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

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Message-ID: <20130901223521.GV13318@ZenIV.linux.org.uk>
Date:	Sun, 1 Sep 2013 23:35:21 +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 03:16:24PM -0700, Linus Torvalds wrote:

> Does not seem to matter. Still 66% mntput_no_expire, 31% path_init.
> And that lg_local_lock() takes 5-6% of CPU, pretty much all of which
> is that single xadd instruction that implements the spinlock.
> 
> This is on /tmp, which is tmpfs. But I don't see how any of that could
> matter. "mntput()" does an unconditional call to mntput_no_expire(),
> and mntput_no_expire() does that br_read_lock() unconditionally too.
> 
> Note that I'm talking about that "cheap" *read* lock being expensive.
> It's the local one, not the global one. So it's not what Waiman saw
> with the global lock. This is a local per-cpu thing.
> 
> That read-lock is supposed to be very cheap - it's just a per-cpu
> spinlock. But it ends up being very expensive for some reason. I'm not
> quite sure why - I don't see any lg_global_lock() calls at all, so...
> 
> 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...
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