Re: [PATCH v2 1/1] mm: Optimizing hugepage zeroing in arm64

[Ira Weiny]

On Tue, Feb 02, 2021 at 01:12:24PM +0530, Prathu Baronia wrote:
> In !HIGHMEM cases, specially in 64-bit architectures, we don't need temp
> mapping of pages. Hence, k(map|unmap)_atomic() acts as nothing more than
> multiple barrier() calls, for example for a 2MB hugepage in
> clear_huge_page() these are called 512 times i.e. to map and unmap each
> subpage that means in total 2048 barrier calls. This called for
> optimization. Simply getting VADDR from page in the form of kmap_local_*
> APIs does the job for us.  We profiled clear_huge_page() using ftrace
> and observed an improvement of 62%.

Nice!

> 
> Setup:-
> Below data has been collected on Qualcomm's SM7250 SoC THP enabled
> (kernel
> v4.19.113) with only CPU-0(Cortex-A55) and CPU-7(Cortex-A76) switched on
> and set to max frequency, also DDR set to perf governor.
> 
> FTRACE Data:-
> 
> Base data:-
> Number of iterations: 48
> Mean of allocation time: 349.5 us
> std deviation: 74.5 us
> 
> v1 data:-
> Number of iterations: 48
> Mean of allocation time: 131 us
> std deviation: 32.7 us
> 
> The following simple userspace experiment to allocate
> 100MB(BUF_SZ) of pages and writing to it gave us a good insight,
> we observed an improvement of 42% in allocation and writing timings.
> -------------------------------------------------------------
> Test code snippet
> -------------------------------------------------------------
>       clock_start();
>       buf = malloc(BUF_SZ); /* Allocate 100 MB of memory */
> 
>         for(i=0; i < BUF_SZ_PAGES; i++)
>         {
>                 *((int *)(buf + (i*PAGE_SIZE))) = 1;
>         }
>       clock_end();
> -------------------------------------------------------------
> 
> Malloc test timings for 100MB anon allocation:-
> 
> Base data:-
> Number of iterations: 100
> Mean of allocation time: 31831 us
> std deviation: 4286 us
> 
> v1 data:-
> Number of iterations: 100
> Mean of allocation time: 18193 us
> std deviation: 4915 us
> 
> Reported-by: Chintan Pandya <chintan.pandya@xxxxxxxxxxx>
> Signed-off-by: Prathu Baronia <prathu.baronia@xxxxxxxxxxx>

Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>

FWIW, I have the same change in a patch in my kmap() changes branch.  However,
my patch also changes clear_highpage(), zero_user_segments(),
copy_user_highpage(), and copy_highpage().

Would changing those help you as well?

Ira

> ---
>  include/linux/highmem.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index d2c70d3772a3..444df139b489 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -146,9 +146,9 @@ static inline void invalidate_kernel_vmap_range(void *vaddr, int size)
>  #ifndef clear_user_highpage
>  static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
>  {
> -	void *addr = kmap_atomic(page);
> +	void *addr = kmap_local_page(page);
>  	clear_user_page(addr, vaddr, page);
> -	kunmap_atomic(addr);
> +	kunmap_local(addr);
>  }
>  #endif
>  
> -- 
> 2.17.1
>