linux-kernel - Re: [PATCH] percpu_counter: Put a reasonable upper bound on percpu_counter

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: <20110906034851.GC18425@mtj.dyndns.org>
Date:	Tue, 6 Sep 2011 12:48:57 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, tytso@....edu,
	adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Put a reasonable upper bound on
 percpu_counter_batch

On Mon, Aug 29, 2011 at 09:46:09PM +1000, Anton Blanchard wrote:
> 
> When testing on a 1024 thread ppc64 box I noticed a large amount of
> CPU time in ext4 code.
> 
> ext4_has_free_blocks has a fast path to avoid summing every free and
> dirty block per cpu counter, but only if the global count shows more
> free blocks than the maximum amount that could be stored in all the
> per cpu counters.
> 
> Since percpu_counter_batch scales with num_online_cpus() and the maximum
> amount in all per cpu counters is percpu_counter_batch * num_online_cpus(),
> this breakpoint grows at O(n^2).
> 
> This issue will also hit with users of percpu_counter_compare which
> does a similar thing for one percpu counter.
> 
> I chose to cap percpu_counter_batch at 1024 as a conservative first
> step, but we may want to reduce it further based on further benchmarking.
> 
> Signed-off-by: Anton Blanchard <anton@...ba.org>

Applied to percpu/for-3.2.

Thanks.

-- 
tejun
--
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/