Re: [RFC PATCH v1 3/6] sched, credit2: improve scheduler fairness

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

On Fri, 2020-06-12 at 06:51 +0200, Jürgen Groß wrote:
> On 12.06.20 02:22, Volodymyr Babchuk wrote:
> > Now we can make corrections for scheduling unit run time, based on
> > data gathered in previous patches. This includes time spent in IRQ
> > handlers and time spent for hypervisor housekeeping tasks. Those time
> > spans needs to be deduced from a total run time.
> > 
> > This patch adds sched_get_time_correction() function which returns
> > time correction value. This value should be subtracted by a scheduler
> > implementation from a total vCPU/shced_unit run time.
> > 
> > TODO: Make the corresponding changes to all other schedulers.
> > 
> > Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
> > ---
> >   xen/common/sched/core.c    | 23 +++++++++++++++++++++++
> >   xen/common/sched/credit2.c |  2 +-
> >   xen/common/sched/private.h | 10 ++++++++++
> >   3 files changed, 34 insertions(+), 1 deletion(-)
> > 
> > diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
> > index d597811fef..a7294ff5c3 100644
> > --- a/xen/common/sched/core.c
> > +++ b/xen/common/sched/core.c
> > @@ -974,6 +974,29 @@ void vcpu_end_hyp_task(struct vcpu *v)
> >   #ifndef NDEBUG
> >       v->in_hyp_task = false;
> >   #endif
> > +
> > +s_time_t sched_get_time_correction(struct sched_unit *u)
> > +{
> > +    unsigned long flags;
> > +    int irq, hyp;
> 
> Using "irq" for a time value is misleading IMO.

Yes, you are right. I'll rename this variables to irq_time and
hyp_time. 

> > +
> > +    while ( true )
> > +    {
> > +        irq = atomic_read(&u->irq_time);
> > +        if ( likely( irq == atomic_cmpxchg(&u->irq_time, irq, 0)) )
> > +            break;
> > +    }
> 
> Just use atomic_xchg()?

Thanks. I somehow missed this macro.

> > +
> > +    while ( true )
> > +    {
> > +        hyp = atomic_read(&u->hyp_time);
> > +        if ( likely( hyp == atomic_cmpxchg(&u->hyp_time, hyp, 0)) )
> > +            break;
> > +    }
> > +
> > +    return irq + hyp;
> 
> Ah, I didn't look into this patch until now.
> 
> You can replace my comments about overflow of an int for patches 1 and 2
> with:
> 
>    Please modify the comment about not overflowing hinting to the value
>    being reset when making scheduling decisions.

Will do.

> And this (of course) needs to be handled in all other schedulers, too.
> 

Yes, the plan is to call this function in all schedulers. I skipped
this in RFC, because I wanted to discuss the general approch. I'll add
support for all other schedulers in the next version.