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

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Date:	Thu, 13 Jun 2013 00:32:24 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	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>, ????????? <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>,
	Fr??d??ric 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 01:26:25PM -0700, Linus Torvalds wrote:

> 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?

What will you do with __d_rcu_to_refcount()?  Any such scheme has to
hold d_lock from zero->non-zero d_count changes, or atomic_dec_and_lock
in dput() won't help at all.  As it is, both comlete_walk() and unlazy_walk()
are grabbing ->d_lock on the dentry we'd reached, so they can call that
sucker.  And that'll give you ->d_lock contention when a bunch of threads
are hitting the same file; I don't see how atomics would avoid that
one...
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