linux-kernel - Re: [rcuwalk] e2a728c2785: -29.4% aim7.2000.jobs-per-min, +71% turbostat.Pkg

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Message-ID: <20140322135412.GF18016@ZenIV.linux.org.uk>
Date:	Sat, 22 Mar 2014 13:54:12 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, lkp@...org
Subject: Re: [rcuwalk] e2a728c2785: -29.4% aim7.2000.jobs-per-min, +71%
 turbostat.Pkg_W

On Sat, Mar 22, 2014 at 07:56:36PM +0800, Fengguang Wu wrote:
> Hi Al,
> 
> We noticed the below changes on
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git untested-mnt_hash
> commit e2a728c2785d6aab45d1a6d2c61e986d385c30bf ("rcuwalk: recheck mount_lock after mountpoint crossing attempts")

*doh*

Sorry about that - what happens is that __follow_mount_rcu() does that
check every time it's called, not on every negative lookup from
__lookup_mnt().  IOW, it's a massive overkill - most of the time
we don't call __lookup_mnt() at all, due to d_mountpoint() being false,
or get a positive from it.

Fixed and force-pushed; should get much better now...
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