From: Vlastimil Babka <vbabka@suse.cz>
To: Andrew Morton <akpm@linux-foundation.org>, qianjun.kernel@gmail.com
Cc: ast@kernel.org, daniel@iogearbox.net, kafai@fb.com,
songliubraving@fb.com, yhs@fb.com, andriin@fb.com,
john.fastabend@gmail.com, kpsingh@chromium.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [PATCH V2 1/1] mm:improve the performance during fork
Date: Wed, 31 Mar 2021 14:11:03 +0200 [thread overview]
Message-ID: <9f012469-ccda-2c95-aa5a-7ca4f6fb2891@suse.cz> (raw)
In-Reply-To: <20210330224406.5e195f3b8b971ff2a56c657d@linux-foundation.org>
On 3/31/21 7:44 AM, Andrew Morton wrote:
> 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
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.
next prev parent reply other threads:[~2021-03-31 12:11 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-29 12:36 [PATCH V2 1/1] mm:improve the performance during fork qianjun.kernel
2021-03-31 5:44 ` Andrew Morton
2021-03-31 12:11 ` Vlastimil Babka [this message]
2021-03-31 14:42 ` Vlastimil Babka
2021-04-06 2:14 ` jun qian
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