Re: [PATCH] mm/page_alloc: make sure __rmqueue() etc. always inline

[Vlastimil Babka <vbabka@suse.cz>]

On 10/13/2017 08:31 AM, Aaron Lu wrote:
> __rmqueue(), __rmqueue_fallback(), __rmqueue_smallest() and
> __rmqueue_cma_fallback() are all in page allocator's hot path and
> better be finished as soon as possible. One way to make them faster
> is by making them inline. But as Andrew Morton and Andi Kleen pointed
> out:
> https://lkml.org/lkml/2017/10/10/1252
> https://lkml.org/lkml/2017/10/10/1279
> To make sure they are inlined, we should use __always_inline for them.
> 
> With the will-it-scale/page_fault1/process benchmark, when using nr_cpu
> processes to stress buddy, the results for will-it-scale.processes with
> and without the patch are:
> 
> On a 2-sockets Intel-Skylake machine:
> 
>  compiler          base        head
> gcc-4.4.7       6496131     6911823 +6.4%
> gcc-4.9.4       7225110     7731072 +7.0%
> gcc-5.4.1       7054224     7688146 +9.0%
> gcc-6.2.0       7059794     7651675 +8.4%
> 
> On a 4-sockets Intel-Skylake machine:
> 
>  compiler          base        head
> gcc-4.4.7      13162890    13508193 +2.6%
> gcc-4.9.4      14997463    15484353 +3.2%
> gcc-5.4.1      14708711    15449805 +5.0%
> gcc-6.2.0      14574099    15349204 +5.3%
> 
> The above 4 compilers are used becuase I've done the tests through Intel's
> Linux Kernel Performance(LKP) infrastructure and they are the available
> compilers there.
> 
> The benefit being less on 4 sockets machine is due to the lock contention
> there(perf-profile/native_queued_spin_lock_slowpath=81%) is less severe
> than on the 2 sockets machine(85%).
> 
> What the benchmark does is: it forks nr_cpu processes and then each
> process does the following:
>     1 mmap() 128M anonymous space;
>     2 writes to each page there to trigger actual page allocation;
>     3 munmap() it.
> in a loop.
> https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault1.c

Are transparent hugepages enabled? If yes, __rmqueue() is called from
rmqueue(), and there's only one page fault (and __rmqueue()) per 512
"writes to each page". If not, __rmqueue() is called from rmqueue_bulk()
in bursts once pcplists are depleted. I guess it's the latter, otherwise
I wouldn't expect a function call to have such visible overhead.

I guess what would help much more would be a bulk __rmqueue_smallest()
to grab multiple pages from the freelists. But can't argue with your
numbers against this patch.

> Binary size wise, I have locally built them with different compilers:
> 
> [aaron@aaronlu obj]$ size */*/mm/page_alloc.o
>    text    data     bss     dec     hex filename
>   37409    9904    8524   55837    da1d gcc-4.9.4/base/mm/page_alloc.o
>   38273    9904    8524   56701    dd7d gcc-4.9.4/head/mm/page_alloc.o
>   37465    9840    8428   55733    d9b5 gcc-5.5.0/base/mm/page_alloc.o
>   38169    9840    8428   56437    dc75 gcc-5.5.0/head/mm/page_alloc.o
>   37573    9840    8428   55841    da21 gcc-6.4.0/base/mm/page_alloc.o
>   38261    9840    8428   56529    dcd1 gcc-6.4.0/head/mm/page_alloc.o
>   36863    9840    8428   55131    d75b gcc-7.2.0/base/mm/page_alloc.o
>   37711    9840    8428   55979    daab gcc-7.2.0/head/mm/page_alloc.o
> 
> Text size increased about 800 bytes for mm/page_alloc.o.

BTW, do you know about ./scripts/bloat-o-meter? :)
With gcc 7.2.1:
> ./scripts/bloat-o-meter base.o mm/page_alloc.o
add/remove: 1/2 grow/shrink: 2/0 up/down: 2493/-1649 (844)
function                                     old     new   delta
get_page_from_freelist                      2898    4937   +2039
steal_suitable_fallback                        -     365    +365
find_suitable_fallback                        31     120     +89
find_suitable_fallback.part                  115       -    -115
__rmqueue                                   1534       -   -1534


> [aaron@aaronlu obj]$ size */*/vmlinux
>    text    data     bss     dec       hex     filename
> 10342757   5903208 17723392 33969357  20654cd gcc-4.9.4/base/vmlinux
> 10342757   5903208 17723392 33969357  20654cd gcc-4.9.4/head/vmlinux
> 10332448   5836608 17715200 33884256  2050860 gcc-5.5.0/base/vmlinux
> 10332448   5836608 17715200 33884256  2050860 gcc-5.5.0/head/vmlinux
> 10094546   5836696 17715200 33646442  201676a gcc-6.4.0/base/vmlinux
> 10094546   5836696 17715200 33646442  201676a gcc-6.4.0/head/vmlinux
> 10018775   5828732 17715200 33562707  2002053 gcc-7.2.0/base/vmlinux
> 10018775   5828732 17715200 33562707  2002053 gcc-7.2.0/head/vmlinux
> 
> Text size for vmlinux has no change though, probably due to function
> alignment.

Yep that's useless to show. These differences do add up though, until
they eventually cross the alignment boundary.

Thanks,
Vlastimil

> 
> Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> ---
>  mm/page_alloc.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0e309ce4a44a..0fe3e2095268 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1794,7 +1794,7 @@ static void prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags
>   * Go through the free lists for the given migratetype and remove
>   * the smallest available page from the freelists
>   */
> -static inline
> +static __always_inline
>  struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
>  						int migratetype)
>  {
> @@ -1838,7 +1838,7 @@ static int fallbacks[MIGRATE_TYPES][4] = {
>  };
>  
>  #ifdef CONFIG_CMA
> -static struct page *__rmqueue_cma_fallback(struct zone *zone,
> +static __always_inline struct page *__rmqueue_cma_fallback(struct zone *zone,
>  					unsigned int order)
>  {
>  	return __rmqueue_smallest(zone, order, MIGRATE_CMA);
> @@ -2219,7 +2219,7 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
>   * deviation from the rest of this file, to make the for loop
>   * condition simpler.
>   */
> -static inline bool
> +static __always_inline bool
>  __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>  {
>  	struct free_area *area;
> @@ -2291,8 +2291,8 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>   * Do the hard work of removing an element from the buddy allocator.
>   * Call me with the zone->lock already held.
>   */
> -static struct page *__rmqueue(struct zone *zone, unsigned int order,
> -				int migratetype)
> +static __always_inline struct page *
> +__rmqueue(struct zone *zone, unsigned int order, int migratetype)
>  {
>  	struct page *page;
>  
> 

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