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    SubjectRe: [RFC][PATCH 6/8] mm: handle_speculative_fault()
    On Tue, 5 Jan 2010 07:26:31 -0800 (PST)
    Linus Torvalds <torvalds@linux-foundation.org> wrote:

    >
    >
    > On Tue, 5 Jan 2010, KAMEZAWA Hiroyuki wrote:
    > > #
    > > # Overhead Command Shared Object Symbol
    > > # ........ ............... ........................ ......
    > > #
    > > 43.23% multi-fault-all [kernel] [k] smp_invalidate_interrupt
    > > 16.27% multi-fault-all [kernel] [k] flush_tlb_others_ipi
    > > 11.55% multi-fault-all [kernel] [k] _raw_spin_lock_irqsave <========(*)
    > > 6.23% multi-fault-all [kernel] [k] intel_pmu_enable_all
    > > 2.17% multi-fault-all [kernel] [k] _raw_spin_unlock_irqrestore
    >
    > Hmm.. The default rwsem implementation shouldn't have any spin-locks in
    > the fast-path. And your profile doesn't seem to have any scheduler
    > footprint, so I wonder what is going on.
    >
    > Oh.
    >
    > Lookie here:
    >
    > - arch/x86/Kconfig.cpu:
    >
    > config X86_XADD
    > def_bool y
    > depends on X86_32 && !M386
    >
    > - arch/x86/Kconfig:
    >
    > config RWSEM_GENERIC_SPINLOCK
    > def_bool !X86_XADD
    >
    > config RWSEM_XCHGADD_ALGORITHM
    > def_bool X86_XADD
    >
    > it looks like X86_XADD only gets enabled on 32-bit builds. Which means
    > that x86-64 in turn seems to end up always using the slower "generic
    > spinlock" version.
    >
    > Are you sure this isn't the reason why your profiles are horrible?
    >
    I think this is the 1st reason but haven't rewrote rwsem itself and tested,
    sorry.

    This is a profile in other test.
    ==
    2.6.33-rc2's score of the same test program is here.

    75.42% multi-fault-all [kernel] [k] _raw_spin_lock_irqsav
    |
    --- _raw_spin_lock_irqsave
    |
    |--49.13%-- __down_read_trylock
    | down_read_trylock
    | do_page_fault
    | page_fault
    | 0x400950
    | |
    | --100.00%-- (nil)
    |
    |--46.92%-- __up_read
    | up_read
    | |
    | |--99.99%-- do_page_fault
    | | page_fault
    | | 0x400950
    | | (nil)
    | --0.01%-- [...]
    ==
    yes, spinlock is from rwsem.

    Why I tried "skipping rwsem" is because I like avoid locking rather than rewrite
    lock itself when I think of the influence of the patch....


    Thanks,
    -Kame






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