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

[Prathu Baronia <prathubaronia2011@gmail.com>]

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 does the job for us.
We profiled clear_huge_page() using ftrace and observed an improvement
of 62%.

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@oneplus.com>
Signed-off-by: Prathu Baronia <prathu.baronia@oneplus.com>
---
 arch/arm64/include/asm/page.h | 3 +++
 arch/arm64/mm/copypage.c      | 8 ++++++++
 2 files changed, 11 insertions(+)

diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
index 012cffc574e8..8f9d005a11bb 100644
--- a/arch/arm64/include/asm/page.h
+++ b/arch/arm64/include/asm/page.h
@@ -35,6 +35,9 @@ void copy_highpage(struct page *to, struct page *from);
 #define clear_user_page(page, vaddr, pg)	clear_page(page)
 #define copy_user_page(to, from, vaddr, pg)	copy_page(to, from)
 
+#define clear_user_highpage clear_user_highpage
+void clear_user_highpage(struct page *page, unsigned long vaddr);
+
 typedef struct page *pgtable_t;
 
 extern int pfn_valid(unsigned long);
diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
index b5447e53cd73..7f5943c6fc12 100644
--- a/arch/arm64/mm/copypage.c
+++ b/arch/arm64/mm/copypage.c
@@ -44,3 +44,11 @@ void copy_user_highpage(struct page *to, struct page *from,
 	flush_dcache_page(to);
 }
 EXPORT_SYMBOL_GPL(copy_user_highpage);
+
+inline void clear_user_highpage(struct page *page, unsigned long vaddr)
+{
+	void *addr = page_address(page);
+
+	clear_user_page(addr, vaddr, page);
+}
+EXPORT_SYMBOL_GPL(clear_user_highpage);
-- 
2.17.1


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