On Mon, Jul 08, 2024 at 02:47:34PM +0200, Vincent Guittot wrote:
On Mon, 8 Jul 2024 at 14:02, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
@@ -8409,6 +8400,15 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
if (cse_is_idle != pse_is_idle)
return;
+ /*
+ * Batch tasks do not preempt non-idle tasks (their preemption
+ * is driven by the tick).
+ * We've done the check about "only one of the entities is idle",
+ * so cse must be non-idle if p is a batch task.
+ */
+ if (unlikely(entity_is_task(pse) && p->policy == SCHED_BATCH))
+ return;
I'm not convinced this condition is right. The current behaviour of
SCHED_BATCH doesn't care about pse, only p.
That is, if p is SCHED_BATCH it will not preempt -- except an
SCHED_IDLE.
So I'm tempted to delete this first part of your condition and have it
be:
if (p->policy == SCHED_BATCH)
return;
That is, suppose you have:
root
|
------------------------
| |
normal_cgroup normal_cgroup
| |
SCHED_BATCH task_A SCHED_BATCH task_B
Then the preemption crud will end up comparing the groups to one another
and still allow A to preempt B -- except we explicitly do not want this.
The 'problem' is that the whole BATCH thing isn't cgroup aware ofcourse,
but I'm not sure we want to go fix that -- esp. not in this patch.
Hmm?
Good question, but do we want to make SCHED_BATCH tasks behave
differently than SCHED_IDLE tasks in a group in this case ?
I suspect we'll have to. People added the idle-cgroup thing, but never
did the same for batch. With the result that they're now fundamentally
different.
With this patch, we don't want task_B to preempt task_A for the case
but task_A will preempt task_B whereas task A is SCHED_IDLE
root
|
------------------------
| |
normal_cgroup idle_cgroup
| |
SCHED_IDLE task_A SCHED_NORMAL task_B
As we only look at the common level of hierarchy between the tasks,
the below will make A to preempt B whereas both are SCHED_IDLE
root
|
------------------------
| |
normal_cgroup normal_cgroup
| |
SCHED_IDLE task_A SCHED_IDLE task_B
So we can make the last test be:
if (unlikely(p->policy != SCHED_NORMAL))
return;
Much like the original condition removed here:
- if (unlikely(p->policy != SCHED_NORMAL) || !sched_feat(WAKEUP_PREEMPTION))
+ if (!sched_feat(WAKEUP_PREEMPTION))
Except now after all that cgroup nonsense. Then the OP case will preempt
because normal_cgroup vs idle_cgroup, my BATCH example will not preempt,
because BATCH != NORMAL, your IDLE example will not preempt either,
because IDLE != NORMAL.