Re: SCHED_BATCH and SCHED_BATCH numbering

[Peter Williams <pwil3058@bigpond.net.au>]

Nick Piggin wrote:
> Peter Williams wrote:
> 
>> Nick Piggin wrote:
>>
>>> Peter Williams wrote:
>>>
>>>> Albert Cahalan wrote:
>>>>
>>>>> Are these going to be numbered consecutively, or might
>>>>> they better be done like the task state? SCHED_FIFO is
>>>>> in fact already treated this way in one place. One might
>>>>> want to test values this way:
>>>>>
>>>>> if(foo & (SCHED_ISO|SCHED_RR|SCHED_FIFO))  ...
>>>>>
>>>>> (leaving aside SCHED_OTHER==0, or just translate
>>>>> that single value for the ABI)
>>>>>
>>>>> I'd like to see these get permenant allocations
>>>>> soon, even if the code doesn't go into the kernel.
>>>>> This is because user-space needs to know the values.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Excellent idea.  The definition of rt_task() could become:
>>>>
>>>> #define rt_task(p) ((p)->policy & (SCHED_RR|SCHED_FIFO))
>>>>
>>>> instead of the highly dodgy:
>>>
>>>
>>
>> I probably should have said "slightly" instead of "highly" here but I 
>> got carried away. :-)
>>
>>>>
>>>> #define rt_task(p) ((p)->prio < MAX_RT_PRIO)
>>>>
>>>
>>> Nothing wrong with that, is there?
>>
>>
>>
>> It's sloppy logic in that "prio" being less than MAX_RT_PRIO is a 
>> consequence of the task being real time not the definition of it.  At 
>> the moment it is a sufficient condition for identifying a task as real 
>> time but that may not always be the case.
> 
> 
> Actually, p->prio < MAX_RT_PRIO iff rt_task(p). This can't change 
> without horribly breaking
> stuff.
> 
>> But, the real issue is, what's the point of having a field, "policy", 
>> that IS the definitive indicator of the task's scheduling policy if 
>> you don't use it?  An rt_task() function/macro defined in terms of the 
>> policy field with this suggested numbering scheme should always be 
>> correct.
>>
>> At the moment rt_task(p) could be defined as ((p)->policy != 
>> SCHED_OTHER) but the addition of SCHED_ISO and SCHED_BATCH would break 
>> that.  Another option would be (((p)->policy == SCHED_FIFO) || 
>> ((p)->policy == SCHED_RR)) but that's a little long winded and 
>> (avoiding it) is probably the reason for the current definition. 
> 
> 
> 
> Conversely, p->prio < MAX_RT_PRIO neatly defines a task as being 
> realtime without worrying
> about what exact policy it is using. However if you add or remove 
> scheduling policies, your
> p->policy method breaks.

Not if Albert's numbering system is used.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce