Messages in this thread | | | Subject | Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt_curr() | From | Kirill Tkhai <> | Date | Thu, 19 Dec 2019 17:25:40 +0300 |
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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
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