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

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Date:   Fri, 22 Mar 2019 12:30:10 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>,
        "linux-alpha@...r.kernel.org" <linux-arm-kernel@...ts.infradead.org>,
        linux-c6x-dev@...ux-c6x.org, uclinux-h8-devel@...ts.sourceforge.jp,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        linux-mips@...r.kernel.org, nios2-dev@...ts.rocketboards.org,
        openrisc@...ts.librecores.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org,
        Linux-sh list <linux-sh@...r.kernel.org>,
        sparclinux@...r.kernel.org, linux-um@...ts.infradead.org,
        linux-xtensa@...ux-xtensa.org,
        linux-arch <linux-arch@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v5 1/3] locking/rwsem: Remove arch specific rwsem files

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