Re: [PATCH 13/15] sched,fair: propagate sum_exec_runtime up the hierarchy

[Rik van Riel <riel@surriel.com>]

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On Thu, 2019-08-29 at 19:20 +0200, Dietmar Eggemann wrote:
> On 28/08/2019 15:14, Rik van Riel wrote:
> > On Wed, 2019-08-28 at 09:51 +0200, Dietmar Eggemann wrote:
> > > On 22/08/2019 04:17, Rik van Riel wrote:
> > > > Now that enqueue_task_fair and dequeue_task_fair no longer
> > > > iterate
> > > > up
> > > > the hierarchy all the time, a method to lazily propagate
> > > > sum_exec_runtime
> > > > up the hierarchy is necessary.
> > > > 
> > > > Once a tick, propagate the newly accumulated exec_runtime up
> > > > the
> > > > hierarchy,
> > > > and feed it into CFS bandwidth control.
> > > > 
> > > > Remove the pointless call to account_cfs_rq_runtime from
> > > > update_curr,
> > > > which is always called with a root cfs_rq.
> > > 
> > > But what about the call to account_cfs_rq_runtime() in
> > > set_curr_task_fair()? Here you always call it with the root
> > > cfs_rq.
> > > Shouldn't this be called also in a loop over all se's until !se-
> > > > parent
> > > (like in propagate_exec_runtime() further below).
> > 
> > I believe that call should be only on the cgroup
> > cfs_rq, with account_cfs_rq_runtime figuring out
> > whether more runtime needs to be obtained from
> > further up in the hierarchy.
> 
> So like this?
> 
> @@ -10248,7 +10248,8 @@ static void set_curr_task_fair(struct rq *rq)
> 
>         set_next_entity(cfs_rq, se);
>         /* ensure bandwidth has been allocated on our new cfs_rq */
> -       account_cfs_rq_runtime(cfs_rq, 0);
> +       if (task_se_in_cgroup(se))
> +               account_cfs_rq_runtime(group_cfs_rq_of_parent(se),
> 0);
>  }
> 
> I fail to understand the second part of your sentence, and
> how is this related to the code in propagate_exec_runtime():
> 
> for_each_sched_entity(se) {
> 
>     propagate_exec_runtime() {
> 
>         if (parent)
>             account_cfs_rq_runtime(cfs_rq, diff);
>     }
> }

I am not sure how that would work for distributing
runtime, since runtime would have to be distributed
downwards and on demand, no?

That seems like a very different code path than
"upwards, and periodically".

Then again, I have not worked out all the details
of reimplementing CFS bandwidth yet...

> > By default we should probably work under the assumption
> > that account_cfs_rq_runtime() will succeed at the current
> > level, and no gymnastics are required to obtain CPU time.
> 
> Maybe this all will become clearer when the reworked CFS Bandwidth
> support is ready ;-) I see this patch as the first part of it.

That is one of the reasons I have not been "fixing"
CFS bandwidth related code in the current patch series.

Having all of those changes in one location seems like
it would be best.

-- 
All Rights Reversed.

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