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

[Uladzislau Rezki <urezki@gmail.com>]

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