Messages in this thread | | | Date | Fri, 22 Mar 2019 12:30:10 -0700 | From | Davidlohr Bueso <> | Subject | Re: [PATCH v5 1/3] locking/rwsem: Remove arch specific rwsem files |
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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
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