Re: [rcuwalk] e2a728c2785: -29.4%, +71% turbostat.Pkg_W

From: Al Viro
Date: Sat Mar 22 2014 - 09:54:26 EST

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


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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