linux-kernel - Re: [PATCH v2] powerpc ticket locks

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Date:	Mon, 10 Feb 2014 16:52:17 +0100
From:	Torsten Duwe <duwe@....de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Scott Wood <scottwood@...escale.com>,
	Tom Musta <tommusta@...il.com>, Ingo Molnar <mingo@...nel.org>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] powerpc ticket locks

On Mon, Feb 10, 2014 at 02:10:23PM +1100, Benjamin Herrenschmidt wrote:
> On Fri, 2014-02-07 at 17:58 +0100, Torsten Duwe wrote:
> >  typedef struct {
> > -       volatile unsigned int slock;
> > -} arch_spinlock_t;
> > +       union {
> > +               __ticketpair_t head_tail;
> > +               struct __raw_tickets {
> > +#ifdef __BIG_ENDIAN__          /* The "tail" part should be in the MSBs */
> > +                       __ticket_t tail, head;
> > +#else
> > +                       __ticket_t head, tail;
> > +#endif
> > +               } tickets;
> > +       };
> > +#if defined(CONFIG_PPC_SPLPAR)
> > +       u32 holder;
> > +#endif
> > +} arch_spinlock_t __aligned(4);
> 
> That's still broken with lockref (which we just merged).
> 
> We must have the arch_spinlock_t and the ref in the same 64-bit word
> otherwise it will break.

Well, as far as I can see you'll just not be able to
USE_CMPXCHG_LOCKREF -- with the appropriate performance hit --
the code just falls back into lock&ref on pSeries.
What again was the intention of directed yield in the first place...?

> We can make it work in theory since the holder doesn't have to be
> accessed atomically, but the practicals are a complete mess ...
> lockref would essentially have to re-implement the holder handling
> of the spinlocks and use lower level ticket stuff.
> 
> Unless you can find a sneaky trick ... :-(

What if I squeeze the bits a little?
4k vCPUs, and 256 physical, as a limit to stay within 32 bits?
At the cost that unlock may become an ll/sc operation again.
I could think about a trick against that.
But alas, hw_cpu_id is 16 bit, which makes a lookup table neccessary :-/

Doing another round of yields for lockrefs now doesn't
sound so bad any more.

Opinions, anyone?

	Torsten

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