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

From: Uladzislau Rezki
Date: Wed Mar 22 2023 - 15:15:21 EST

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.

--
Uladzislau Rezki