Re: [PATCH 2/2] mm/vmalloc: Use kvmalloc to allocate the table of pages

From: Uladzislau Rezki
Date: Tue Mar 23 2021 - 08:05:19 EST

On Mon, Mar 22, 2021 at 11:03:11PM +0000, Matthew Wilcox wrote:
> On Mon, Mar 22, 2021 at 11:36:19PM +0100, Uladzislau Rezki wrote:
> > On Mon, Mar 22, 2021 at 07:38:20PM +0000, Matthew Wilcox (Oracle) wrote:
> > > If we're trying to allocate 4MB of memory, the table will be 8KiB in size
> > > (1024 pointers * 8 bytes per pointer), which can usually be satisfied
> > > by a kmalloc (which is significantly faster). Instead of changing this
> > > open-coded implementation, just use kvmalloc().
> > >
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > ---
> > > mm/vmalloc.c | 7 +------
> > > 1 file changed, 1 insertion(+), 6 deletions(-)
> > >
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 96444d64129a..32b640a84250 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -2802,13 +2802,8 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> > > gfp_mask |= __GFP_HIGHMEM;
> > >
> > > /* Please note that the recursion is strictly bounded. */
> > > - if (array_size > PAGE_SIZE) {
> > > - pages = __vmalloc_node(array_size, 1, nested_gfp, node,
> > > + pages = kvmalloc_node_caller(array_size, nested_gfp, node,
> > > area->caller);
> > > - } else {
> > > - pages = kmalloc_node(array_size, nested_gfp, node);
> > > - }
> > > -
> > > if (!pages) {
> > > free_vm_area(area);
> > > return NULL;
> > > --
> > > 2.30.2
> > Makes sense to me. Though i expected a bigger difference:
> >
> > # patch
> > single CPU, 4MB allocation, loops: 1000000 avg: 85293854 usec
> >
> > # default
> > single CPU, 4MB allocation, loops: 1000000 avg: 89275857 usec
>
> Well, 4.5% isn't something to leave on the table ... but yeah, I was
> expecting more in the 10-20% range. It may be more significant if
> there's contention on the spinlocks (like if this crazy ksmbd is calling
> vmalloc(4MB) on multiple nodes simultaneously).
>
Yep, it can be that simultaneous allocations will show even bigger
improvements because of lock contention. Anyway there is an advantage
in switching to SLAB - 5% is also a win :)

>
> I suspect the vast majority of the time is spent calling alloc_pages_node()
> 1024 times. Have you looked at Mel's patch to do ... well, exactly what
> vmalloc() wants?
>
<snip>
- 97.37% 0.00% vmalloc_test/0 [kernel.vmlinux] [k] ret_from_fork ◆
ret_from_fork ▒
kthread ▒
- 0xffffffffc047373b ▒
- 52.67% 0xffffffffc047349f ▒
__vmalloc_node ▒
- __vmalloc_node_range ▒
- 45.25% __alloc_pages_nodemask ▒
- 37.59% get_page_from_freelist ▒
4.34% __list_del_entry_valid ▒
3.67% __list_add_valid ▒
1.52% prep_new_page ▒
1.20% check_preemption_disabled ▒
3.75% map_kernel_range_noflush ▒
- 0.64% kvmalloc_node_caller ▒
__kmalloc_track_caller ▒
memset_orig ▒
- 44.61% 0xffffffffc047348d ▒
- __vunmap ▒
- 35.56% free_unref_page ▒
- 22.48% free_pcppages_bulk ▒
- 4.21% __mod_zone_page_state ▒
2.78% check_preemption_disabled ▒
0.80% __this_cpu_preempt_check ▒
2.24% __list_del_entry_valid ▒
1.84% __list_add_valid ▒
- 6.55% free_unref_page_commit ▒
2.47% check_preemption_disabled ▒
1.36% __list_add_valid ▒
3.10% free_unref_page_prepare.part.88 ▒
0.72% free_pcp_prepare ▒
- 6.26% remove_vm_area ▒
6.18% unmap_kernel_range_noflush ▒
2.31% __free_pages
<snip>

__alloc_pages_nodemask() consumes lot of cycles because it is called
one time per a page and like you mentioned, for 4MB request it is invoked
1024 times!

>
> https://lore.kernel.org/linux-mm/20210322091845.16437-1-mgorman@xxxxxxxxxxxxxxxxxxx/
>
I saw it. It would be good to switch to the bulk interface for vmalloc
once it is settled and mainlined. Apart of that, i find it also useful
for the kvfree_rcu() code in a context of page-cache refilling :)

>
> > One question. Should we care much about fragmentation? I mean
> > with the patch, allocations > 2MB will do request to SLAB bigger
> > then PAGE_SIZE.
>
> We're pretty good about allocating memory in larger chunks these days.
> Looking at my laptop's slabinfo,
> kmalloc-8k 219 232 8192 4 8 : tunables 0 0 0 : sla
> bdata 58 58 0
>
> That's using 8 pages per slab, so that's order-3 allocations. There's a
> few more of those:
>
> $ sudo grep '8 :' /proc/slabinfo |wc
> 42 672 4508
>
> so I have confidence that kvmalloc() will manage to use kmalloc up to 16MB
> vmalloc allocations, and after that it'll tend to fall back to vmalloc.
>

Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>

Thanks!

--
Vlad Rezki