Re: [PATCH v3 04/11] mm: vmalloc: Remove global vmap_area_root rb-tree

Wen Gu
2024-01-06T09:17:49Z

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On 2024/1/5 18:50, Uladzislau Rezki wrote:

> Hello, Wen Gu.
>
>>
>> Hi Uladzislau Rezki,
>>

<...>

>> Fortunately, thank you for this patch set, the global vmap_area_lock was
>> removed and per node lock vn->busy.lock is introduced. it is really helpful:
>>
>> In 48 CPUs qemu environment, the Requests/s increased by 5 times:
>> - nginx
>> - wrk -c 1000 -t 96 -d 30 http://127.0.0.1:80
>>
>> vzalloced shmem vzalloced shmem(with this patch set)
>> Requests/sec 113536.56 583729.93
>>
>>
> Thank you for the confirmation that your workload is improved. The "nginx"
> is 5 times better!
>

Yes, thank you very much for the improvement!

>> But it also has some overhead, compared to using kzalloced shared memory
>> or unsetting CONFIG_HARDENED_USERCOPY, which won't involve finding vmap area:
>>
>> kzalloced shmem vzalloced shmem(unset CONFIG_HARDENED_USERCOPY)
>> Requests/sec 831950.39 805164.78
>>
>>
> The CONFIG_HARDENED_USERCOPY prevents coping "wrong" memory regions. That is
> why if it is a vmalloced memory it wants to make sure it is really true,
> if not user-copy is aborted.
>
> So there is an extra work that involves finding a VA associated with an address.
>

Yes, and lock contention in finding VA is likely to be a performance bottleneck,
which is mitigated a lot by your work.

>> So, as a newbie in Linux-mm, I would like to ask for some suggestions:
>>
>> Is it possible to further eliminate the overhead caused by lock contention
>> in find_vmap_area() in this scenario (maybe this is asking too much), or the
>> only way out is not setting CONFIG_HARDENED_USERCOPY or not using vzalloced
>> buffer in the situation where cocurrent kernel-userspace-copy happens?
>>
> Could you please try below patch, if it improves this series further?
> Just in case:
>

Thank you! I tried the patch, and it seems that the wait for rwlock_t
also exists, as much as using spinlock_t. (The flamegraph is attached.
Not sure why the read_lock waits so long, given that there is no frequent
write_lock competition)

vzalloced shmem(spinlock_t) vzalloced shmem(rwlock_t)
Requests/sec 583729.93 460007.44

So I guess the overhead in finding vmap area is inevitable here and the
original spin_lock is fine in this series.

Thanks again for your help!

Best regards,
Wen Gu

> <snip>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index e30dabf68263..40acf53cadfb 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -772,7 +772,7 @@ static DEFINE_PER_CPU(struct vmap_area *, ne_fit_preload_node);
> struct rb_list {
> struct rb_root root;
> struct list_head head;
> - spinlock_t lock;
> + rwlock_t lock;
> };
>
> struct vmap_pool {
> @@ -947,19 +947,19 @@ find_vmap_area_exceed_addr_lock(unsigned long addr, struct vmap_area **va)
> for (i = 0; i < nr_vmap_nodes; i++) {
> vn = &vmap_nodes[i];
>
> - spin_lock(&vn->busy.lock);
> + read_lock(&vn->busy.lock);
> va_lowest = __find_vmap_area_exceed_addr(addr, &vn->busy.root);
> if (va_lowest) {
> if (!va_node || va_lowest->va_start < (*va)->va_start) {
> if (va_node)
> - spin_unlock(&va_node->busy.lock);
> + read_unlock(&va_node->busy.lock);
>
> *va = va_lowest;
> va_node = vn;
> continue;
> }
> }
> - spin_unlock(&vn->busy.lock);
> + read_unlock(&vn->busy.lock);
> }
>
> return va_node;
> @@ -1695,9 +1695,9 @@ static void free_vmap_area(struct vmap_area *va)
> /*
> * Remove from the busy tree/list.
> */
> - spin_lock(&vn->busy.lock);
> + write_lock(&vn->busy.lock);
> unlink_va(va, &vn->busy.root);
> - spin_unlock(&vn->busy.lock);
> + write_unlock(&vn->busy.lock);
>
> /*
> * Insert/Merge it back to the free tree/list.
> @@ -1901,9 +1901,9 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>
> vn = addr_to_node(va->va_start);
>
> - spin_lock(&vn->busy.lock);
> + write_lock(&vn->busy.lock);
> insert_vmap_area(va, &vn->busy.root, &vn->busy.head);
> - spin_unlock(&vn->busy.lock);
> + write_unlock(&vn->busy.lock);
>
> BUG_ON(!IS_ALIGNED(va->va_start, align));
> BUG_ON(va->va_start < vstart);
> @@ -2123,10 +2123,10 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end,
> if (RB_EMPTY_ROOT(&vn->lazy.root))
> continue;
>
> - spin_lock(&vn->lazy.lock);
> + write_lock(&vn->lazy.lock);
> WRITE_ONCE(vn->lazy.root.rb_node, NULL);
> list_replace_init(&vn->lazy.head, &vn->purge_list);
> - spin_unlock(&vn->lazy.lock);
> + write_unlock(&vn->lazy.lock);
>
> start = min(start, list_first_entry(&vn->purge_list,
> struct vmap_area, list)->va_start);
> @@ -2223,9 +2223,9 @@ static void free_vmap_area_noflush(struct vmap_area *va)
> vn = is_vn_id_valid(vn_id) ?
> id_to_node(vn_id):addr_to_node(va->va_start);
>
> - spin_lock(&vn->lazy.lock);
> + write_lock(&vn->lazy.lock);
> insert_vmap_area(va, &vn->lazy.root, &vn->lazy.head);
> - spin_unlock(&vn->lazy.lock);
> + write_unlock(&vn->lazy.lock);
>
> trace_free_vmap_area_noflush(va_start, nr_lazy, nr_lazy_max);
>
> @@ -2272,9 +2272,9 @@ struct vmap_area *find_vmap_area(unsigned long addr)
> do {
> vn = &vmap_nodes[i];
>
> - spin_lock(&vn->busy.lock);
> + read_lock(&vn->busy.lock);
> va = __find_vmap_area(addr, &vn->busy.root);
> - spin_unlock(&vn->busy.lock);
> + read_unlock(&vn->busy.lock);
>
> if (va)
> return va;
> @@ -2293,11 +2293,11 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> do {
> vn = &vmap_nodes[i];
>
> - spin_lock(&vn->busy.lock);
> + write_lock(&vn->busy.lock);
> va = __find_vmap_area(addr, &vn->busy.root);
> if (va)
> unlink_va(va, &vn->busy.root);
> - spin_unlock(&vn->busy.lock);
> + write_unlock(&vn->busy.lock);
>
> if (va)
> return va;
> @@ -2514,9 +2514,9 @@ static void free_vmap_block(struct vmap_block *vb)
> BUG_ON(tmp != vb);
>
> vn = addr_to_node(vb->va->va_start);
> - spin_lock(&vn->busy.lock);
> + write_lock(&vn->busy.lock);
> unlink_va(vb->va, &vn->busy.root);
> - spin_unlock(&vn->busy.lock);
> + write_unlock(&vn->busy.lock);
>
> free_vmap_area_noflush(vb->va);
> kfree_rcu(vb, rcu_head);
> @@ -2942,9 +2942,9 @@ static void setup_vmalloc_vm(struct vm_struct *vm, struct vmap_area *va,
> {
> struct vmap_node *vn = addr_to_node(va->va_start);
>
> - spin_lock(&vn->busy.lock);
> + read_lock(&vn->busy.lock);
> setup_vmalloc_vm_locked(vm, va, flags, caller);
> - spin_unlock(&vn->busy.lock);
> + read_unlock(&vn->busy.lock);
> }
>
> static void clear_vm_uninitialized_flag(struct vm_struct *vm)
> @@ -4214,19 +4214,19 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
>
> next_va:
> next = va->va_end;
> - spin_unlock(&vn->busy.lock);
> + read_unlock(&vn->busy.lock);
> } while ((vn = find_vmap_area_exceed_addr_lock(next, &va)));
>
> finished_zero:
> if (vn)
> - spin_unlock(&vn->busy.lock);
> + read_unlock(&vn->busy.lock);
>
> /* zero-fill memory holes */
> return count - remains + zero_iter(iter, remains);
> finished:
> /* Nothing remains, or We couldn't copy/zero everything. */
> if (vn)
> - spin_unlock(&vn->busy.lock);
> + read_unlock(&vn->busy.lock);
>
> return count - remains;
> }
> @@ -4563,11 +4563,11 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
> for (area = 0; area < nr_vms; area++) {
> struct vmap_node *vn = addr_to_node(vas[area]->va_start);
>
> - spin_lock(&vn->busy.lock);
> + write_lock(&vn->busy.lock);
> insert_vmap_area(vas[area], &vn->busy.root, &vn->busy.head);
> setup_vmalloc_vm_locked(vms[area], vas[area], VM_ALLOC,
> pcpu_get_vm_areas);
> - spin_unlock(&vn->busy.lock);
> + write_unlock(&vn->busy.lock);
> }
>
> /*
> @@ -4687,7 +4687,7 @@ bool vmalloc_dump_obj(void *object)
>
> vn = addr_to_node((unsigned long)objp);
>
> - if (spin_trylock(&vn->busy.lock)) {
> + if (read_trylock(&vn->busy.lock)) {
> va = __find_vmap_area(addr, &vn->busy.root);
>
> if (va && va->vm) {
> @@ -4697,7 +4697,7 @@ bool vmalloc_dump_obj(void *object)
> success = true;
> }
>
> - spin_unlock(&vn->busy.lock);
> + read_unlock(&vn->busy.lock);
> }
>
> if (success)
> @@ -4742,13 +4742,13 @@ static void show_purge_info(struct seq_file *m)
> for (i = 0; i < nr_vmap_nodes; i++) {
> vn = &vmap_nodes[i];
>
> - spin_lock(&vn->lazy.lock);
> + read_lock(&vn->lazy.lock);
> list_for_each_entry(va, &vn->lazy.head, list) {
> seq_printf(m, "0x%pK-0x%pK %7ld unpurged vm_area\n",
> (void *)va->va_start, (void *)va->va_end,
> va->va_end - va->va_start);
> }
> - spin_unlock(&vn->lazy.lock);
> + read_unlock(&vn->lazy.lock);
> }
> }
>
> @@ -4762,7 +4762,7 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
> for (i = 0; i < nr_vmap_nodes; i++) {
> vn = &vmap_nodes[i];
>
> - spin_lock(&vn->busy.lock);
> + read_lock(&vn->busy.lock);
> list_for_each_entry(va, &vn->busy.head, list) {
> if (!va->vm) {
> if (va->flags & VMAP_RAM)
> @@ -4808,7 +4808,7 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
> show_numa_info(m, v);
> seq_putc(m, '\n');
> }
> - spin_unlock(&vn->busy.lock);
> + read_unlock(&vn->busy.lock);
> }
>
> /*
> @@ -4902,11 +4902,11 @@ static void vmap_init_nodes(void)
> vn = &vmap_nodes[n];
> vn->busy.root = RB_ROOT;
> INIT_LIST_HEAD(&vn->busy.head);
> - spin_lock_init(&vn->busy.lock);
> + rwlock_init(&vn->busy.lock);
>
> vn->lazy.root = RB_ROOT;
> INIT_LIST_HEAD(&vn->lazy.head);
> - spin_lock_init(&vn->lazy.lock);
> + rwlock_init(&vn->lazy.lock);
>
> for (i = 0; i < MAX_VA_SIZE_PAGES; i++) {
> INIT_LIST_HEAD(&vn->pool[i].head);
> <snip>
>
> Thank you!
>
> --
> Uladzislau Rezki

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