netdev - Re: iptables very slow after commit 784544739a25c30637397ace5489eeb6e15d7d49

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: <20090411080554.67a17410@nehalam>
Date:	Sat, 11 Apr 2009 08:05:54 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	jeff.chua.linux@...il.com, dada1@...mosbay.com, jengelh@...ozas.de,
	kaber@...sh.net, r000n@...0n.net,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: iptables very slow after commit
 784544739a25c30637397ace5489eeb6e15d7d49

On Sat, 11 Apr 2009 09:08:54 +0200
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > 	I will nevertheless suggest the following egregious hack to
> > 	get a consistent sample of one counter for some other CPU:
> > 
> > 	a.	Disable interrupts
> > 	b.	Atomically exchange the bottom 32 bits of the
> > 		counter with the value zero.
> > 	c.	Atomically exchange the top 32 bits of the counter
> > 		with the value zero.
> > 	d.	Concatenate the values obtained in (b) and (c), which
> > 		is the snapshot value.
> 
> Note, i have recently implemented full atomic64_t support on 32-bit 
> x86, for the perfcounters code, based on the CMPXCHG8B instruction.
> 
> Which, while not the lightest of instructions, is still much better 
> than the sequence above.
> 
> So i think a better approach would be to also add a dumb generic 
> implementation for atomic64_t (using a global lock or so), and then 
> generic code could just assume that atomic64_t always exists.
> 
> It is far nicer - and faster as well - as the hack above, even on 
> 32-bit x86.
> 
> 	Ingo

The iptables counters are write mostly, read rarely so they don't
fit the seq counter or atomic use case. Also, it is important
to get a consistent snapshot of the whole set not just each
individual counter.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html