linux-kernel - Re: [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

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Date:	Sat, 10 May 2014 16:13:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

On Fri, May 09, 2014 at 09:19:32PM -0400, Waiman Long wrote:
> On 05/08/2014 03:06 PM, Peter Zijlstra wrote:
> >On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote:
> >>If unfair lock is supported, the lock acquisition loop at the end of
> >>the queue_spin_lock_slowpath() function may need to detect the fact
> >>the lock can be stolen. Code are added for the stolen lock detection.
> >>
> >>A new qhead macro is also defined as a shorthand for mcs.locked.
> >NAK, unfair should be a pure test-and-set lock.
> 
> I have performance data showing that a simple test-and-set lock does not
> scale well. That is the primary reason of ditching the test-and-set lock and
> use a more complicated scheme which scales better.

Nobody should give a fuck about scalability in this case anyway.

Also, as I explained/asked earlier:

  lkml.kernel.org/r/20140314083001.GN27965@...ns.programming.kicks-ass.net

Lock holder preemption is _way_ worse with any kind of queueing. You've
not explained how the simple 3 cpu example in that email gets better
performance than a test-and-set lock.

> Also, it will be hard to
> make the unfair test-and-set lock code to coexist nicely with PV spinlock
> code.

That's just complete crap as the test-and-set lock is like 3 lines of
code.

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