linux-kernel - Re: [RFC v1 0/5] fs/locks: Use plain percpu spinlocks instead of lglock to protect file

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Date:	Tue, 24 Feb 2015 16:06:43 -0500
From:	Jeff Layton <jlayton@...chiereds.net>
To:	Daniel Wagner <daniel.wagner@...-carit.de>
Cc:	Andi Kleen <andi@...stfloor.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, John Kacur <jkacur@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"J. Bruce Fields" <bfields@...ldses.org>
Subject: Re: [RFC v1 0/5] fs/locks: Use plain percpu spinlocks instead of
 lglock to protect file_lock

On Tue, 24 Feb 2015 16:58:26 +0100
Daniel Wagner <daniel.wagner@...-carit.de> wrote:

> On 02/20/2015 05:05 PM, Andi Kleen wrote:
> > Daniel Wagner <daniel.wagner@...-carit.de> writes:
> >>
> >> I am looking at how to get rid of lglock. Reason being -rt is not too
> >> happy with that lock, especially that it uses arch_spinlock_t and
> > 
> > AFAIK it could just use normal spinlock. Have you tried that?
> 
> I have tried it. At least fs/locks.c didn't blow up. The benchmark
> results (lockperf) indicated that using normal spinlocks is even
> slightly faster. Simply converting felt like cheating. It might be
> necessary for the other user (kernel/stop_machine.c). Currently it looks
> like there is some additional benefit getting lglock away in fs/locks.c.
> 

What would that benefit be?

lglocks are basically percpu spinlocks. Fixing some underlying
infrastructure that provides that seems like it might be a better
approach than declaring them "manually" and avoiding them altogether.

Note that you can still do basically what you're proposing here with
lglocks as well. Avoid using lg_global_* and just lock each one in
turn.

That said, now that I've thought about this, I'm not sure that's really
something we want to do when accessing /proc/locks. If you lock each
one in turn, then you aren't freezing the state of the file_lock_list
percpu lists. Won't that mean that you aren't necessarily getting a
consistent view of the locks on those lists when you cat /proc/locks?

I think having a consistent view there might trump any benefit to
performance. Reading /proc/locks is a *very* rare activity in the big
scheme of things.

I do however like the idea of moving more to be protected by the
lglocks, and minimizing usage of the blocked_lock_lock.

-- 
Jeff Layton <jlayton@...chiereds.net>
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