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

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

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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