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

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