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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:

<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>

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) <urezki@gmail.com>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Uladzislau Rezki (Sony) 2023-03-30 21:06:38 +02:00 committed by Andrew Morton
parent 62f31bd4dc
commit 062eacf57a
1 changed files with 63 additions and 8 deletions
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71
mm/vmalloc.c
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@ -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
* |------|------|------|------|------|------|...<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.
*
* 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. */