Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks

[Yuyang Du]

On Tue, Jun 21, 2016 at 10:41:19AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 20, 2016 at 03:49:34PM +0100, Dietmar Eggemann wrote:
> > On 20/06/16 13:35, Vincent Guittot wrote:
> 
> > > It will go through wake_up_new_task and post_init_entity_util_avg
> > > during its fork which is enough to set last_update_time. Then, it will
> > > use the switched_to_fair if the task becomes a fair one
> > 
> > Oh I see. We want to make sure that every task (even when forked as
> > !fair) has a last_update_time value != 0, when becoming fair one day.
> 
> Right, see 2 below. I need to write a bunch of comments explaining PELT
> proper, as well as document these things.
> 
> The things we ran into with these patches were that:
> 
>  1) You need to update the cfs_rq _before_ any entity attach/detach
>     (and might need to update_tg_load_avg when update_cfs_rq_load_avg()
>     returns true).

This is intrinsically an additional update, not a fix to anything. I
don't think it is a must, but I am fine with it.
 
>  2) (fair) entities are always attached, switched_from/to deal with !fair.
 
Yes, me too.

>  3) cpu migration is the only exception and uses the last_update_time=0
>     thing -- because refusal to take second rq->lock.

Task's last_update_time means this task is detached from fair queue. This
(re)definition is by all means much better than migrating. No?

> Which is why I dislike Yuyang's patches, they create more exceptions
> instead of applying existing rules (albeit undocumented).
> 
> Esp. 1 is important, because while for mathematically consistency you
> don't actually need to do this, you only need the entities to be
> up-to-date with the cfs rq when you attach/detach, but that forgets the
> temporal aspect of _when_ you do this.
 
Yes, temporally at any instant the avgs are outdated. But, I can have it,
and what if I have it?

I am thinking about document this really well, like "An art of load tracking:
accuracy, overhead, and usefulness", seriously.