoT6e3sgcSzhIL9ydC4M1n69RiLg/ET9g62OjYvkJDYaIz9jLuiV+BCyUN/UpjFZtcEqePsUMwIDesy2pZBRIWZysuSAEDQGRJFZ4eiiQ2Mp2uAG0NMq7TK8ZM/SDoHVRnqXrTisIGYACRfTRufjVYr1YPIL1NxkwYAqf+sajSbHIWiA2MjeiQS4WfKWYNotcnGyNwq1RTWVfi95sNKzWe71eaOEx2YlbNzYyW X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Mar 27, 2023 at 07:01:25PM +0200, Uladzislau Rezki (Sony) wrote: > 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 > ... > > > 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. > > v1 - v2: > - Add more comments(Andrew Morton req.) > - Switch to WARN_ON_ONCE(Lorenzo Stoakes req.) > > v2 -> v3: > - Fix a kernel-doc complain(Matthew Wilcox) > > Signed-off-by: Uladzislau Rezki (Sony) > --- > mm/vmalloc.c | 85 +++++++++++++++++++++++++++++++++++++++------------- > 1 file changed, 64 insertions(+), 21 deletions(-) > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > index 978194dc2bb8..821256ecf81c 100644 > --- a/mm/vmalloc.c > +++ b/mm/vmalloc.c > @@ -1908,9 +1908,22 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr) > #define VMAP_BLOCK 0x2 /* mark out the vmap_block sub-type*/ > #define VMAP_FLAGS_MASK 0x3 > > +/* > + * We should probably have a fallback mechanism to allocate virtual memory > + * out of partially filled vmap blocks. However vmap block sizing should be > + * fairly reasonable according to the vmalloc size, so it shouldn't be a > + * big problem. > + */ > 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 { > @@ -1928,24 +1941,46 @@ 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 store them into a per-cpu xarray. A hash > + * function is addr_to_vbq() whereas a key is a vb->va->va_start > + * value. > + * > + * Please note, a vmap_block_queue, which is a per-cpu, is not > + * serialized by a raw_smp_processor_id() current CPU, instead > + * it is chosen based on a CPU-index it belongs to, i.e. it is > + * a hash-table. > + * > + * 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. > */ OK so if I understand this correctly, you're overloading the per-CPU vmap_block_queue array to use as a simple hash based on the address and relying on the xa_lock() in xa_insert() to serialise in case of contention? I like the general heft of your comment but I feel this could be spelled out a little more clearly, something like:- In order to have fast access to any vmap_block object associated with a specific address, we use a hash. Rather than waste space on defining a new hash table we take advantage of the fact we already have a static per-cpu array vmap_block_queue. This is already used for per-CPU access to the block queue, however we overload this to _also_ act as a vmap_block hash. The hash function is addr_to_vbq() which hashes on vb->va->va_start. This then uses per_cpu() to lookup the _index_ rather than the _cpu_. Each vmap_block_queue contains an xarray of vmap blocks which are indexed on the same key as the hash (vb->va->va_start). xarray read acceses are protected by RCU lock and inserts are protected by a spin lock so there is no risk of a race here. An example: ... Feel free to cut this down as needed :) but I do feel it's important to _explicitly_ point out that we're overloading this as it's quite confusing at face value. > -static DEFINE_XARRAY(vmap_blocks); > +static struct vmap_block_queue * > +addr_to_vbq(unsigned long addr) > +{ > + int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus(); > > -/* > - * We should probably have a fallback mechanism to allocate virtual memory > - * out of partially filled vmap blocks. However vmap block sizing should be > - * fairly reasonable according to the vmalloc size, so it shouldn't be a > - * big problem. > - */ > + return &per_cpu(vmap_block_queue, index); > +} > > -static unsigned long addr_to_vb_idx(unsigned long addr) > +static unsigned long > +addr_to_vb_va_start(unsigned long addr) > { > - addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1); > - addr /= VMAP_BLOCK_SIZE; > - return addr; > + return rounddown(addr, VMAP_BLOCK_SIZE); > } > > static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off) > @@ -1953,7 +1988,7 @@ static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off) > unsigned long addr; > > addr = va_start + (pages_off << PAGE_SHIFT); > - BUG_ON(addr_to_vb_idx(addr) != addr_to_vb_idx(va_start)); > + WARN_ON_ONCE(addr_to_vb_va_start(addr) != va_start); > return (void *)addr; > } > > @@ -1970,7 +2005,6 @@ 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; > - unsigned long vb_idx; > int node, err; > void *vaddr; > > @@ -2003,8 +2037,8 @@ 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); > > - vb_idx = addr_to_vb_idx(va->va_start); > - err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask); > + vbq = addr_to_vbq(va->va_start); > + err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask); I might be being pedantic here, but shortly after this code you reassign vbq:- vbq = addr_to_vbq(va->va_start); err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask); if (err) { kfree(vb); free_vmap_area(va); return ERR_PTR(err); } vbq = raw_cpu_ptr(&vmap_block_queue); Which is confusing at a glance, as you're using it once as a hash lookup and again for its 'true purpose'. I wonder whether it would be better overall, since you always follow a vbq lookup explicitly with an operation on vmap_blocks, to just add a helper that returned a pointer to the xarray? e.g. (untested code here :):- static struct xarray *get_vblock_array(unsigned long addr) { struct vmap_block_queue *vbq; int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus(); vbq = &per_cpu(vmap_block_queue, index); return &vbq->vblocks; } And replace addr_to_vbq() with this. That'd also make the mechanism of this hash lookup super explicit. > if (err) { > kfree(vb); > free_vmap_area(va); > @@ -2021,9 +2055,11 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask) > > static void free_vmap_block(struct vmap_block *vb) > { > + struct vmap_block_queue *vbq; > struct vmap_block *tmp; > > - tmp = xa_erase(&vmap_blocks, addr_to_vb_idx(vb->va->va_start)); > + vbq = addr_to_vbq(vb->va->va_start); > + tmp = xa_erase(&vbq->vmap_blocks, vb->va->va_start); > BUG_ON(tmp != vb); > > spin_lock(&vmap_area_lock); > @@ -2135,6 +2171,7 @@ static void vb_free(unsigned long addr, unsigned long size) > unsigned long offset; > unsigned int order; > struct vmap_block *vb; > + struct vmap_block_queue *vbq; > > BUG_ON(offset_in_page(size)); > BUG_ON(size > PAGE_SIZE*VMAP_MAX_ALLOC); > @@ -2143,7 +2180,10 @@ 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)); > + > + vbq = addr_to_vbq(addr); > + vb = xa_load(&vbq->vmap_blocks, addr_to_vb_va_start(addr)); > + > spin_lock(&vb->lock); > bitmap_clear(vb->used_map, offset, (1UL << order)); > spin_unlock(&vb->lock); > @@ -3486,6 +3526,7 @@ static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags > { > char *start; > struct vmap_block *vb; > + struct vmap_block_queue *vbq; > unsigned long offset; > unsigned int rs, re, n; > > @@ -3503,7 +3544,8 @@ static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags > * 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)); > + vbq = addr_to_vbq((unsigned long) addr); > + vb = xa_load(&vbq->vmap_blocks, addr_to_vb_va_start((unsigned long) addr)); > if (!vb) > goto finished; > > @@ -4272,6 +4314,7 @@ 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. */ > -- > 2.30.2 >