linux-kernel - Re: [RFC v3 2/6] Improve the tracking of active utilisation

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Date:   Thu, 10 Nov 2016 13:45:19 +0100
From:   luca abeni <luca.abeni@...tn.it>
To:     Juri Lelli <juri.lelli@....com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC v3 2/6] Improve the tracking of active utilisation

On Thu, 10 Nov 2016 12:34:15 +0000
Juri Lelli <juri.lelli@....com> wrote:
[...]
> > Ok; I'll think about some possible solution for this race... If I do
> > not find any simple way to solve it, I'll add a "contending" flag,
> > which allows to know if the inactive timer handler already executed
> > or not.
> >   
> 
> Right, this might help.
> 
> Another thing that I was thinking of is whether we can use the return
> value of hrtimer_try_to_cancel() to decide what to do:
> 
>  - if it returns 0 it means that the callback exectuted or the timer
> was never set, so nothing to do (as in nothing to sub_running_bw from)
>  - if it returns 1 we succedeed, so we need to actively sub_running_bw
>  - if -1 we can assume that it will eventually do sub_running_bw() so
> we don't need to care explicitly
This is about what I did in the initial version of the patch, but I
found some problems... I'll try to have another look at this approach.


> Now I guess the problem is that the task can be migrated while his
> inactive_timer is set (by select_task_rq_dl or by other classes load
> balacing if setscheduled to a different class). Can't we store a back
> reference to the rq from which the inactive_timer was queued and use
> that to sub_running_bw() from? It seems that we might end up with some
> "shadow" bandwidth, say when we do a wakeup migration, but maybe this
> is something we can tolerate? Just thinking aloud. :)
I'll think about this...


> > BTW, talking about sched_dl_entity flags: I see there are three
> > different int fields "dl_throttled, "dl_boosted" and "dl_yielded";
> > any reason for doing this instead of having a "dl_flags" field and
> > setting its different bits when the entity is throttled, boosted or
> > yielded? In other words: if I need this "contending" flag, should I
> > add a new "dl_contending" field?
> >   
> 
> I think you might want to add a clean-up patch to your series (or a
> separate one) fixing the current situation, and the build on to adding
> the new flag if needed.
Ok, can do this. I just wanted to know if there is a reason for having
different "dl_*" fields instead of a "flags" field (I do not know,
maybe performance?) or if it is just an historical accident and this
can be changed.



			Thanks,
				Luca