Re: [Xen-devel] [PATCH v3 34/47] xen/sched: add fall back to idle vcpu when scheduling unit

[Jürgen Groß <jgross@xxxxxxxx>]

On 25.09.19 15:11, Jan Beulich wrote:
> On 25.09.2019 14:58, Jürgen Groß wrote:
> > On 24.09.19 12:53, Jan Beulich wrote:
> > > On 14.09.2019 10:52, Juergen Gross wrote:
> > > > @@ -2015,7 +2079,8 @@ static void sched_slave(void)
> > > >    
> > > >        pcpu_schedule_unlock_irq(lock, cpu);
> > > >    
> > > > -    sched_context_switch(vprev, sched_unit2vcpu_cpu(next, cpu), now);
> > > > +    sched_context_switch(vprev, sched_unit2vcpu_cpu(next, cpu),
> > > > +                         is_idle_unit(next) && !is_idle_unit(prev), now);
> > > >    }
> > > >    
> > > >    /*
> > > > @@ -2075,7 +2140,8 @@ static void schedule(void)
> > > >        pcpu_schedule_unlock_irq(lock, cpu);
> > > >    
> > > >        vnext = sched_unit2vcpu_cpu(next, cpu);
> > > > -    sched_context_switch(vprev, vnext, now);
> > > > +    sched_context_switch(vprev, vnext,
> > > > +                         !is_idle_unit(prev) && is_idle_unit(next), now);
> > > >    }
> > > As a minor remark, I think between such constructs it would be good
> > > if there was no difference, unless there's a reason to have one. Yet
> > > if there was a reason, it surely would want to be spelled out.
> > I guess you mean changing the parameters of sched_context_switch()? I
> > can do that.
> Well, yes, the two sides of the && in one of them.
Ah, okay.

> > > > --- a/xen/include/asm-x86/smp.h
> > > > +++ b/xen/include/asm-x86/smp.h
> > > > @@ -76,6 +76,9 @@ void set_nr_sockets(void);
> > > >    /* Representing HT and core siblings in each socket. */
> > > >    extern cpumask_t **socket_cpumask;
> > > >    
> > > > +#define get_cpu_current(cpu) \
> > > > +    (get_cpu_info_from_stack((unsigned long)stack_base[cpu])->current_vcpu)
> > > I don't think this can go without a comment clarifying under what
> > > (pretty narrow I think) conditions this is legitimate to use.
> > Okay. I'll add a comment like: "to be used only while no context switch
> > can occur on the cpu".
> To be crystal clear, I'd append ", i.e. by certain scheduling code only".
Fine with me.


Juergen


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