On 24/05/18 10:04, Patrick Bellasi wrote: [...] > From 84bb8137ce79f74849d97e30871cf67d06d8d682 Mon Sep 17 00:00:00 2001 > From: Patrick Bellasi <patrick.bellasi@xxxxxxx> > Date: Wed, 23 May 2018 16:33:06 +0100 > Subject: [PATCH 1/1] cgroup/cpuset: disable sched domain rebuild when not > required > > The generate_sched_domains() already addresses the "special case for 99% > of systems" which require a single full sched domain at the root, > spanning all the CPUs. However, the current support is based on an > expensive sequence of operations which destroy and recreate the exact > same scheduling domain configuration. > > If we notice that: > > 1) CPUs in "cpuset.isolcpus" are excluded from load balancing by the > isolcpus= kernel boot option, and will never be load balanced > regardless of the value of "cpuset.sched_load_balance" in any > cpuset. > > 2) the root cpuset has load_balance enabled by default at boot and > it's the only parameter which userspace can change at run-time. > > we know that, by default, every system comes up with a complete and > properly configured set of scheduling domains covering all the CPUs. > > Thus, on every system, unless the user explicitly disables load balance > for the top_cpuset, the scheduling domains already configured at boot > time by the scheduler/topology code and updated in consequence of > hotplug events, are already properly configured for cpuset too. > > This configuration is the default one for 99% of the systems, > and it's also the one used by most of the Android devices which never > disable load balance from the top_cpuset. > > Thus, while load balance is enabled for the top_cpuset, > destroying/rebuilding the scheduling domains at every cpuset.cpus > reconfiguration is a useless operation which will always produce the > same result. > > Let's anticipate the "special" optimization within: > > rebuild_sched_domains_locked() > > thus completely skipping the expensive: > > generate_sched_domains() > partition_sched_domains() > > for all the cases we know that the scheduling domains already defined > will not be affected by whatsoever value of cpuset.cpus. [...] > + /* Special case for the 99% of systems with one, full, sched domain */ > + if (!top_cpuset.isolation_count && > + is_sched_load_balance(&top_cpuset)) > + goto out; > + Mmm, looks like we still need to destroy e recreate if there is a new_topology (see arch_update_cpu_topology() in partition_sched_ domains). Maybe we could move the check you are proposing in update_cpumasks_ hier() ? -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
