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From:   qianjun.kernel@gmail.com
To:     akpm@linux-foundation.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        jun qian <qianjun.kernel@gmail.com>
Subject: [PATCH 1/1] mm:improve the performance during fork
Date:   Tue, 22 Dec 2020 20:19:04 +0800
Message-Id: <20201222121904.50845-1-qianjun.kernel@gmail.com>
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From: jun qian <qianjun.kernel@gmail.com>

In our project, Many business delays come from fork, so
we started looking for the reason why fork is time-consuming.
I used the ftrace with function_graph to trace the fork, found
that the vm_normal_page will be called tens of thousands and
the execution time of this vm_normal_page function is only a
few nanoseconds. And the vm_normal_page is not a inline function.
So I think if the function is inline style, it maybe reduce the
call time overhead.

I did the following experiment:

I have wrote the c test code, pls ignore the memory leak :)
Before fork, I will malloc 4G bytes, then acculate the fork
time.

int main()
{
        char *p;
        unsigned long long i=0;
        float time_use=0;
        struct timeval start;
        struct timeval end;

        for(i=0; i<LEN; i++) {
                p = (char *)malloc(4096);
                if (p == NULL) {
                        printf("malloc failed!\n");
                        return 0;
                }
                p[0] = 0x55;
        }
        gettimeofday(&start,NULL);
        fork();
        gettimeofday(&end,NULL);

        time_use=(end.tv_sec * 1000000 + end.tv_usec) -
                (start.tv_sec * 1000000 + start.tv_usec);
        printf("time_use is %.10f us\n",time_use);

        return 0;
}

We need to compare the changes in the size of vmlinux, the time of
fork in inline and non-inline cases, and the vm_normal_page will be
called in many function. So we also need to compare this function's
size. For examples, the do_wp_page will call vm_normal_page, so I
also calculated it's size.

		  inline           non-inline       diff
vmlinux size      9709248 bytes    9709824 bytes    -576 bytes
fork time         23475ns          24638ns          -4.7%
do_wp_page size   972              743              +229

According to the above test data, I think inline vm_normal_page can
reduce fork execution time.

Signed-off-by: jun qian <qianjun.kernel@gmail.com>
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 7d608765932b..a689bb5d3842 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -591,7 +591,7 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
  * PFNMAP mappings in order to support COWable mappings.
  *
  */
-struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
+inline struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
 			    pte_t pte)
 {
 	unsigned long pfn = pte_pfn(pte);
-- 
2.18.2