Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock and vmap_block->lock

Uladzislau Rezki
2023-03-23T12:48:05Z

On Wed, Mar 22, 2023 at 08:15:09PM +0100, Uladzislau Rezki wrote:
> On Wed, Mar 22, 2023 at 07:01:59PM +0100, Uladzislau Rezki wrote:
> > On Wed, Mar 22, 2023 at 05:47:28PM +0000, Matthew Wilcox wrote:
> > > On Wed, Mar 22, 2023 at 02:18:19PM +0100, Uladzislau Rezki wrote:
> > > > Hello, Dave.
> > > >
> > > > >
> > > > > I'm travelling right now, but give me a few days and I'll test this
> > > > > against the XFS workloads that hammer the global vmalloc spin lock
> > > > > really, really badly. XFS can use vm_map_ram and vmalloc really
> > > > > heavily for metadata buffers and hit the global spin lock from every
> > > > > CPU in the system at the same time (i.e. highly concurrent
> > > > > workloads). vmalloc is also heavily used in the hottest path
> > > > > throught the journal where we process and calculate delta changes to
> > > > > several million items every second, again spread across every CPU in
> > > > > the system at the same time.
> > > > >
> > > > > We really need the global spinlock to go away completely, but in the
> > > > > mean time a shared read lock should help a little bit....
> > > > >
> > > > Could you please share some steps how to run your workloads in order to
> > > > touch vmalloc() code. I would like to have a look at it in more detail
> > > > just for understanding the workloads.
> > > >
> > > > Meanwhile my grep agains xfs shows:
> > > >
> > > > <snip>
> > > > urezki@pc638:~/data/raid0/coding/linux-rcu.git/fs/xfs$ grep -rn vmalloc ./
> > >
> > > You're missing:
> > >
> > > fs/xfs/xfs_buf.c: bp->b_addr = vm_map_ram(bp->b_pages, bp->b_page_count,
> > >
> > > which i suspect is the majority of Dave's workload. That will almost
> > > certainly take the vb_alloc() path.
> > >
> > Then it has nothing to do with vmalloc contention(i mean global KVA allocator), IMHO.
> > Unless:
> >
> > <snip>
> > void *vm_map_ram(struct page **pages, unsigned int count, int node)
> > {
> > unsigned long size = (unsigned long)count << PAGE_SHIFT;
> > unsigned long addr;
> > void *mem;
> >
> > if (likely(count <= VMAP_MAX_ALLOC)) {
> > mem = vb_alloc(size, GFP_KERNEL);
> > if (IS_ERR(mem))
> > return NULL;
> > addr = (unsigned long)mem;
> > } else {
> > struct vmap_area *va;
> > va = alloc_vmap_area(size, PAGE_SIZE,
> > VMALLOC_START, VMALLOC_END, node, GFP_KERNEL);
> > if (IS_ERR(va))
> > return NULL;
> > <snip>
> >
> > number of pages > VMAP_MAX_ALLOC.
> >
> > That is why i have asked about workloads because i would like to understand
> > where a "problem" is. A vm_map_ram() access the global vmap space but it happens
> > when a new vmap block is required and i also think it is not a problem.
> >
> > But who knows, therefore it makes sense to have a lock at workload.
> >
> There is a lock-stat statistics for vm_map_ram()/vm_unmap_ram() test.
> I did it on 64 CPUs system with running 64 threads doing mapping/unmapping
> of 1 page. Each thread does 10 000 000 mapping + unmapping in a loop:
>
> <snip>
> root@pc638:/home/urezki# cat /proc/lock_stat
> lock_stat version 0.4
> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> vmap_area_lock: 2554079 2554276 0.06 213.61 11719647.67 4.59 2986513 3005712 0.05 67.02 3573323.37 1.19
> --------------
> 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 0.05 77.74 849325.39 0.98 3005156 3005709 0.12 31.11 1920242.82 0.64
> -------------------
> 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
>
> ....................................................................................................................................................................................................
>
> free_vmap_area_lock: 394960 394961 0.05 124.78 448241.23 1.13 1514508 1515077 0.12 30.48 1179167.01 0.78
> -------------------
> free_vmap_area_lock 385970 [<00000000955bd641>] alloc_vmap_area+0xe5/0x910
> free_vmap_area_lock 4692 [<00000000230abf7e>] __purge_vmap_area_lazy+0x10a/0x7d0
> free_vmap_area_lock 4299 [<00000000eed9ff9e>] alloc_vmap_area+0x497/0x910
> -------------------
> free_vmap_area_lock 371734 [<00000000955bd641>] alloc_vmap_area+0xe5/0x910
> free_vmap_area_lock 17007 [<00000000230abf7e>] __purge_vmap_area_lazy+0x10a/0x7d0
> free_vmap_area_lock 6220 [<00000000eed9ff9e>] alloc_vmap_area+0x497/0x910
>
> .....................................................................................................................................................................................................
>
> purge_vmap_area_lock: 169307 169312 0.05 31.08 81655.21 0.48 1514794 1515078 0.05 67.73 912391.12 0.60
> --------------------
> purge_vmap_area_lock 166409 [<0000000050938075>] free_vmap_area_noflush+0x65/0x370
> purge_vmap_area_lock 2903 [<00000000fb8b57f7>] __purge_vmap_area_lazy+0x47/0x7d0
> --------------------
> purge_vmap_area_lock 167511 [<0000000050938075>] free_vmap_area_noflush+0x65/0x370
> purge_vmap_area_lock 1801 [<00000000fb8b57f7>] __purge_vmap_area_lazy+0x47/0x7d0
> <snip>
>
> alloc_vmap_area is a top and second one is xa_lock. But the test i have
> done is pretty high concurrent scenario.
>
<snip>
From 32c38d239c6de3f1d3accf97d9d4944ecaa4bccd Mon Sep 17 00:00:00 2001
From: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Date: Thu, 23 Mar 2023 13:07:27 +0100
Subject: [PATCH] mm: vmalloc: Remove 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. Under
stress test the lock-stat shows that a "vmap_blocks.xa_lock"
lock is a second in a list when it comes to contentions:

<snip>
----------------------------------------
class name con-bounces contentions ...
----------------------------------------
...
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 is considered as 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>

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
mm/vmalloc.c | 54 +++++++++++++++++++++++++++++-----------------------
1 file changed, 30 insertions(+), 24 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 978194dc2bb8..b1e549d152b2 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1911,6 +1911,7 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
struct vmap_block_queue {
spinlock_t lock;
struct list_head free;
+ struct xarray vmap_blocks;
};

struct vmap_block {
@@ -1927,25 +1928,22 @@ struct vmap_block {
/* Queue of free and dirty vmap blocks, for allocation and flushing purposes */
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.
- */
-static DEFINE_XARRAY(vmap_blocks);
-
-/*
- * 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.
- */
+static struct vmap_block_queue *
+addr_to_vbq(unsigned long addr)
+{
+ int cpu = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
+ return &per_cpu(vmap_block_queue, cpu);
+}

-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;
+ /* Check if aligned. */
+ if (IS_ALIGNED(addr, VMAP_BLOCK_SIZE))
+ return addr;
+
+ /* A start address of block an address belongs to. */
+ return rounddown(addr, VMAP_BLOCK_SIZE);
}

static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
@@ -1953,7 +1951,7 @@ 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));
+ BUG_ON(addr_to_vb_va_start(addr) != addr_to_vb_va_start(va_start));
return (void *)addr;
}

@@ -1970,7 +1968,6 @@ 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 +2000,8 @@ 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);
if (err) {
kfree(vb);
free_vmap_area(va);
@@ -2021,9 +2018,11 @@ 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 +2134,7 @@ 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 +2143,10 @@ 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 +3489,7 @@ 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 +3507,8 @@ 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 +4277,7 @@ 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
<snip>

Any thoughts patch?

I do not consider it as a big improvement in performance. But, it tends
to remove completely a contention on the "xa_lock" + it refactor slightly
the per-cpu allocator. XFS workloads can be improved, though.

--
Uladzislau Rezki