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authorNick Piggin <npiggin@suse.de>2009-01-20 04:24:26 +0100
committerIngo Molnar <mingo@elte.hu>2009-01-20 13:14:23 +0100
commit92181f190b649f7ef2b79cbf5c00f26ccc66da2a (patch)
treec5c15a4d998d8f21dfa389559408268f917696ac /init
parent0ce1c383681370964e7f77dd44506aeb3a6ba657 (diff)
x86: optimise x86's do_page_fault (C entry point for the page fault path)
Impact: cleanup, restructure code to improve assembly gcc isn't _all_ that smart about spilling registers to stack or reusing stack slots, even with branch annotations. do_page_fault contained a lot of functionality, so split unlikely paths into their own functions, and mark them as noinline just to be sure. I consider this actually to be somewhat of a cleanup too: the main function now contains about half the number of lines so the normal path is easier to read, while the error cases are also nicely split away. Also, ensure the order of arguments to functions is always the same: regs, addr, error_code. This can reduce code size a tiny bit, and just looks neater too. And add a couple of branch annotations. Before: do_page_fault: subq $360, %rsp #, After: do_page_fault: subq $56, %rsp #, bloat-o-meter: add/remove: 8/0 grow/shrink: 0/1 up/down: 2222/-1680 (542) function old new delta __bad_area_nosemaphore - 506 +506 no_context - 474 +474 vmalloc_fault - 424 +424 spurious_fault - 358 +358 mm_fault_error - 272 +272 bad_area_access_error - 89 +89 bad_area - 89 +89 bad_area_nosemaphore - 10 +10 do_page_fault 2464 784 -1680 Yes, the total size increases by 542 bytes, due to the extra function calls. But these will very rarely be called (except for vmalloc_fault) in a normal workload. Importantly, do_page_fault is less than 1/3rd it's original size, and touches far less stack. Existing gotos and branch hints did move a lot of the infrequently used text out of the fastpath, but that's even further improved after this patch. Signed-off-by: Nick Piggin <npiggin@suse.de> Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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