Re: [Xen-devel] [PATCH v5 01/19] xen/sched: add code to sync scheduling of all vcpus of a sched unit

[Jürgen Groß <jgross@xxxxxxxx>]

On 30.09.19 12:39, Jan Beulich wrote:
> On 30.09.2019 12:38, Andrew Cooper wrote:
> > On 30/09/2019 11:36, Jan Beulich wrote:
> > > On 30.09.2019 07:21, Juergen Gross wrote:
> > > > When switching sched units synchronize all vcpus of the new unit to be
> > > > scheduled at the same time.
> > > >
> > > > A variable sched_granularity is added which holds the number of vcpus
> > > > per schedule unit.
> > > >
> > > > As tasklets require to schedule the idle unit it is required to set the
> > > > tasklet_work_scheduled parameter of do_schedule() to true if any cpu
> > > > covered by the current schedule() call has any pending tasklet work.
> > > >
> > > > For joining other vcpus of the schedule unit we need to add a new
> > > > softirq SCHED_SLAVE_SOFTIRQ in order to have a way to initiate a
> > > > context switch without calling the generic schedule() function
> > > > selecting the vcpu to switch to, as we already know which vcpu we
> > > > want to run. This has the other advantage not to loose any other
> > > > concurrent SCHEDULE_SOFTIRQ events.
> > > >
> > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > > Reviewed-by: Dario Faggioli <dfaggioli@xxxxxxxx>
> > > x86 and applicable common code parts
> > > Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> > >
> > > However, ...
> > >
> > > > +static void sched_context_switch(struct vcpu *vprev, struct vcpu *vnext,
> > > > +                                 s_time_t now)
> > > > +{
> > > > +    if ( unlikely(vprev == vnext) )
> > > >       {
> > > > -        pcpu_schedule_unlock_irq(lock, cpu);
> > > >           TRACE_4D(TRC_SCHED_SWITCH_INFCONT,
> > > > -                 next->domain->domain_id, next->unit_id,
> > > > -                 now - prev->state_entry_time,
> > > > -                 prev->next_time);
> > > > -        trace_continue_running(next->vcpu_list);
> > > > -        return continue_running(prev->vcpu_list);
> > > > +                 vnext->domain->domain_id, vnext->sched_unit->unit_id,
> > > > +                 now - vprev->runstate.state_entry_time,
> > > > +                 vprev->sched_unit->next_time);
> > > > +        sched_context_switched(vprev, vnext);
> > > > +        trace_continue_running(vnext);
> > > > +        return continue_running(vprev);
> > > >       }
> > > ... I don't recall if there weren't compiler (clang?) versions not
> > > allowing (or at least warning about) use of this extension.
> > Which extension?
> "return" with an expression of "void" type.
It was there in the original code, too:

http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/common/schedule.c;h=fd587622f4c3ee13d57334f90b1eab4b17031c0b;hb=refs/heads/staging-4.12#l1536


Juergen


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