Re: [PATCH v2 5/7] xen: credit1: increase efficiency and scalability of load balancing.

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Fri, 2017-04-07 at 15:38 +0100, George Dunlap wrote:
> On 06/04/17 09:16, Dario Faggioli wrote:
> > --- a/xen/common/sched_credit.c
> > +++ b/xen/common/sched_credit.c
> > @@ -1738,14 +1800,13 @@ csched_load_balance(struct csched_private
> > *prv, int cpu,
> >                   * could cause a deadlock if the peer CPU is also
> > load
> >                   * balancing and trying to lock this CPU.
> >                   */
> > -                spinlock_t *lock =
> > pcpu_schedule_trylock(peer_cpu);
> > +                lock = pcpu_schedule_trylock(peer_cpu);
> >                  SCHED_STAT_CRANK(steal_trylock);
> >                  if ( !lock )
> >                  {
> >                      SCHED_STAT_CRANK(steal_trylock_failed);
> >                      TRACE_2D(TRC_CSCHED_STEAL_CHECK, peer_cpu, /*
> > skipp'n */ 0);
> > -                    peer_cpu = cpumask_cycle(peer_cpu, &workers);
> > -                    continue;
> > +                    goto next_cpu;
> 
> Wait -- does this mean that before this patch, this effectively
> busy-waited until peer_cpu was actually free (since peer_cpu was
> never
> incremented)?
> 
Err, what do you mean? peer_cpu is updated here, with the result of
cpumask_cycle(), run on workers, starting from the the current value of
peer_cpu itself.

So, no, we don't busy wait try-locking on the same pcpu, we try all of
them, one after the other (in a node-wise fashion, thanks to the outer
loop).

If this is what you were asking....

> I like the idea in general, but I'm not a fan of the current way of
> doing the accounting -- it seems like too many special cases.  Let me
> have a think about this and come back to it.
> 
Ok. I'm not sure I see what you mean with 'accounting' in this context,
but, yeah, go ahead and let me know. :-)

Thanks and Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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