Messages in this thread | | | Date | Tue, 21 Jun 2016 12:51:39 +0800 | From | Yuyang Du <> | Subject | Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks |
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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Yes, temporally at any instant the avgs are outdated.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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