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

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Date:	Fri, 30 Aug 2013 11:27:01 +0200
From:	Sedat Dilek <sedat.dilek@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Waiman Long <waiman.long@...com>, Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	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 Fri, Aug 30, 2013 at 9:55 AM, Sedat Dilek <sedat.dilek@...il.com> wrote:
> On Fri, Aug 30, 2013 at 5:54 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Thu, Aug 29, 2013 at 8:12 PM, Waiman Long <waiman.long@...com> wrote:
>>> On 08/29/2013 07:42 PM, Linus Torvalds wrote:
>>>>
>>>> Waiman? Mind looking at this and testing? Linus
>>>
>>> Sure, I will try out the patch tomorrow morning and see how it works out for
>>> my test case.
>>
>> Ok, thanks, please use this slightly updated patch attached here.
>>
>> It improves on the previous version in actually handling the
>> "unlazy_walk()" case with native lockref handling, which means that
>> one other not entirely odd case (symlink traversal) avoids the d_lock
>> contention.
>>
>> It also refactored the __d_rcu_to_refcount() to be more readable, and
>> adds a big comment about what the heck is going on. The old code was
>> clever, but I suspect not very many people could possibly understand
>> what it actually did. Plus it used nested spinlocks because it wanted
>> to avoid checking the sequence count twice. Which is stupid, since
>> nesting locks is how you get really bad contention, and the sequence
>> count check is really cheap anyway. Plus the nesting *really* didn't
>> work with the whole lockref model.
>>
>> With this, my stupid thread-lookup thing doesn't show any spinlock
>> contention even for the "look up symlink" case.
>>
>> It also avoids the unnecessary aligned u64 for when we don't actually
>> use cmpxchg at all.
>>
>> It's still one single patch, since I was working on lots of small
>> cleanups. I think it's pretty close to done now (assuming your testing
>> shows it performs fine - the powerpc numbers are promising, though),
>> so I'll split it up into proper chunks rather than random commit
>> points. But I'm done for today at least.
>>
>> NOTE NOTE NOTE! My test coverage really has been pretty pitiful. You
>> may hit cases I didn't test. I think it should be *stable*, but maybe
>> there's some other d_lock case that your tuned waiting hid, and that
>> my "fastpath only for unlocked case" version ends up having problems
>> with.
>>
>
> Following this thread with half an eye... Was that "unsigned" stuff
> fixed (someone pointed to it).
> How do you call that test-patch (subject)?
> I would like to test it on my SNB ultrabook with your test-case script.
>

Here on Ubuntu/precise v12.04.3 AMD64 I get these numbers for total loops:

lockref:  w/o patch | w/ patch
======================
Run #1: 2.688.094 | 2.643.004
Run #2: 2.678.884 | 2.652.787
Run #3: 2.686.450 | 2.650.142
Run #4: 2.688.435 | 2.648.409
Run #5: 2.693.770 | 2.651.514

Average: 2687126,6 VS. 2649171,2 ( −37955,4 )

- Sedat -
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