On 3/31/21 7:44 AM, Andrew Morton wrote:
> On Mon, 29 Mar 2021 20:36:35 +0800 qianjun.kernel@xxxxxxxxx wrote:
>
>> From: jun qian <qianjun.kernel@xxxxxxxxx>
>>
>> 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.
>> --- 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.
