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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

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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