Hi Mathieu, On Thu, 12 Oct 2017 10:57:09 -0600 Mathieu Poirier <[email protected]> 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/[email protected] > > > > 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
