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[86.156.84.164]) by smtp.gmail.com with ESMTPSA id e11-20020a5d65cb000000b002c55de1c72bsm17003870wrw.62.2023.03.23.14.46.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Mar 2023 14:46:01 -0700 (PDT) Date: Thu, 23 Mar 2023 21:46:00 +0000 From: Lorenzo Stoakes To: "Uladzislau Rezki (Sony)" Cc: Andrew Morton , linux-mm@kvack.org, LKML , Baoquan He , Christoph Hellwig , Matthew Wilcox , Dave Chinner , Oleksiy Avramchenko Subject: Re: [PATCH 1/1] mm: vmalloc: Remove a global vmap_blocks xarray Message-ID: <9ef9690d-cd79-4024-b7f1-eae2916cf5a2@lucifer.local> References: <20230323192111.1501308-1-urezki@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230323192111.1501308-1-urezki@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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: > > > ---------------------------------------- > 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 > ... > > > 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: > > > ... > &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 > ... > > 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) > --- > 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 >