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SubjectRe: CPU affinity & IPI latency

On 16-Jul-2001 Mike Kravetz wrote:
> On Fri, Jul 13, 2001 at 11:25:21PM -0400, Hubertus Franke wrote:
>>
>> Mike, could we utilize the existing mechanism such as has_cpu.
>>
>
> I like it. Especially the way you eliminated the situation where
> we would have multiple tasks waiting for schedule. Hope this is
> not a frequent situation!!! The only thing I don't like is the
> use of has_cpu to prevent the task from being scheduled. Right
> now, I can't think of any problems with it. However, in the past
> I have been bit by using fields for purposes other than what they
> were designed for.

How about this ( draft ) :


struct task_struct {
...
struct task_struct * wlist_next;
...
};


static union {
struct schedule_data {
struct task_struct * curr;
struct task_struct * wlist;
cycles_t last_schedule;
} schedule_data;
char __pad [SMP_CACHE_BYTES];
} aligned_data [NR_CPUS] __cacheline_aligned = { {{&init_task,0}}};


static inline struct task_struct * wpick(aligned_data * ad) {
struct task_struct * tsk = ad->wlist;
if (tsk) {
ad->wlist = tsk->wlist_next;
add_to_runqueue(tsk);
}
return tsk;
}

static inline void wpush(aligned_data * ad, struct task_struct * tsk) {
if (task_on_runqueue(tsk))
list_del(&tsk->run_list);
tsk->wlist_next = ad->wlist;
ad->wlist = tsk;
}


asmlinkage void schedule(void)
{
...
if ((next = wpick(sched_data)))
goto ...;
...
}

In reschedule_idle() when before sending the IPI we do a wpush().
We modify aligned_data->wlist and tsk->wlist_next under runqueue_lock so we
don't need another one.
A slight change is needed to reschedule_idle() to handle the new field.
Pros to this solution are 1) we are not going to give other fields a different
meaning 2) when the idle will call schedule it'll pick the task w/o rescan.




- Davide

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