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

From: Uladzislau Rezki
Date: Thu Mar 23 2023 - 08:48:04 EST

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>