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) <[email protected]> > > > --- > > > 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/[email protected]/
>
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) <[email protected]>
Thanks!
--
Vlad Rezki
