Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt_curr()

[Kirill Tkhai]

On 19.12.2019 17:02, Peter Zijlstra wrote:
> On Thu, Dec 19, 2019 at 02:12:42PM +0100, Peter Zijlstra wrote:
>> On Thu, Dec 19, 2019 at 03:39:14PM +0300, Kirill Tkhai wrote:
>>> In kernel/sched/Makefile files, describing different sched classes, already
>>> go in the order from the lowest priority class to the highest priority class:
>>>
>>> idle.o fair.o rt.o deadline.o stop_task.o
>>>
>>> The documentation of GNU linker says, that section appears in the order
>>> they are seen during link time (see [1]):
>>>
>>>> Normally, the linker will place files and sections matched by wildcards
>>>> in the order in which they are seen during the link. You can change this
>>>> by using the SORT keyword, which appears before a wildcard pattern
>>>> in parentheses (e.g., SORT(.text*)).
>>>
>>> So, we may expect const variables from idle.o will go before ro variables
>>> from fair.o in RO_DATA section, while ro variables from fair.o will go
>>> before ro variables from rt.o, etc.
>>>
>>> (Also, it looks like the linking order is already used in kernel, e.g.
>>>  in drivers/md/Makefile)
>>>
>>> Thus, we may introduce an optimization based on xxx_sched_class addresses
>>> in these two hot scheduler functions: pick_next_task() and check_preempt_curr().
>>>
>>> One more result of the patch is that size of object file becomes a little
>>> less (excluding added BUG_ON(), which goes in __init section):
>>>
>>> $size kernel/sched/core.o
>>>          text     data      bss	    dec	    hex	filename
>>> before:  66446    18957	    676	  86079	  1503f	kernel/sched/core.o
>>> after:   66398    18957	    676	  86031	  1500f	kernel/sched/core.o
>>
>> Does LTO preserve this behaviour? I've never quite dared do this exact
>> optimization.
> 
> Also, ld.lld seems a popular option.

I asked on their IRC. Oh, it looks like no way is for this.

About the link: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/sections.html

(17:19:25) nbjoerg: but it is not guarenteed behavior
(17:19:50) nbjoerg: if for some strange reason you really need to enforce relative orders of global objects, put them in consecutively named sections