Messages in this thread | | | Date | Thu, 19 Dec 2019 14:12:42 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt_curr() |
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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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