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

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Date:	Sun, 1 Sep 2013 15:16:24 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
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 1, 2013 at 2:23 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> How much of that is due to br_write_lock() taken in mntput_no_expire()
> for no good reason?  IOW, could you try shmem.c patch I've sent yesterday
> and see how much effect does it have?[1]  Basically, we get it grabbed
> exclusive on each final fput() of a struct file created by shmem_file_setup(),
> which is _not_ a rare event.  And the only reason for that is not having
> shm_mnt marked long-living, even though its refcount never hits 0...

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.

Odd. What am I missing?

            Linus
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