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

[Uladzislau Rezki <urezki@gmail.com>]

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) <willy@infradead.org>
> > > ---
> > >  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/20210322091845.16437-1-mgorman@techsingularity.net/
>
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) <urezki@gmail.com>

Thanks!

--
Vlad Rezki