On Fri, May 01, 2020 at 02:28:55PM +0530, Prathu Baronia wrote:
> Platform and setup conditions:
> Qualcomm's SM8150 platform under controlled conditions(i.e. only CPU0 and 6
> turned on and set to max frequency, and DDR set to performance governor).
> ---------------------------------------------------------------------------
>
> ---------------------------------------------------------------------------
> Summary:
> We observed a ~61% improvement in executon time of clearing a hugepage
> in the case of arm64 if we increase the granularity i.e. the chunk size
> to 64KB from 4KB for each chunk clearing subroutine call.
> ---------------------------------------------------------------------------
>
> For the base build:
>
> clear_huge_page() ftrace profile
> --------------------------------
> - CPU0:
> - Samples: 95
> - Mean: 242.099 us
> - Std dev: 45.0096 us
That's one hell of a deviation. Any idea what's going on there?
> - CPU6:
> - Samples: 61
> - Mean: 258.372 us
> - Std dev: 22.0754 us
>
> With patches [PATCH {1,2,3}/4] provided at the end where we just revert the
> forward-reverse traversal code we observed:
>
> clear_huge_page() ftrace profile
> --------------------------------
> - CPU0:
> - Samples: 77
> - Mean: 234.568
> - Std dev: 6.52
> - CPU6:
> - Samples: 81
> - Mean: 259.437
> - Std dev: 19.25
>
> We were expecting a bit of an improvement for arm64's case because of our
> hypothesis that reverse traversal is considerably slower in arm64 but after Will
> Deacon's test code which showed similar timings for forward and reverse
> traversals we digged a bit deeper into this.
>
> I found that In the case of arm64 a page is cleared using a special clear_page.S
> assembly routine instead of an explicit call to memset. With the below patch we
> bypassed the assembly routine and oberserved improvement in execution time of
> clear_huge_page on CPU0.
>
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index ea5cdbd8c2c3..a0a97a95aee8 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -158,7 +158,7 @@ do {
> \
> static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
> {
> void *addr = kmap_atomic(page);
> - clear_user_page(addr, vaddr, page);
> + memset(addr, 0x0, PAGE_SIZE);
> kunmap_atomic(addr);
> }
> #endif
>
> For reference I will call the above patch v-exp.
>
> When v-exp is applied on base we observed:
>
> clear_huge_page() ftrace profile
> --------------------------------
> - CPU0:
> - Samples: 71
> - Mean: 124.657 us
> - Std dev: 0.494165 us
This doesn't make any sense to me. memset() of zero is special-cased to
use the DC ZVA instruction in a loop:
3:
dc zva, dst
add dst, dst, zva_len_x
subs count, count, zva_len_x
b.ge 3b
which is basically the same as clear_page():
1: dc zva, x0
add x0, x0, x1
tst x0, #(PAGE_SIZE - 1)
b.ne 1b
Are you able to reproduce this in userspace?
Will
