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

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Mar 23, 2021 at 02:07:22PM +0000, Matthew Wilcox wrote:
> On Tue, Mar 23, 2021 at 02:39:48PM +0100, Uladzislau Rezki wrote:
> > On Tue, Mar 23, 2021 at 12:39:13PM +0000, Matthew Wilcox 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);
> > >  	}
> > > 
> > Will check it today!
> > 
> > > 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.
> > >  
> > > > Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > > 
> > > > Thanks!
> > > 
> > > Thank you!
> > You are welcome. A small nit:
> > 
> >   CC      mm/vmalloc.o
> > mm/vmalloc.c: In function ‘__vmalloc_area_node’:
> > mm/vmalloc.c:2492:14: warning: passing argument 4 of ‘kvmalloc_node_caller’ makes integer from pointer without a cast [-Wint-conversion]
> >           area->caller);
> >           ~~~~^~~~~~~~
> > In file included from mm/vmalloc.c:12:
> > ./include/linux/mm.h:782:7: note: expected ‘long unsigned int’ but argument is of type ‘const void *’
> >  void *kvmalloc_node_caller(size_t size, gfp_t flags, int node,
> 
> Oh, thank you!  I confused myself by changing the type halfway through.
> vmalloc() uses void * to match __builtin_return_address while most
> of the rest of the kernel uses unsigned long to match _RET_IP_.
> I'll submit another patch to convert vmalloc to use _RET_IP_.
> 
Thanks!

> > As for the bulk-array interface. I have checked the:
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-bulk-rebase-v6r2
> > 
> > applied the patch that is in question + below one:
> > 
> > <snip>
> > @@ -2503,25 +2498,13 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> >         area->pages = pages;
> >         area->nr_pages = nr_pages;
> >  
> > -       for (i = 0; i < area->nr_pages; i++) {
> > -               struct page *page;
> > -
> > -               if (node == NUMA_NO_NODE)
> > -                       page = alloc_page(gfp_mask);
> > -               else
> > -                       page = alloc_pages_node(node, gfp_mask, 0);
> > -
> > -               if (unlikely(!page)) {
> > -                       /* Successfully allocated i pages, free them in __vfree() */
> > -                       area->nr_pages = i;
> > -                       atomic_long_add(area->nr_pages, &nr_vmalloc_pages);
> > -                       goto fail;
> > -               }
> > -               area->pages[i] = page;
> > -               if (gfpflags_allow_blocking(gfp_mask))
> > -                       cond_resched();
> > +       ret = alloc_pages_bulk_array(gfp_mask, area->nr_pages, area->pages);
> > +       if (ret == nr_pages)
> > +               atomic_long_add(area->nr_pages, &nr_vmalloc_pages);
> > +       else {
> > +               area->nr_pages = ret;
> > +               goto fail;
> >         }
> > -       atomic_long_add(area->nr_pages, &nr_vmalloc_pages);
> > <snip>
> > 
> > single CPU, 4MB allocation, 1000000 avg: 70639437 usec
> > single CPU, 4MB allocation, 1000000 avg: 89218654 usec
> > 
> > and now we get ~21% delta. That is very good :)
> 
> Amazing!  That's great news for Mel's patch as well as the kvmalloc
> change.
> 
Cool! I am glad if it gives some points to the bulk-array interface :)

--
Vlad Rezki