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Message-ID: <9ef9690d-cd79-4024-b7f1-eae2916cf5a2@lucifer.local>
Date: Thu, 23 Mar 2023 21:46:00 +0000
From: Lorenzo Stoakes <lstoakes@...il.com>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
LKML <linux-kernel@...r.kernel.org>, Baoquan He <bhe@...hat.com>,
Christoph Hellwig <hch@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
Dave Chinner <david@...morbit.com>,
Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH 1/1] mm: vmalloc: Remove a global vmap_blocks xarray
On Thu, Mar 23, 2023 at 08:21:11PM +0100, 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>
>
Nice! Really good to see contention reduced, but in addition I'm a huge fan
of us removing the global state in vmalloc and this is a good start.
I've noticed a small perf regression after 3 runs of ./test_vmalloc.sh
performance from an average of 119356136169 cycles to 120404645782 or +0.9%
but this doesn't seem especially egregious.
> 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.
>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
> mm/vmalloc.c | 50 ++++++++++++++++++++++++++------------------------
> 1 file changed, 26 insertions(+), 24 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 978194dc2bb8..13b5342bed9a 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 +1928,18 @@ 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.
> - */
Doesn't this comment still apply? Or is the idea of returning the "cookie"
not really viable?
> -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.
> - */
Again, is this comment no longer relevant?
> +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);
> +}
Andrew's already commented on this, so I won't dwell but it does seem odd
to subdivide by number of possible CPUs rather than just use the actual
CPU. I guess your response to his question will also answer mine :)
>
> -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;
> + /* A start address of block an address belongs to. */
A nit, but might be worth referring to the assert in vmap_block_vaddr(), as
this comment seems a bit redundant otherwise as it is implied by the code
it comments.
> + return rounddown(addr, VMAP_BLOCK_SIZE);
> }
>
> static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
> @@ -1953,7 +1947,7 @@ 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));
Maybe nitty, but perhaps better to WARN_ON() here to avoid BUG_ON proliferation?
And can't this be the below?
WARN_ON(addr_to_vb_va_start(addr) != va_start);
> return (void *)addr;
> }
>
> @@ -1970,7 +1964,6 @@ 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 +1996,8 @@ 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);
This seems actually like a nice subtle improvement in that we are now
indexing always on va_start explicitly and will always load using
addr_to_vb_va_start().
> if (err) {
> kfree(vb);
> free_vmap_area(va);
> @@ -2021,9 +2014,11 @@ 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 +2130,7 @@ 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 +2139,10 @@ 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 +3485,7 @@ 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 +3503,8 @@ 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 +4273,7 @@ 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
>
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