linux-ext4 - Re: [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked

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Message-Id: <20120608150253.e42464a6.akpm@linux-foundation.org>
Date:	Fri, 8 Jun 2012 15:02:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Boaz Harrosh <bharrosh@...asas.com>,
	Tao Ma <boyu.mt@...bao.com>, Nick Piggin <npiggin@...nel.dk>,
	"Dmitry V. Levin" <ldv@...linux.org>,
	v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-afs@...ts.infradead.org,
	linux-btrfs@...r.kernel.org, ceph-devel@...r.kernel.org,
	linux-cifs@...r.kernel.org, samba-technical@...ts.samba.org,
	codalist@...EMANN.coda.cs.cmu.edu, ecryptfs@...r.kernel.org,
	osd-dev@...n-osd.org, linux-ext4@...r.kernel.org,
	fuse-devel@...ts.sourceforge.net, linux-mtd@...ts.infradead.org,
	jfs-discussion@...ts.sourceforge.net, logfs@...fs.org,
	linux-nfs@...r.kernel.org, linux-nilfs@...r.kernel.org,
	linux-ntfs-dev@...ts.sourceforge.net, ocfs2-devel@....oracle.com,
	reiserfs-devel@...r.kernel.org
Subject: Re: [RFC, PATCH, RESEND] fs: push rcu_barrier() from
 deactivate_locked_super() to filesystems

On Sat,  9 Jun 2012 00:41:03 +0300
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:

> There's no reason to call rcu_barrier() on every deactivate_locked_super().
> We only need to make sure that all delayed rcu free inodes are flushed
> before we destroy related cache.
> 
> Removing rcu_barrier() from deactivate_locked_super() affects some
> fas paths. E.g. on my machine exit_group() of a last process in IPC
> namespace takes 0.07538s. rcu_barrier() takes 0.05188s of that time.

What an unpleasant patch.  Is final-process-exiting-ipc-namespace a
sufficiently high-frequency operation to justify the change?

I don't really understand what's going on here.  Are you saying that
there is some filesystem against which we run deactivate_locked_super()
during exit_group(), and that this filesystem doesn't use rcu-freeing
of inodes?  The description needs this level of detail, please.


The implementation would be less unpleasant if we could do the
rcu_barrier() in kmem_cache_destroy().  I can't see a way of doing that
without adding a dedicated slab flag, which would require editing all
the filesystems anyway.


(kmem_cache_destroy() already has an rcu_barrier().  Can we do away
with the private rcu games in the vfs and switch to
SLAB_DESTROY_BY_RCU?)
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