linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Date:	Tue, 5 Jan 2010 07:26:31 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Minchan Kim <minchan.kim@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, cl@...ux-foundation.org,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [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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