Re: [RFC PATCH 2/5] x86,smp: proportional backoff for ticket spinlocks

[Raghavendra K T]

On 01/03/2013 05:12 PM, Michel Lespinasse wrote:
> On Thu, Jan 3, 2013 at 3:35 AM, Raghavendra KT
> <raghavendra.kt.linux@xxxxxxxxx> wrote:
> > [Ccing IBM id]
> > On Thu, Jan 3, 2013 at 10:52 AM, Rik van Riel <riel@xxxxxxxxxx> wrote:
> > > Simple fixed value proportional backoff for ticket spinlocks.
> > > By pounding on the cacheline with the spin lock less often,
> > > bus traffic is reduced. In cases of a data structure with
> > > embedded spinlock, the lock holder has a better chance of
> > > making progress.
> > >
> > > If we are next in line behind the current holder of the
> > > lock, we do a fast spin, so as not to waste any time when
> > > the lock is released.
> > >
> > > The number 50 is likely to be wrong for many setups, and
> > > this patch is mostly to illustrate the concept of proportional
> > > backup. The next patch automatically tunes the delay value.
> > >
> > > Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> > > Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
> > > ---
> > >   arch/x86/kernel/smp.c |   23 ++++++++++++++++++++---
> > >   1 files changed, 20 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> > > index 20da354..9c56fe3 100644
> > > --- a/arch/x86/kernel/smp.c
> > > +++ b/arch/x86/kernel/smp.c
> > > @@ -117,11 +117,28 @@ static bool smp_no_nmi_ipi = false;
> > >    */
> > >   void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
> > >   {
> > > +       __ticket_t head = inc.head, ticket = inc.tail;
> > > +       __ticket_t waiters_ahead;
> > > +       unsigned loops;
> > > +
> > >          for (;;) {
> > > -               cpu_relax();
> > > -               inc.head = ACCESS_ONCE(lock->tickets.head);
> > > +               waiters_ahead = ticket - head - 1;
> >                                               ^^^^^^^^^^^^^^
> > Just wondering,
> > Does wraparound affects this?
>
> The result gets stored in waiters_ahead, which is unsigned and has
> same bit size as ticket and head. So, this takes care of the
> wraparound issue.
>
> In other words, you may have to add 1<<8 or 1<<16 if the integer
> difference was negative; but you get that for free by just computing
> the difference as a 8 or 16 bit unsigned value.

Michael,
Sorry for the noise and for missing the simple math :) and Thanks for 
explanation.

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