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

[Matthew Wilcox]

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@xxxxxxxxx>
> > > 
> > > 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_.

> 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.

(there's an entirely separate issue that they really shouldn't be
allocating 4MB of memory, but we can at least make what we have faster).