Re: SCHED_BATCH and SCHED_BATCH numbering

[Nick Piggin <nickpiggin@yahoo.com.au>]

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.