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

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Date:	Tue, 11 Jun 2013 03:14:01 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	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>,
	sbw@....edu
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On Tue, Jun 11, 2013 at 03:53:17PM +0800, Lai Jiangshan wrote:
> On 06/11/2013 08:51 AM, Linus Torvalds wrote:
> > On Mon, Jun 10, 2013 at 5:44 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> >>
> >> OK, I haven't found a issue here yet, but youss are beiing trickssy! We
> >> don't like trickssy, and we must find precccciouss!!!
> > 
> > .. and I personally have my usual reservations. I absolutely hate
> > papering over scalability issues, and historically whenever people
> > have ever thought that we want complex spinlocks, the problem has
> > always been that the locking sucks.
> > 
> > So reinforced by previous events, I really feel that code that needs
> > this kind of spinlock is broken and needs to be fixed, rather than
> > actually introduce tricky spinlocks.
> > 
> > So in order to merge something like this, I want (a) numbers for real
> > loads and (b) explanations for why the spinlock users cannot be fixed.
> > 
> > Because "we might hit loads" is just not good enough. I would counter
> > with "hiding problems causes more of them".
> > 
> 
> Hi, all
> 
> Off-topic, although I am in this community for several years,
> I am not exactly clear with this problem.
> 
> 1) In general case, which lock is the most competitive in the kernel? what it protects for?
> 2) In which special case, which lock is the most competitive in the kernel? what it protects for?
> 3) In general case, which list is the most hot list?
> 4) In which special case, which list is the most hot list?

Others would know better than I, but mmap_sem has been called out as a
prime offender for some workloads.  There is of course some debate as
to whether the fault lies mmap_sem or with the workloads.  There have
been some efforts to solve this one on LKML, plus some in academia have
worked on this as well:

http://people.csail.mit.edu/nickolai/papers/clements-bonsai.pdf
http://pdos.csail.mit.edu/papers/radixvm:eurosys13.pdf

And IIRC this was the subject of a session at a recent minisummit.

There are a few locks within the RCU implementation that have popped
up from time to time on very large systems, but I have dealt with those
and have plans for each should it become a problem.  The plans probably
won't survive first contact with a real workload, but having thought
things through is very helpful.

							Thanx, Paul

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