CC: gcc-help@xxxxxxxxxxx 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.