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:
>
> <snip>
> ----------------------------------------
> 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
> ...
> <snip>
>
> 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:
>
> <snip>
> ...
> &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
> ...
> <snip>
>
> 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) <urezki@xxxxxxxxx>
> ---
> 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
> + * |------|------|------|------|------|------|...<vmap address space>
> + * 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
>
