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

On 3/31/21 7:44 AM, Andrew Morton wrote:

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

quoted

From: jun qian <redacted>

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

I got this:

./scripts/bloat-o-meter memory.o.before mm/memory.o
add/remove: 0/0 grow/shrink: 1/3 up/down: 285/-86 (199)
Function                                     old     new   delta
copy_pte_range                              2095    2380    +285
vm_normal_page                               168     163      -5
do_anonymous_page                           1039    1003     -36
do_swap_page                                1835    1790     -45
Total: Before=42411, After=42610, chg +0.47%

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()

Hm but the inline only affects the users within mm/memory.c, unless the kernel
is built with link time optimization (LTO), which is not AFAIK not the standard yet.

quoted

--- 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.

AFAIK it doesn't completely ignore it, just takes it as a hint in addition to
its own heuristics. So adding the keyword might flip the decision to inline in
some cases, but is not guaranteed to.