Re: [PATCH 0/7] sched/deadline: fix cpusets bandwidth accounting

[Luca Abeni]

Hi Mathieu,

On Thu, 12 Oct 2017 10:57:09 -0600
Mathieu Poirier <mathieu.poirier@xxxxxxxxxx> wrote:
[...]
> >> Regardless of how we proceed (using existing CPUset list or new ones) we
> >> need to deal with DL tasks that span more than one root domain,  something
> >> that will typically happen after a CPUset operation.  For example, if we
> >> split the number of available CPUs on a system in two CPUsets and then turn
> >> off the 'sched_load_balance' flag on the parent CPUset, DL tasks in the
> >> parent CPUset will end up spanning two root domains.
> >>
> >> One way to deal with this is to prevent CPUset operations from happening
> >> when such condition is detected, as enacted in this set.  Although simple
> >> this approach feels brittle and akin to a "whack-a-mole" game.  A better
> >> and more reliable approach would be to teach the DL scheduler to deal with
> >> tasks that span multiple root domains, a serious and substantial
> >> undertaking.
> >>
> >> I am sending this as a starting point for discussion.  I would be grateful
> >> if you could take the time to comment on the approach and most importantly
> >> provide input on how to deal with the open issue underlined above.  
> >
> > Right, so teaching DEADLINE about arbitrary affinities is 'interesting'.
> >
> > Although the rules proposed by Tomasso; if found sufficient; would
> > greatly simplify things. Also the online semi-partition approach to SMP
> > could help with that.  
> 
> The "rules" proposed by Tomasso, are you referring to patches or the
> deadline/cgroup extension work that he presented at OSPM?

No, that is an unrelated thing... Tommaso previously proposed some
improvements to the admission control mechanism to take arbitrary
affinities into account.


I think Tommaso's proposal is similar to what I previously proposed in
this thread (to admit a SCHED_DEADLINE task with utilization
u = runtime / period and affinity to N runqueues, we can account u / N
to each one of the runqueues, and check if the sum of the utilizations
on each runqueue is < 1).

As previously noticed by Peter, this might have some scalability issues
(a naive implementation would lock the root domain while iterating on
all the runqueues). Few days ago, I was discussing with Tommaso about a
possible solution based on not locking the root domain structure, and
eventually using a roll-back strategy if the status of the root domain
changes while we are updating it. I think in a previous email you
mentioned RCU, which might result in a similar solution.

Anyway, I am adding Tommaso in cc so that he can comment more.


> I'd also be
> interested to know more about this "online semi-partition approach to
> SMP" you mentioned.

It is basically an implementation (and extension to arbitrary
affinities) of this work:
http://drops.dagstuhl.de/opus/volltexte/2017/7165/


				Luca

> Maybe that's a conversation we could have at the
> upcoming RT summit in Prague.
> 
> >
> > But yes, that's fairly massive surgery. For now I think we'll have to
> > live and accept the limitations. So failing the various cpuset
> > operations when they violate rules seems fine. Relaxing rules is always
> > easier than tightening them (later).  
> 
> Agreed.
> 
> >
> > One 'series' you might be interested in when respinning these is:
> >
> >   https://lkml.kernel.org/r/20171011094833.pdp4torvotvjdmkt@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > By doing synchronous domain rebuild we loose a bunch of funnies.  
> 
> Getting rid of the asynchronous nature of the hotplug path would be a
> delight - I'll start keeping track of that effort as well.
> 
> Thanks for the review,
> Mathieu