Re: [PATCH v2] sched/fair: Preserve PLACE_DEADLINE_INITIAL deadline

[Daniel Jordan]

Hi Chenyu,

On Wed, Oct 04, 2023 at 11:46:21PM +0800, Chen Yu wrote:
> Hi Daniel,
> 
> On 2023-10-04 at 09:09:08 -0400, Daniel Jordan wrote:
> > An entity is supposed to get an earlier deadline with
> > PLACE_DEADLINE_INITIAL when it's forked, but the deadline gets
> > overwritten soon after in enqueue_entity() the first time a forked
> > entity is woken so that PLACE_DEADLINE_INITIAL is effectively a no-op.
> > 
> > Placing in task_fork_fair() seems unnecessary since none of the values
> > that get set (slice, vruntime, deadline) are used before they're set
> > again at enqueue time, so get rid of that (and with it all of
> > task_fork_fair()) and just pass ENQUEUE_INITIAL to enqueue_entity() via
> > wake_up_new_task().
> > 
> > Fixes: e8f331bcc270 ("sched/smp: Use lag to simplify cross-runqueue placement")
> > Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> > ---
> > 
> > v2
> >  - place_entity() seems like the only reason for task_fork_fair() to exist
> >    after the recent removal of sysctl_sched_child_runs_first, so take out
> >    the whole function.
> 
> At first glance I thought if we remove task_fork_fair(), do we lose one chance to
> update the parent task's statistic in update_curr()?  We might get out-of-date
> parent task's deadline and make preemption decision based on the stale data in
> wake_up_new_task() -> wakeup_preempt() -> pick_eevdf(). But after a second thought,
> I found that wake_up_new_task() -> enqueue_entity() itself would invoke update_curr(),
> so this should not be a problem.
>
> Then I was wondering why can't we just skip place_entity() in enqueue_entity()
> if ENQUEUE_WAKEUP is not set, just like the code before e8f331bcc270? In this
> way the new fork task's deadline will not be overwritten by wake_up_new_task()->
> enqueue_entity(). Then I realized that, after e8f331bcc270, the task's vruntime
> and deadline are all calculated by place_entity() rather than being renormalised
> to cfs_rq->min_vruntime in enqueue_entity(), so we can not simply skip place_entity()
> in enqueue_entity().

This all made me wonder if the order of update_curr() for the parent and
place_entity() for the child matters.  And it does, since placing uses
avg_vruntime(), which wants an up-to-date vruntime for current and
min_vruntime for cfs_rq.  Good that 'curr' in enqueue_entity() is false
on fork so that the parent's vruntime is up to date, but it seems
placing should always happen after update_curr().

> Per my understanding, this patch looks good,
> 
> Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>

Thanks!

Daniel