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    SubjectRe: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks
    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    Yes, temporally at any instant the avgs are outdated.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.

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