Re: [PATCH 2/2] mm/vmalloc: Use kvmalloc to allocate the table of pages
From: Uladzislau Rezki
Date: Wed Mar 24 2021 - 14:42:08 EST
On Tue, Mar 23, 2021 at 09:39:24PM +0100, Uladzislau Rezki wrote:
> > On Tue, Mar 23, 2021 at 01:04:36PM +0100, Uladzislau Rezki wrote:
> > > On Mon, Mar 22, 2021 at 11:03:11PM +0000, Matthew Wilcox wrote:
> > > > 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>
> > > - __vmalloc_node_range
> > > - 45.25% __alloc_pages_nodemask
> > > - 37.59% get_page_from_freelist
> > [...]
> > > - 44.61% 0xffffffffc047348d
> > > - __vunmap
> > > - 35.56% free_unref_page
> >
> > Hmm! I hadn't been thinking about the free side of things.
> > Does this make a difference?
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 4f5f8c907897..61d5b769fea0 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -2277,16 +2277,8 @@ static void __vunmap(const void *addr, int deallocate_pages)
> > vm_remove_mappings(area, deallocate_pages);
> >
> > if (deallocate_pages) {
> > - int i;
> > -
> > - for (i = 0; i < area->nr_pages; i++) {
> > - struct page *page = area->pages[i];
> > -
> > - BUG_ON(!page);
> > - __free_pages(page, 0);
> > - }
> > + release_pages(area->pages, area->nr_pages);
> > atomic_long_sub(area->nr_pages, &nr_vmalloc_pages);
> > -
> > kvfree(area->pages);
> > }
> >
> Same test. 4MB allocation on a single CPU:
>
> default: loops: 1000000 avg: 93601889 usec
> patch: loops: 1000000 avg: 98217904 usec
>
> <snip default>
> - __vunmap
> - 41.17% free_unref_page
> - 28.42% free_pcppages_bulk
> - 6.38% __mod_zone_page_state
> 4.79% check_preemption_disabled
> 2.63% __list_del_entry_valid
> 2.63% __list_add_valid
> - 7.50% free_unref_page_commit
> 2.15% check_preemption_disabled
> 2.01% __list_add_valid
> 2.31% free_unref_page_prepare.part.86
> 0.70% free_pcp_prepare
> <snip default>
>
> <snip patch>
> - __vunmap
> - 45.36% release_pages
> - 37.70% free_unref_page_list
> - 24.70% free_pcppages_bulk
> - 5.42% __mod_zone_page_state
> 4.23% check_preemption_disabled
> 2.31% __list_add_valid
> 2.07% __list_del_entry_valid
> - 7.58% free_unref_page_commit
> 2.47% check_preemption_disabled
> 1.75% __list_add_valid
> 3.43% free_unref_page_prepare.part.86
> - 2.39% mem_cgroup_uncharge_list
> uncharge_page
> <snip patch>
>
> It is obvious that the default version is slightly better. It requires
> less things to be done comparing with release_pages() variant.
>
> >
> > release_pages does a bunch of checks that are unnecessary ... we could
> > probably just do:
> >
> > LIST_HEAD(pages_to_free);
> >
> > for (i = 0; i < area->nr_pages; i++) {
> > struct page *page = area->pages[i];
> > if (put_page_testzero(page))
> > list_add(&page->lru, &pages_to_free);
> > }
> > free_unref_page_list(&pages_to_free);
> >
> > but let's see if the provided interface gets us the performance we want.
> >
> I will test it tomorrow. From the first glance it looks like a more light version :)
>
Here we go:
<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 4f5f8c907897..349024768ba6 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2254,6 +2254,7 @@ static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
static void __vunmap(const void *addr, int deallocate_pages)
{
struct vm_struct *area;
+ LIST_HEAD(pages_to_free);
if (!addr)
return;
@@ -2282,11 +2283,12 @@ static void __vunmap(const void *addr, int deallocate_pages)
for (i = 0; i < area->nr_pages; i++) {
struct page *page = area->pages[i];
- BUG_ON(!page);
- __free_pages(page, 0);
+ if (put_page_testzero(page))
+ list_add(&page->lru, &pages_to_free);
}
- atomic_long_sub(area->nr_pages, &nr_vmalloc_pages);
+ free_unref_page_list(&pages_to_free);
+ atomic_long_sub(area->nr_pages, &nr_vmalloc_pages);
kvfree(area->pages);
}
<snip>
# patch
4MB allocation, single cpu, loops: 1000000 avg: 89065758 usec
4MB allocation, single cpu, loops: 1000000 avg: 90258523 usec
4MB allocation, single cpu, loops: 1000000 avg: 89363057 usec
4MB allocation, single cpu, loops: 1000000 avg: 89271685 usec
4MB allocation, single cpu, loops: 1000000 avg: 89247375 usec
# default
4MB allocation, single cpu, loops: 1000000 avg: 89258814 usec
4MB allocation, single cpu, loops: 1000000 avg: 89364194 usec
4MB allocation, single cpu, loops: 1000000 avg: 89226816 usec
4MB allocation, single cpu, loops: 1000000 avg: 89247360 usec
4MB allocation, single cpu, loops: 1000000 avg: 89330116 usec
Do not see any difference.
See below some profiling regarding cache misses:
<snip>
- __vunmap
- 32.15% free_unref_page_list
- 23.54% free_pcppages_bulk
- 6.33% __mod_zone_page_state
4.65% check_preemption_disabled
<snip>
free_unref_page_list():
│ free_unref_page_list():
│ffffffff8125152a: mov 0x8(%rbp),%rax
31.81 │ffffffff8125152e: lea 0x8(%rbp),%r12
│ffffffff81251532: mov %rbp,%r14
14.40 │ffffffff81251535: lea -0x8(%rax),%rbp
(gdb) l *0xffffffff8125152e
0xffffffff8125152e is in free_unref_page_list (mm/page_alloc.c:3271).
3266 struct page *page, *next;
3267 unsigned long flags, pfn;
3268 int batch_count = 0;
3269
3270 /* Prepare pages for freeing */
3271 list_for_each_entry_safe(page, next, list, lru) {
3272 pfn = page_to_pfn(page);
3273 if (!free_unref_page_prepare(page, pfn))
3274 list_del(&page->lru);
3275 set_page_private(page, pfn);
(gdb)
free_pcppages_bulk():
│ PageBuddy():
0.59 │ffffffff8124f523: mov 0x30(%rax),%edi
13.59 │ffffffff8124f526: and $0xf0000080,%edi
(gdb) l *0xffffffff8124f526
0xffffffff8124f526 is in free_pcppages_bulk (./include/linux/page-flags.h:742).
737
738 /*
739 * PageBuddy() indicates that the page is free and in the buddy system
740 * (see mm/page_alloc.c).
741 */
742 PAGE_TYPE_OPS(Buddy, buddy)
743
744 /*
745 * PageOffline() indicates that the page is logically offline although the
746 * containing section is online. (e.g. inflated in a balloon driver or
(gdb)
Looks like it would be good to have a free_pages_bulk_array() :)
--
Vlad Rezki