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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

>>> --- 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".

Jan

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