[PATCH v3 07/11] mm: vmalloc: Offload free_vmap_area_lock lock

Uladzislau Rezki (Sony)
2024-01-02T18:47:18Z

Concurrent access to a global vmap space is a bottle-neck.
We can simulate a high contention by running a vmalloc test
suite.

To address it, introduce an effective vmap node logic. Each
node behaves as independent entity. When a node is accessed
it serves a request directly(if possible) from its pool.

This model has a size based pool for requests, i.e. pools are
serialized and populated based on object size and real demand.
A maximum object size that pool can handle is set to 256 pages.

This technique reduces a pressure on the global vmap lock.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
mm/vmalloc.c | 387 +++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 342 insertions(+), 45 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 9b2f1b0cac9d..fa4ab2bbbc5b 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -775,7 +775,22 @@ struct rb_list {
spinlock_t lock;
};

+struct vmap_pool {
+ struct list_head head;
+ unsigned long len;
+};
+
+/*
+ * A fast size storage contains VAs up to 1M size.
+ */
+#define MAX_VA_SIZE_PAGES 256
+
static struct vmap_node {
+ /* Simple size segregated storage. */
+ struct vmap_pool pool[MAX_VA_SIZE_PAGES];
+ spinlock_t pool_lock;
+ bool skip_populate;
+
/* Bookkeeping data of this node. */
struct rb_list busy;
struct rb_list lazy;
@@ -784,6 +799,8 @@ static struct vmap_node {
* Ready-to-free areas.
*/
struct list_head purge_list;
+ struct work_struct purge_work;
+ unsigned long nr_purged;
} single;

static struct vmap_node *vmap_nodes = &single;
@@ -802,6 +819,61 @@ addr_to_node(unsigned long addr)
return &vmap_nodes[addr_to_node_id(addr)];
}

+static inline struct vmap_node *
+id_to_node(unsigned int id)
+{
+ return &vmap_nodes[id % nr_vmap_nodes];
+}
+
+/*
+ * We use the value 0 to represent "no node", that is why
+ * an encoded value will be the node-id incremented by 1.
+ * It is always greater then 0. A valid node_id which can
+ * be encoded is [0:nr_vmap_nodes - 1]. If a passed node_id
+ * is not valid 0 is returned.
+ */
+static unsigned int
+encode_vn_id(unsigned int node_id)
+{
+ /* Can store U8_MAX [0:254] nodes. */
+ if (node_id < nr_vmap_nodes)
+ return (node_id + 1) << BITS_PER_BYTE;
+
+ /* Warn and no node encoded. */
+ WARN_ONCE(1, "Encode wrong node id (%u)\n", node_id);
+ return 0;
+}
+
+/*
+ * Returns an encoded node-id, the valid range is within
+ * [0:nr_vmap_nodes-1] values. Otherwise nr_vmap_nodes is
+ * returned if extracted data is wrong.
+ */
+static unsigned int
+decode_vn_id(unsigned int val)
+{
+ unsigned int node_id = (val >> BITS_PER_BYTE) - 1;
+
+ /* Can store U8_MAX [0:254] nodes. */
+ if (node_id < nr_vmap_nodes)
+ return node_id;
+
+ /* If it was _not_ zero, warn. */
+ WARN_ONCE(node_id != UINT_MAX,
+ "Decode wrong node id (%d)\n", node_id);
+
+ return nr_vmap_nodes;
+}
+
+static bool
+is_vn_id_valid(unsigned int node_id)
+{
+ if (node_id < nr_vmap_nodes)
+ return true;
+
+ return false;
+}
+
static __always_inline unsigned long
va_size(struct vmap_area *va)
{
@@ -1623,6 +1695,104 @@ preload_this_cpu_lock(spinlock_t *lock, gfp_t gfp_mask, int node)
kmem_cache_free(vmap_area_cachep, va);
}

+static struct vmap_pool *
+size_to_va_pool(struct vmap_node *vn, unsigned long size)
+{
+ unsigned int idx = (size - 1) / PAGE_SIZE;
+
+ if (idx < MAX_VA_SIZE_PAGES)
+ return &vn->pool[idx];
+
+ return NULL;
+}
+
+static bool
+node_pool_add_va(struct vmap_node *n, struct vmap_area *va)
+{
+ struct vmap_pool *vp;
+
+ vp = size_to_va_pool(n, va_size(va));
+ if (!vp)
+ return false;
+
+ spin_lock(&n->pool_lock);
+ list_add(&va->list, &vp->head);
+ WRITE_ONCE(vp->len, vp->len + 1);
+ spin_unlock(&n->pool_lock);
+
+ return true;
+}
+
+static struct vmap_area *
+node_pool_del_va(struct vmap_node *vn, unsigned long size,
+ unsigned long align, unsigned long vstart,
+ unsigned long vend)
+{
+ struct vmap_area *va = NULL;
+ struct vmap_pool *vp;
+ int err = 0;
+
+ vp = size_to_va_pool(vn, size);
+ if (!vp || list_empty(&vp->head))
+ return NULL;
+
+ spin_lock(&vn->pool_lock);
+ if (!list_empty(&vp->head)) {
+ va = list_first_entry(&vp->head, struct vmap_area, list);
+
+ if (IS_ALIGNED(va->va_start, align)) {
+ /*
+ * Do some sanity check and emit a warning
+ * if one of below checks detects an error.
+ */
+ err |= (va_size(va) != size);
+ err |= (va->va_start < vstart);
+ err |= (va->va_end > vend);
+
+ if (!WARN_ON_ONCE(err)) {
+ list_del_init(&va->list);
+ WRITE_ONCE(vp->len, vp->len - 1);
+ } else {
+ va = NULL;
+ }
+ } else {
+ list_move_tail(&va->list, &vp->head);
+ va = NULL;
+ }
+ }
+ spin_unlock(&vn->pool_lock);
+
+ return va;
+}
+
+static struct vmap_area *
+node_alloc(unsigned long size, unsigned long align,
+ unsigned long vstart, unsigned long vend,
+ unsigned long *addr, unsigned int *vn_id)
+{
+ struct vmap_area *va;
+
+ *vn_id = 0;
+ *addr = vend;
+
+ /*
+ * Fallback to a global heap if not vmalloc or there
+ * is only one node.
+ */
+ if (vstart != VMALLOC_START || vend != VMALLOC_END ||
+ nr_vmap_nodes == 1)
+ return NULL;
+
+ *vn_id = raw_smp_processor_id() % nr_vmap_nodes;
+ va = node_pool_del_va(id_to_node(*vn_id), size, align, vstart, vend);
+ *vn_id = encode_vn_id(*vn_id);
+
+ if (va)
+ *addr = va->va_start;
+
+ return va;
+}
+
/*
* Allocate a region of KVA of the specified size and alignment, within the
* vstart and vend.
@@ -1637,6 +1807,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
struct vmap_area *va;
unsigned long freed;
unsigned long addr;
+ unsigned int vn_id;
int purged = 0;
int ret;

@@ -1647,11 +1818,23 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
return ERR_PTR(-EBUSY);

might_sleep();
- gfp_mask = gfp_mask & GFP_RECLAIM_MASK;

- va = kmem_cache_alloc_node(vmap_area_cachep, gfp_mask, node);
- if (unlikely(!va))
- return ERR_PTR(-ENOMEM);
+ /*
+ * If a VA is obtained from a global heap(if it fails here)
+ * it is anyway marked with this "vn_id" so it is returned
+ * to this pool's node later. Such way gives a possibility
+ * to populate pools based on users demand.
+ *
+ * On success a ready to go VA is returned.
+ */
+ va = node_alloc(size, align, vstart, vend, &addr, &vn_id);
+ if (!va) {
+ gfp_mask = gfp_mask & GFP_RECLAIM_MASK;
+
+ va = kmem_cache_alloc_node(vmap_area_cachep, gfp_mask, node);
+ if (unlikely(!va))
+ return ERR_PTR(-ENOMEM);
+ }

/*
* Only scan the relevant parts containing pointers to other objects
@@ -1660,10 +1843,12 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
kmemleak_scan_area(&va->rb_node, SIZE_MAX, gfp_mask);

retry:
- preload_this_cpu_lock(&free_vmap_area_lock, gfp_mask, node);
- addr = __alloc_vmap_area(&free_vmap_area_root, &free_vmap_area_list,
- size, align, vstart, vend);
- spin_unlock(&free_vmap_area_lock);
+ if (addr == vend) {
+ preload_this_cpu_lock(&free_vmap_area_lock, gfp_mask, node);
+ addr = __alloc_vmap_area(&free_vmap_area_root, &free_vmap_area_list,
+ size, align, vstart, vend);
+ spin_unlock(&free_vmap_area_lock);
+ }

trace_alloc_vmap_area(addr, size, align, vstart, vend, addr == vend);

@@ -1677,7 +1862,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
va->va_start = addr;
va->va_end = addr + size;
va->vm = NULL;
- va->flags = va_flags;
+ va->flags = (va_flags | vn_id);

vn = addr_to_node(va->va_start);

@@ -1770,63 +1955,135 @@ static DEFINE_MUTEX(vmap_purge_lock);
static void purge_fragmented_blocks_allcpus(void);
static cpumask_t purge_nodes;

-/*
- * Purges all lazily-freed vmap areas.
- */
-static unsigned long
-purge_vmap_node(struct vmap_node *vn)
+static void
+reclaim_list_global(struct list_head *head)
{
- unsigned long num_purged_areas = 0;
- struct vmap_area *va, *n_va;
+ struct vmap_area *va, *n;

- if (list_empty(&vn->purge_list))
- return 0;
+ if (list_empty(head))
+ return;

spin_lock(&free_vmap_area_lock);
+ list_for_each_entry_safe(va, n, head, list)
+ merge_or_add_vmap_area_augment(va,
+ &free_vmap_area_root, &free_vmap_area_list);
+ spin_unlock(&free_vmap_area_lock);
+}
+
+static void
+decay_va_pool_node(struct vmap_node *vn, bool full_decay)
+{
+ struct vmap_area *va, *nva;
+ struct list_head decay_list;
+ struct rb_root decay_root;
+ unsigned long n_decay;
+ int i;
+
+ decay_root = RB_ROOT;
+ INIT_LIST_HEAD(&decay_list);
+
+ for (i = 0; i < MAX_VA_SIZE_PAGES; i++) {
+ struct list_head tmp_list;
+
+ if (list_empty(&vn->pool[i].head))
+ continue;
+
+ INIT_LIST_HEAD(&tmp_list);
+
+ /* Detach the pool, so no-one can access it. */
+ spin_lock(&vn->pool_lock);
+ list_replace_init(&vn->pool[i].head, &tmp_list);
+ spin_unlock(&vn->pool_lock);
+
+ if (full_decay)
+ WRITE_ONCE(vn->pool[i].len, 0);
+
+ /* Decay a pool by ~25% out of left objects. */
+ n_decay = vn->pool[i].len >> 2;
+
+ list_for_each_entry_safe(va, nva, &tmp_list, list) {
+ list_del_init(&va->list);
+ merge_or_add_vmap_area(va, &decay_root, &decay_list);
+
+ if (!full_decay) {
+ WRITE_ONCE(vn->pool[i].len, vn->pool[i].len - 1);
+
+ if (!--n_decay)
+ break;
+ }
+ }
+
+ /* Attach the pool back if it has been partly decayed. */
+ if (!full_decay && !list_empty(&tmp_list)) {
+ spin_lock(&vn->pool_lock);
+ list_replace_init(&tmp_list, &vn->pool[i].head);
+ spin_unlock(&vn->pool_lock);
+ }
+ }
+
+ reclaim_list_global(&decay_list);
+}
+
+static void purge_vmap_node(struct work_struct *work)
+{
+ struct vmap_node *vn = container_of(work,
+ struct vmap_node, purge_work);
+ struct vmap_area *va, *n_va;
+ LIST_HEAD(local_list);
+
+ vn->nr_purged = 0;
+
list_for_each_entry_safe(va, n_va, &vn->purge_list, list) {
unsigned long nr = (va->va_end - va->va_start) >> PAGE_SHIFT;
unsigned long orig_start = va->va_start;
unsigned long orig_end = va->va_end;
+ unsigned int vn_id = decode_vn_id(va->flags);

- /*
- * Finally insert or merge lazily-freed area. It is
- * detached and there is no need to "unlink" it from
- * anything.
- */
- va = merge_or_add_vmap_area_augment(va, &free_vmap_area_root,
- &free_vmap_area_list);
-
- if (!va)
- continue;
+ list_del_init(&va->list);

if (is_vmalloc_or_module_addr((void *)orig_start))
kasan_release_vmalloc(orig_start, orig_end,
va->va_start, va->va_end);

atomic_long_sub(nr, &vmap_lazy_nr);
- num_purged_areas++;
+ vn->nr_purged++;
+
+ if (is_vn_id_valid(vn_id) && !vn->skip_populate)
+ if (node_pool_add_va(vn, va))
+ continue;
+
+ /* Go back to global. */
+ list_add(&va->list, &local_list);
}
- spin_unlock(&free_vmap_area_lock);

- return num_purged_areas;
+ reclaim_list_global(&local_list);
}

/*
* Purges all lazily-freed vmap areas.
*/
-static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
+static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end,
+ bool full_pool_decay)
{
- unsigned long num_purged_areas = 0;
+ unsigned long nr_purged_areas = 0;
+ unsigned int nr_purge_helpers;
+ unsigned int nr_purge_nodes;
struct vmap_node *vn;
int i;

lockdep_assert_held(&vmap_purge_lock);
+
+ /*
+ * Use cpumask to mark which node has to be processed.
+ */
purge_nodes = CPU_MASK_NONE;

for (i = 0; i < nr_vmap_nodes; i++) {
vn = &vmap_nodes[i];

INIT_LIST_HEAD(&vn->purge_list);
+ vn->skip_populate = full_pool_decay;
+ decay_va_pool_node(vn, full_pool_decay);

if (RB_EMPTY_ROOT(&vn->lazy.root))
continue;
@@ -1845,17 +2102,45 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
cpumask_set_cpu(i, &purge_nodes);
}

- if (cpumask_weight(&purge_nodes) > 0) {
+ nr_purge_nodes = cpumask_weight(&purge_nodes);
+ if (nr_purge_nodes > 0) {
flush_tlb_kernel_range(start, end);

+ /* One extra worker is per a lazy_max_pages() full set minus one. */
+ nr_purge_helpers = atomic_long_read(&vmap_lazy_nr) / lazy_max_pages();
+ nr_purge_helpers = clamp(nr_purge_helpers, 1U, nr_purge_nodes) - 1;
+
for_each_cpu(i, &purge_nodes) {
- vn = &nodes[i];
- num_purged_areas += purge_vmap_node(vn);
+ vn = &vmap_nodes[i];
+
+ if (nr_purge_helpers > 0) {
+ INIT_WORK(&vn->purge_work, purge_vmap_node);
+
+ if (cpumask_test_cpu(i, cpu_online_mask))
+ schedule_work_on(i, &vn->purge_work);
+ else
+ schedule_work(&vn->purge_work);
+
+ nr_purge_helpers--;
+ } else {
+ vn->purge_work.func = NULL;
+ purge_vmap_node(&vn->purge_work);
+ nr_purged_areas += vn->nr_purged;
+ }
+ }
+
+ for_each_cpu(i, &purge_nodes) {
+ vn = &vmap_nodes[i];
+
+ if (vn->purge_work.func) {
+ flush_work(&vn->purge_work);
+ nr_purged_areas += vn->nr_purged;
+ }
}
}

- trace_purge_vmap_area_lazy(start, end, num_purged_areas);
- return num_purged_areas > 0;
+ trace_purge_vmap_area_lazy(start, end, nr_purged_areas);
+ return nr_purged_areas > 0;
}

/*
@@ -1866,14 +2151,14 @@ static void reclaim_and_purge_vmap_areas(void)
{
mutex_lock(&vmap_purge_lock);
purge_fragmented_blocks_allcpus();
- __purge_vmap_area_lazy(ULONG_MAX, 0);
+ __purge_vmap_area_lazy(ULONG_MAX, 0, true);
mutex_unlock(&vmap_purge_lock);
}

static void drain_vmap_area_work(struct work_struct *work)
{
mutex_lock(&vmap_purge_lock);
- __purge_vmap_area_lazy(ULONG_MAX, 0);
+ __purge_vmap_area_lazy(ULONG_MAX, 0, false);
mutex_unlock(&vmap_purge_lock);
}

@@ -1884,9 +2169,10 @@ static void drain_vmap_area_work(struct work_struct *work)
*/
static void free_vmap_area_noflush(struct vmap_area *va)
{
- struct vmap_node *vn = addr_to_node(va->va_start);
unsigned long nr_lazy_max = lazy_max_pages();
unsigned long va_start = va->va_start;
+ unsigned int vn_id = decode_vn_id(va->flags);
+ struct vmap_node *vn;
unsigned long nr_lazy;

if (WARN_ON_ONCE(!list_empty(&va->list)))
@@ -1896,10 +2182,14 @@ static void free_vmap_area_noflush(struct vmap_area *va)
PAGE_SHIFT, &vmap_lazy_nr);

/*
- * Merge or place it to the purge tree/list.
+ * If it was request by a certain node we would like to
+ * return it to that node, i.e. its pool for later reuse.
*/
+ vn = is_vn_id_valid(vn_id) ?
+ id_to_node(vn_id):addr_to_node(va->va_start);
+
spin_lock(&vn->lazy.lock);
- merge_or_add_vmap_area(va, &vn->lazy.root, &vn->lazy.head);
+ insert_vmap_area(va, &vn->lazy.root, &vn->lazy.head);
spin_unlock(&vn->lazy.lock);

trace_free_vmap_area_noflush(va_start, nr_lazy, nr_lazy_max);
@@ -2408,7 +2698,7 @@ static void _vm_unmap_aliases(unsigned long start, unsigned long end, int flush)
}
free_purged_blocks(&purge_list);

- if (!__purge_vmap_area_lazy(start, end) && flush)
+ if (!__purge_vmap_area_lazy(start, end, false) && flush)
flush_tlb_kernel_range(start, end);
mutex_unlock(&vmap_purge_lock);
}
@@ -4576,7 +4866,7 @@ static void vmap_init_free_space(void)
static void vmap_init_nodes(void)
{
struct vmap_node *vn;
- int i;
+ int i, j;

for (i = 0; i < nr_vmap_nodes; i++) {
vn = &vmap_nodes[i];
@@ -4587,6 +4877,13 @@ static void vmap_init_nodes(void)
vn->lazy.root = RB_ROOT;
INIT_LIST_HEAD(&vn->lazy.head);
spin_lock_init(&vn->lazy.lock);
+
+ for (j = 0; j < MAX_VA_SIZE_PAGES; j++) {
+ INIT_LIST_HEAD(&vn->pool[j].head);
+ WRITE_ONCE(vn->pool[j].len, 0);
+ }
+
+ spin_lock_init(&vn->pool_lock);
}
}

--
2.39.2