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SubjectRe: [RFC][PATCH 6/8] mm: handle_speculative_fault()


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?

Linus


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