On Fri, 22 Mar 2019, Linus Torvalds wrote:
Some of them _might_ be performance-critical. There's the one on mmap_sem in the fault handling path, for example. And yes, I'd expect the normal case to very much be "no other readers or writers" for that one.
Yeah, the mmap_sem case in the fault path is really expecting an unlocked state. To the point that four archs have added branch predictions, ie: 92181f190b6 (x86: optimise x86's do_page_fault (C entry point for the page fault path)) b15021d994f (powerpc/mm: Add a bunch of (un)likely annotations to do_page_fault) And using PROFILE_ANNOTATED_BRANCHES shows pretty clearly: (without resetting the counters) correct incorrect % Function File Line ------- --------- - -------- ---- ---- 4603685 34 0 do_user_addr_fault fault.c 1416 (bootup) 382327745 449 0 do_user_addr_fault fault.c 1416 (kernel build) 399446159 461 0 do_user_addr_fault fault.c 1416 (redis benchmark) It would probably wouldn't harm doing the unlikely() for all archs, or alternatively, add likely() to the atomic_long_try_cmpxchg_acquire in patch 3 and do it implicitly but maybe that would be less flexible(?) Thanks, Davidlohr
