Re: Preemptable Ticket Spinlock

[Rik van Riel]

On 04/22/2013 04:08 PM, Peter Zijlstra wrote:
> On Mon, 2013-04-22 at 15:56 -0400, Rik van Riel wrote:
> > On 04/22/2013 03:49 PM, Peter Zijlstra wrote:
> > > On Mon, 2013-04-22 at 08:52 -0400, Rik van Riel wrote:
> >
> > > > If the native spin_lock code has been called already at
> > > > that time, the native code would still need to be modified
> > > > to increment the ticket number by 2, so we end up with a
> > > > compatible value in each spin lock's .tickets field, and
> > > > prevent a deadlock after we switch over to the paravirt
> > > > variant.
> > >
> > > I thought the stuff already made it upstream, but apparently not; the
> > > lastest posting I'm aware of is here:
> > >
> > >     https://lkml.org/lkml/2012/5/2/105
> > >
> > > That stuff changes the normal ticket increment as well..
> >
> > Jiannan,
> >
> > It looks like the patch above could make a good patch
> > 1 (or 2) in your patch series :)
>
> I much prefer the entire series from Jeremy since it maintains the
> ticket semantics and doesn't degrade the lock to unfair under
> contention.
>
> Now I suppose there's a reason its not been merged yet and I suspect
> its !paravirt hotpath impact which wasn't rightly justified or somesuch
> so maybe someone can work on that or so.. dunno.

IIRC one of the reasons was that the performance improvement wasn't
as obvious.  Rescheduling VCPUs takes a fair amount of time, quite
probably more than the typical hold time of a spinlock.

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