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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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