linux-kernel - Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge regression in performance

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Message-ID: <alpine.LFD.2.00.1003231242420.18017@i5.linux-foundation.org>
Date:	Tue, 23 Mar 2010 12:52:44 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Anton Starikov <ant.starikov@...il.com>,
	Ingo Molnar <mingo@...e.hu>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge regression
 in performance



On Tue, 23 Mar 2010, Andrew Morton wrote:
> 
> You should be able to simply set CONFIG_RWSEM_GENERIC_SPINLOCK=n,
> CONFIG_RWSEM_XCHGADD_ALGORITHM=y by hand, as I mentioned earlier?

No. Doesn't work. The XADD code simply never worked on x86-64, which is 
why those three commits I pointed at are required.

Oh, and you need one more commit (at least) in addition to the three I 
already mentioned - the one that actually adds the x86-64 wrappers and 
Kconfig option:

	bafaecd x86-64: support native xadd rwsem implementation

so the minimal list of commits (on top of 2.6.33) is at least

	59c33fa x86-32: clean up rwsem inline asm statements
	5d0b723 x86: clean up rwsem type system
	bafaecd x86-64: support native xadd rwsem implementation
	1838ef1 x86-64, rwsem: 64-bit xadd rwsem implementation

and I just verified that they at least cherry-pick cleanly (in that 
order). I _think_ it would be good to also do

	0d1622d x86-64, rwsem: Avoid store forwarding hazard in __downgrade_write

but that one is a small detail, not anything fundamentally important.

			Linus
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