linux-kernel - [Q] ld: Does LTO reorder ro variables in two files?

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Date:   Thu, 19 Dec 2019 17:04:49 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Peter Zijlstra <peterz@...radead.org>, gcc-help@....gnu.org
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: [Q] ld: Does LTO reorder ro variables in two files?

CC: gcc-help@....gnu.org

Hi, gcc guys,

this thread starts here: https://lkml.org/lkml/2019/12/19/403

There are two const variables:

   struct sched_class idle_sched_class
and
   struct sched_class fair_sched_class,

which are declared in two files idle.c and fair.c.

1)In Makefile the order is: idle.o fair.o
2)the variables go to the same ro section
3)there is no SORT(.*) keyword in linker script.

Is it always true, that after linkage &idle_sched_class < &fair_sched_class?

Thanks!
Kirill

On 19.12.2019 16:12, 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.