Messages in this thread | | | Date | Mon, 30 Jun 2008 13:47:21 -0700 | From | Max Krasnyansky <> | Subject | Re: [PATCH 2/2] S390 topology: don't use kthread() for arch_reinit_sched_domains() |
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Paul Jackson wrote: > Max wrote: >> When a CPU goes off line overall partitioning does not change we just >> need to update current domains and remove the CPU that is no longer >> available. > > I don't believe that this is correct. > > If one had say just the following three cpusets in a system: > > /dev/cpuset # sched_load_balance == 0 > /dev/cpuset/alpha # sched_load_balance == 1 > /dev/cpuset/beta # sched_load_balance == 1 > > where the 'cpus' of alpha and beta overlapped by a single CPU, > and if one then took that single CPU offline, then the overall > partitioning of the system would change, from a single sched > domain covering the combined 'cpus' of alpha and beta, to two > separate sched domains, handling the 'cpus' of alpha and beta > separately. > >> When a CPU goes online it always ends up in the root cpuset, >> which means it can be added to the first load-balanced sched domain. > > Also not correct, but at least in this case, one might be able > to avoid doing a full fledged 'rebuild_sched_domains()' call, > by the following reasoning. > > When bringing CPUs online, either the top cpuset has > sched_load_balance set (1) or off (0). If it is set, then one > has a single sched domain covering all online CPUs, and yes one > could just add the new CPU to that sched domain. If off, then > the newly online CPU would only be in the top cpuset, which does > not by itself put that CPU in any sched domain, and that new CPU > should not be added to -any- cpuset. > > However, since we have to handle the offline case as well as the online > case, and since that case requires (to the best of my understanding) > calling rebuild_sched_domains(), I think it is best to just call that > routine in all cases. > > An earlier version of this sched domain code always attempted to > incrementally adjust sched domains to online and offline events, > and that code ended up being a maintenance nightmare. I will be most > reluctant to attempt to go back to such calculations.
Makes perfect sense. Thanx for the explanation. You've just saved me a bunch of time :). I'll give up on that idea then and instead will go and play with Paul M.'s latest patch.
btw We should update s390 to not call arch_init_sched_domains() directly once we know that rebuild_sched_domains() is safe to call from most contexts.
Max
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