Re: [Xen-devel] [PATCH] xen: credit1: fix tickling when it happens from a remote pCPU

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Thu, 2015-09-24 at 03:29 -0600, Jan Beulich wrote:
> > > > On 24.09.15 at 06:31, <dario.faggioli@xxxxxxxxxx> wrote:
> > --- a/xen/common/sched_credit.c
> > +++ b/xen/common/sched_credit.c

> >  #define csched_balance_mask (CSCHED_PCPU(smp_processor_id())
> > ->balance_mask)
> >  
> > +#define csched_balance_mask_cpu(c) (CSCHED_PCPU(c)->balance_mask)
> 
> csched_runq_steal() gets called with peer_cpu's runqueue lock held
> afaics, but uses smp_processor_id()'s balance_mask. I.e. it looks to
> me that what JÃrgen suggested as an option is actually a requirement.
> 
And I'm very much agreeable on taking his suggestion, because I
actually like it.

Correctness should not be an issue, though. In fact, here is the story
about csched_runq_steal():

 schedule()
   cpu = smp_processor_id()
   lock = pcpu_schedule_lock_irq(cpu);
   sched = this_cpu(scheduler);
   next_slice = sched->do_schedule(sched, ...); 
    |
    --> csched_schedule()
          cpu = smp_processor_id();
          snext = csched_load_balance(..., cpu, ...);
            peer_cpu = xxx;
            lock = pcpu_schedule_trylock(peer_cpu);
            speer = csched_runq_steal(peer_cpu, cpu, ...);
              csched_balance_cpumask(..., csched_balance_mask);
            pcpu_schedule_unlock(lock, peer_cpu);
   pcpu_schedule_unlock_irq(lock, cpu);

So, we can safely use smp_processor_id()'s scratch space, as we own its
runqueue lock too.

In any case, thanks a lot for having a look.
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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