From mboxrd@z Thu Jan  1 00:00:00 1970
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount
Date: Fri, 30 Aug 2013 08:28:33 -0700
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Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Waiman Long <Waiman.Long@hp.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@redhat.com>,
	Miklos Szeredi <mszeredi@suse.cz>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andi Kleen <andi@firstfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@hp.com>,
	"Norton, Scott J" <scott.norton@hp.com>,
	Michael Neuling <michael.neuling@au1.ibm.com>
To: Ingo Molnar <mingo@kernel.org>
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On Fri, Aug 30, 2013 at 12:16 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
>> > BTW. Do you have your test case at hand ?
>>
>> My test-case is a joke. It's explicitly *trying* to get as much
>> contention as possible on a dentry, by just starting up a lot of threads
>> that look up one single pathname (the same one for everybody). It
>> defaults to using /tmp for this, but you can specify the filename.
>
> Waiman's tests seemed to use sufficiently generic and varied workloads
> (AIM7) and they showed pretty nice unconditional improvements with his
> variant of this scheme, so I think testing with your simple testcase that
> intentionally magnifies the scalability issue is 100% legit and may in
> fact help tune the changes more accurately, because it has less inherent
> noise.

Yes. However, what I am (not very) worried about is that people will
hit some particular codepath that ends up having bad behavior.

I think I covered all the normal hotpaths in pathname lookup, which is
why I'm not *that* worried, but it's still the case that my silly
test-case is very limited. It's limited for a good *reason* (to try to
show the worst-case scalability problem), but it's limited.

              Linus