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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

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