linux-kernel - Re: [PATCH RFC ticketlock] Auto-queued ticketlock

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Date:	Wed, 12 Jun 2013 13:26:25 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr.bueso@...com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	赖江山 <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>, niv@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Darren Hart <darren@...art.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Silas Boyd-Wickizer <sbw@....edu>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On Wed, Jun 12, 2013 at 1:03 PM, Davidlohr Bueso <davidlohr.bueso@...com> wrote:
>
> According to him:
>
> "the short workload calls security functions like getpwnam(),
> getpwuid(), getgrgid() a couple of times. These functions open
> the /etc/passwd or /etc/group files, read their content and close the
> files.

Ahh, ok. So yeah, it's multiple threads all hitting the same file.

I guess that /etc/passwd case is historically interesting, but I'm not
sure we really want to care too deeply..

> I did a quick attempt at this (patch attached).

Yeah, that's wrong, although it probably approximates the dget() case
(but incorrectly).

One of the points behind using an atomic d_count is that then dput() should do

   if (!atomic_dec_and_lock(&dentry->d_count, &dentry->d_count))
      return;

at the very top of the function. It can avoid taking the lock entirely
if the count doesn't go down to zero, which would be a common case if
you have lots of users opening the same file. While still protecting
d_count from ever going to zero while the lock is held.

Your

+       if (atomic_read(&dentry->d_count) > 1) {
+               atomic_dec(&dentry->d_count);
+               return;
+       }
+       spin_lock(&dentry->d_lock);

pattern is fundamentally racy, but it's what "atomic_dec_and_lock()"
should do race-free.

For similar reasons, I think you need to still maintain the d_lock in
d_prune_aliases etc. That's a slow-path, so the fact that we add an
atomic sequence there doesn't much matter.

However, one optimization missing from your patch is obvious in the
profile. "dget_parent()" also needs to be optimized - you still have
that as 99% of the spin-lock case. I think we could do something like

   rcu_read_lock();
   parent = ACCESS_ONCE(dentry->d_parent);
   if (atomic_inc_nonzero(&parent->d_count))
      return parent;
   .. get d_lock and do it the slow way ...
   rcu_read_unlock();

to locklessly get the parent pointer. We know "parent" isn't going
away (dentries are rcu-free'd and we hold the rcu read lock), and I
think that we can optimistically take *any* parent dentry that
happened to be valid at one point. As long as the refcount didn't go
down to zero. Al?

With dput and dget_parent() both being lockless for the common case,
you might get rid of the d_lock contention entirely for that load. I
dunno. And I should really think more about that dget_parent() thing a
bit more, but I cannot imagine how it could not be right (because even
with the current d_lock model, the lock is gotten *within*
dget_parent(), so the caller can never know if it gets a new or an old
parent, so there is no higher-level serialization going on - and we
might as well return *either* the new or the old as such).

I really want Al to double-check me if we decide to try going down
this hole. But the above two fixes to your patch should at least
approximate the d_lock changes, even if I'd have to look more closely
at the other details of your patch..

                Linus
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