Re: [PATCH v2 3/7] xen: credit: consider tickled pCPUs as busy.

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

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On Fri, 2017-04-07 at 11:56 +0100, George Dunlap wrote:
> On 06/04/17 09:16, Dario Faggioli wrote:
> > Currently, it can happen that __runq_tickle(),
> > running on pCPU 2 because vCPU x woke up, decides
> > to tickle pCPU 3, because it's idle. Just after
> > that, but before pCPU 3 manages to schedule and
> > pick up x, either __runq_tickel() or
> > __csched_cpu_pick(), running on pCPU 6, sees that
> > idle pCPUs are 0, 1 and also 3, and for whatever
> > reason it also chooses 3 for waking up (or
> > migrating) vCPU y.
> > 
> > When pCPU 3 goes through the scheduler, it will
> > pick up, say, vCPU x, and y will sit in its
> > runqueue, even if there are idle pCPUs.
> > 
> > Alleviate this by marking a pCPU to be idle
> > right away when tickling it (like, e.g., it happens
> > in Credit2).
> > 
> > Note that this does not eliminate the race. That
> > is not possible without introducing proper locking
> > for the cpumasks the scheduler uses. It significantly
> > reduces the window during which it can happen, though.
> > 
> > Introduce proper locking for the cpumask can, in
> > theory, be done, and may be investigated in future.
> > It is a significant amount of work to do it properly
> > (e.g., avoiding deadlock), and it is likely to adversely
> > affect scalability, and so it may be a path it is just
> > not worth following.
> > 
> > Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
> > ---
> > Cc: George Dunlap <george.dunlap@eu.citrix.com>
> > Cc: Anshul Makkar <anshul.makkar@citrix.com>
> > ---
> >  xen/common/sched_credit.c |   22 +++++++++++++++++++---
> >  1 file changed, 19 insertions(+), 3 deletions(-)
> > 
> > diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
> > index 5a3f13f..c753089 100644
> > --- a/xen/common/sched_credit.c
> > +++ b/xen/common/sched_credit.c
> > @@ -489,7 +489,17 @@ static inline void __runq_tickle(struct
> > csched_vcpu *new)
> >                  __trace_var(TRC_CSCHED_TICKLE, 1, sizeof(cpu),
> > &cpu);
> >          }
> >  
> > -        /* Send scheduler interrupts to designated CPUs */
> > +        /*
> > +         * Mark the designated CPUs as busy and send them all the
> > scheduler
> > +         * interrupt. We need the for_each_cpu for dealing with
> > the
> > +         * !opt_tickle_one_idle case. We must use
> > cpumask_clear_cpu() and
> > +         * can't use cpumask_andnot(), because prv->idlers needs
> > atomic access.
> > +         *
> > +         * In the default (and most common) case, when
> > opt_rickle_one_idle is
> > +         * true, the loop does only one step, and only one bit is
> > cleared.
> > +         */
> > +        for_each_cpu(cpu, &mask)
> > +            cpumask_clear_cpu(cpu, prv->idlers);
> >          cpumask_raise_softirq(&mask, SCHEDULE_SOFTIRQ);
> >      }
> >      else
> > @@ -985,6 +995,8 @@ csched_vcpu_acct(struct csched_private *prv,
> > unsigned int cpu)
> >              SCHED_VCPU_STAT_CRANK(svc, migrate_r);
> >              SCHED_STAT_CRANK(migrate_running);
> >              set_bit(_VPF_migrating, &current->pause_flags);
> > +            ASSERT(!cpumask_test_cpu(cpu,
> > +                                     CSCHED_PRIV(per_cpu(scheduler
> > , cpu))->idlers));
> 
> What are these about?  Is this just to double-check that the "idler
> accounting" logic is correct?
>
No, raising the SCHEDULE_SOFTIRQ (which is what we do here, right below
the added ASSERT(), and also in the other place where I added another
one) basically means tickling the cpu we raise it for.

Therefore, according to the very purpose of this patch, we should clear
the bit corresponding to the cpu itself in the idler mask. In practise,
that is not necessary, because it (in both cases) that happens to be
cleared already.

The ASSERT()-s are for both making this explicit, and check/enforce it.

I appreciate it's not that clear, though. I'll add comments.

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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