linux-kernel - Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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Message-ID: <20130901224459.GW13318@ZenIV.linux.org.uk>
Date:	Sun, 1 Sep 2013 23:44:59 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Sedat Dilek <sedat.dilek@...il.com>,
	Waiman Long <waiman.long@...com>,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On Sun, Sep 01, 2013 at 11:35:21PM +0100, Al Viro wrote:
> > I wonder if there is some false sharing going on. But I don't see that
> > either, this is the percpu offset map afaik:
> > 
> >   000000000000f560 d files_lglock_lock
> >   000000000000f564 d nr_dentry
> >   000000000000f568 d last_ino
> >   000000000000f56c d nr_unused
> >   000000000000f570 d nr_inodes
> >   000000000000f574 d vfsmount_lock_lock
> >   000000000000f580 d bh_accounting
> > 
> > and I don't see anything there that would get cross-cpu accesses, so
> > there shouldn't be any cacheline bouncing. That's the whole point of
> > percpu variables, after all.
> 
> Hell knows...  Are you sure you don't see br_write_lock() at all?  I don't
> see anything else that would cause cross-cpu traffic with that layout...

GRRR...  I see something else:
void file_sb_list_del(struct file *file)
{
        if (!list_empty(&file->f_u.fu_list)) {
                lg_local_lock_cpu(&files_lglock, file_list_cpu(file));
                list_del_init(&file->f_u.fu_list);
                lg_local_unlock_cpu(&files_lglock, file_list_cpu(file));
        }
}
will cheerfully cause cross-CPU traffic.  If that's what is going on, the
earlier patch I've sent (not putting non-regulars and files opened r/o
on ->s_list) should reduce the cacheline bouncing on that cacheline.
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