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

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Message-ID: <20210323203924.GA52881@pc638.lan>
Date:   Tue, 23 Mar 2021 21:39:24 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 2/2] mm/vmalloc: Use kvmalloc to allocate the table of
 pages

> 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 :)

--
Vlad Rezki