On Tue, Apr 23, 2013 at 04:59:02PM +0200, Vincent Guittot wrote: > On my smp platform which is made of 5 cores in 2 clusters, I have the > nr_busy_cpu field of sched_group_power struct that is not null when the > platform is fully idle. The root cause is: > During the boot sequence, some CPUs reach the idle loop and set their > NOHZ_IDLE flag while waiting for others CPUs to boot. But the nr_busy_cpus > field is initialized later with the assumption that all CPUs are in the busy > state whereas some CPUs have already set their NOHZ_IDLE flag. > > More generally, the NOHZ_IDLE flag must be initialized when new sched_domains > are created in order to ensure that NOHZ_IDLE and nr_busy_cpus are aligned. > > This condition can be ensured by adding a synchronize_rcu between the > destruction of old sched_domains and the creation of new ones so the NOHZ_IDLE > flag will not be updated with old sched_domain once it has been initialized. > But this solution introduces a additionnal latency in the rebuild sequence > that is called during cpu hotplug. > > As suggested by Frederic Weisbecker, another solution is to have the same > rcu lifecycle for both NOHZ_IDLE and sched_domain struct. > A new nohz_idle field is added to sched_domain so both status and sched_domain > will share the same RCU lifecycle and will be always synchronized. > In addition, there is no more need to protect nohz_idle against concurrent > access as it is only modified by 2 exclusive functions called by local cpu. > > This solution has been prefered to the creation of a new struct with an extra > pointer indirection for sched_domain.
> Signed-off-by: Vincent Guittot <vincent.guit...@linaro.org> Acked-by: Peter Zijlstra <a.p.zijls...@chello.nl> Thanks for persisting! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
