Messages in this thread | | | Date | Tue, 1 Jul 2008 12:46:20 +0200 | From | "Dmitry Adamushko" <> | Subject | Re: [sched-devel, patch-rfc] rework of "prioritize non-migratable tasks over migratable ones" |
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2008/6/16 Peter Zijlstra <a.p.zijlstra@chello.nl>: > On Mon, 2008-06-16 at 19:59 +0200, Dmitry Adamushko wrote: > >> One way or another, we have different aritifacts (and mine have likely >> more) but conceptually, both "violates" POSIX if a strict round-robin >> scheduling is required. > > http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_08.html#tag_02_08_04_01 > > Is quite strict on what FIFO should do, and I know of two points where > we deviate and should work to match. >
btw., rt group scheduling seems to well, slightly wreck this (per-rq) FIFO model as well.
say, group_A has N SCHED_FIFO tasks of equal prio. So far so good, they all run strictly one after another.
Now group_B gets task_S. On a group layer, group_B gets enqueued after group_A.
This changes when a current task (that belongs to group_A) relinquishes a CPU: dequeue_stack -> __enqueue_rt_entity() will place group_A in the tail of its list.
So the next task to run is task_S, although group_A migth have plenty of tasks of the same prio that were enqueued ealrier.
We can't get a strict FIFO ordering with this pure tree-like hierarchy.
btw #2,
Gregory, our new modification also doesn't work nicely with group-scheduling.
We may place a task in the head of its queue, yes. But its group will still remain where it was.
rt_se->nr_cpus_allowed just has no adequat sense for groups and __enqueue_rt_entity() always places a group at the tail.
IOW, even if check_preempt_curr_rt() calls resched_task() based on analysis of the newly arrived task 'p', 'p' won't be necessarily picked up by pick_next_task_rt(). Although, there is a way to fix it.
-- Best regards, Dmitry Adamushko
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