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 17:12:58 -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 4:30 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Hrm...  It excludes sharing between the locks, all right.  AFAICS, that
> won't exclude sharing with plain per-cpu vars, will it?

Yes it will. DEFINE_PER_CPU_SHARED_ALIGNED not only aligns the data,
it also puts it in a separate section with only other aligned data
entries. So now the percpu address map around it looks like this:

  ...
  0000000000013a80 d call_single_queue
  0000000000013ac0 d cfd_data
  0000000000013b00 d files_lglock_lock
  0000000000013b40 d vfsmount_lock_lock
  0000000000013b80 d file_lock_lglock_lock
  0000000000013bc0 D softnet_data
  0000000000013d40 D __per_cpu_end
   ..

So there shouldn't be anything to share falsely with.

I'd like to say that the profile is bad, but this is *so* consistent,
and the profile data really looks perfectly fine in every other way.
I'm using "-e cycles:pp", so it's using hardware profiling and all the
other functions really look correct.

It *is* one of the few locked accesses remaining, and it's clearly
getting called a lot (three calls per system call: two mntput's  - one
for the root path, one for the result path, and one from path_init ->
rcu_walk_init), but with up to 8% CPU time for basically that one
"lock xadd" instruction is damn odd. I can't see how that could happen
without seriously nasty cacheline bouncing, but I can't see how *that*
can happen when all the accesses seem to be from the current CPU.

This is a new Haswell-based machine that I put together yesterdat, and
I haven't used it for profiling before. So maybe it _is_ something odd
with the profiling after all, and atomic serializing instructions get
incorrect profile counts.

             Linus
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