linux-ext4 - Re: [PATCH] percpu_counter: Fix __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: <20081208142226.c7b46f04.akpm@linux-foundation.org>
Date:	Mon, 8 Dec 2008 14:22:26 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Theodore Tso <tytso@....edu>
Cc:	dada1@...mosbay.com, linux-kernel@...r.kernel.org,
	davem@...emloft.net, a.p.zijlstra@...llo.nl, cmm@...ibm.com,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

On Mon, 8 Dec 2008 17:12:41 -0500
Theodore Tso <tytso@....edu> wrote:

> Actually, if all popular architectures had a hardware-implemented
> atomic_t, I wonder how much ext4 really needs the percpu counter,
> especially given ext4's multiblock allocator; with ext3, given that
> each block allocation required taking a per-filesystem spin lock,
> optimizing away that spinlock was far more important for improving
> ext3's scalability.  But with the multiblock allocator, it may that
> we're going through a lot more effort than what is truly necessary.

I expect that the performance numbers for the percpu counters in the
superblock are buried away in the historical git changelogs somewhere. 
I don't recall how much difference it made.

An atomic_inc() of an fs-wide counter will have similar cost to
spin_lock() of an fs-wide lock.

If the multiblock allocator can avoid doing one atomic_inc() for each
block and can instead do atomic_add(large_value, &counter) then yes,
I'm sure that an fs-wide atomic_long_t would be OK.

Of course, similar changes should be made in trucate, etc.
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html