Re: [PATCH V2 1/1] mm:improve the performance during fork

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 29 Mar 2021 20:36:35 +0800 qianjun.kernel@gmail.com wrote:

> 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:
> 
> use the bpftrace tool to trace the fork time :
> 
> bpftrace -e 'kprobe:_do_fork/comm=="redis-server"/ {@st=nsecs;} \
> kretprobe:_do_fork /comm=="redis-server"/{printf("the fork time \
> is %d us\n", (nsecs-@st)/1000)}'
> 
> no inline vm_normal_page:
> result:
> the fork time is 40743 us
> the fork time is 41746 us
> the fork time is 41336 us
> the fork time is 42417 us
> the fork time is 40612 us
> the fork time is 40930 us
> the fork time is 41910 us
> 
> inline vm_normal_page:
> result:
> the fork time is 39276 us
> the fork time is 38974 us
> the fork time is 39436 us
> the fork time is 38815 us
> the fork time is 39878 us
> the fork time is 39176 us
> 
> In the same test environment, we can get 3% to 4% of
> performance improvement.
> 
> note:the test data is from the 4.18.0-193.6.3.el8_2.v1.1.x86_64,
> because my product use this version kernel to test the redis
> server, If you need to compare the latest version of the kernel
> test data, you can refer to the version 1 Patch.
> 
> We need to compare the changes in the size of vmlinux:
>                   inline           non-inline       diff
> vmlinux size      9709248 bytes    9709824 bytes    -576 bytes
> 

I get very different results with gcc-7.2.0:

q:/usr/src/25> size mm/memory.o
   text    data     bss     dec     hex filename
  74898    3375      64   78337   13201 mm/memory.o-before
  75119    3363      64   78546   132d2 mm/memory.o-after

That's a somewhat significant increase in code size, and larger code
size has a worsened cache footprint.

Not that this is necessarily a bad thing for a function which is
tightly called many times in succession as is vm__normal_page()

> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -592,7 +592,7 @@ 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);

I'm a bit surprised this made any difference - rumour has it that
modern gcc just ignores `inline' and makes up its own mind.  Which is
why we added __always_inline.