Re: [PATCH 0/5] kstrdup optimization

Andrzej Hajda
2015-01-14T08:06:33Z

On 01/14/2015 12:37 AM, Andrew Morton wrote:
> On Mon, 12 Jan 2015 10:18:38 +0100 Andrzej Hajda <a.hajda@samsung.com> wrote:
>
>> Hi,
>>
>> kstrdup if often used to duplicate strings where neither source neither
>> destination will be ever modified. In such case we can just reuse the source
>> instead of duplicating it. The problem is that we must be sure that
>> the source is non-modifiable and its life-time is long enough.
>>
>> I suspect the good candidates for such strings are strings located in kernel
>> .rodata section, they cannot be modifed because the section is read-only and
>> their life-time is equal to kernel life-time.
>>
>> This small patchset proposes alternative version of kstrdup - kstrdup_const,
>> which returns source string if it is located in .rodata otherwise it fallbacks
>> to kstrdup.
>> To verify if the source is in .rodata function checks if the address is between
>> sentinels __start_rodata, __end_rodata. I guess it should work with all
>> architectures.
>>
>> The main patch is accompanied by four patches constifying kstrdup for cases
>> where situtation described above happens frequently.
>>
>> As I have tested the patchset on mobile platform (exynos4210-trats) it saves
>> 3272 string allocations. Since minimal allocation is 32 or 64 bytes depending
>> on Kconfig options the patchset saves respectively about 100KB or 200KB of memory.
> That's a lot of memory. I wonder where it's all going to. sysfs,
> probably?

Stats from tested platform.
By caller:
2260 __kernfs_new_node
631 clk_register+0xc8/0x1b8
318 clk_register+0x34/0x1b8
51 kmem_cache_create
12 alloc_vfsmnt

By string (with count >= 5):
883 power
876 subsystem
135 parameters
132 device
61 iommu_group
44 sclk_mpll
42 aclk100
41 driver
36 sclk_vpll
35 none
34 sclk_epll
34 aclk160
32 sclk_hdmi24m
31 xxti
31 xusbxti
31 sclk_usbphy0
30 sclk_hdmiphy
28 bdi
28 aclk133
14 sclk_apll
14 aclk200
9 module
9 fin_pll
5 div_core2

>
> What the heck does (the cheerily undocumented) KERNFS_STATIC_NAME do
> and can we remove it if this patchset is in place?
>
>

The only call path when this flag is set starts from
sysfs_add_file_mode_ns function.
But I guess this function can be called also for non-const names.

Regards
Andrzej