Re: [PATCH v5 1/3] locking/rwsem: Remove arch specific rwsem files

[Davidlohr Bueso <dave@stgolabs.net>]

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