On 11 October 2017 at 10:02, Peter Zijlstra <[email protected]> wrote: > On Wed, Aug 16, 2017 at 03:20:36PM -0600, Mathieu Poirier wrote: > >> In this set the problem is addressed by relying on existing list of tasks >> (sleeping or not) already maintained by CPUsets. > > Right, that's a much saner approach :-)
Luca and Juri had the same opinion so let's continue with that solution. > >> OPEN ISSUE: >> >> 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? I'd also be interested to know more about this "online semi-partition approach to SMP" you mentioned. 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
