linux-kernel - Re: AIM7 40% regression with 2.6.26-rc1

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20080507172246.GA13262@elte.hu>
Date:	Wed, 7 May 2008 19:22:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Matthew Wilcox <matthew@....cx>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"J. Bruce Fields" <bfields@...i.umich.edu>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: AIM7 40% regression with 2.6.26-rc1

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> Which is why I'm 100% convinced it's not even worth saving the old 
> code. It needs to use mutexes, or spinlocks. I bet it has *nothing* to 
> do with "slow path" other than the fact that it gets to that slow path 
> much more these days.

i think your theory should be easy to test: Yanmin, could you turn on 
CONFIG_MUTEX_DEBUG=y and check by how much AIM7 regresses?

Because in the CONFIG_MUTEX_DEBUG=y case the mutex debug code does 
exactly that: it doesnt use the single-instruction fastpath [it uses 
asm-generic/mutex-null.h] but always drops into the slowpath (to be able 
to access debug state). That debug code is about as expensive as the 
generic semaphore code's current fastpath. (perhaps even more 
expensive.)

There's far more normal mutex fastpath use during an AIM7 run than any 
BKL use. So if it's due to any direct fastpath overhead and the 
resulting widening of the window for the real slowdown, we should see a 
severe slowdown on AIM7 with CONFIG_MUTEX_DEBUG=y. Agreed?

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/