Messages in this thread | | | Date | Thu, 4 Apr 2013 16:24:23 +0200 | Subject | Re: [PATCH Resend v5] sched: fix init NOHZ_IDLE flag | From | Frederic Weisbecker <> |
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2013/4/3 Vincent Guittot <vincent.guittot@linaro.org>: > 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. I have introduce > a new sched_domain_rq struct that is the entry point for both sched_domains > and objects that must follow the same lifecycle like NOHZ_IDLE flags. They > will share the same RCU lifecycle and will be always synchronized. > > The synchronization is done at the cost of : > - an additional indirection for accessing the first sched_domain level > - an additional indirection and a rcu_dereference before accessing to the > NOHZ_IDLE flag. > > Change since v4: > - link both sched_domain and NOHZ_IDLE flag in one RCU object so > their states are always synchronized. > > Change since V3; > - NOHZ flag is not cleared if a NULL domain is attached to the CPU > - Remove patch 2/2 which becomes useless with latest modifications > > Change since V2: > - change the initialization to idle state instead of busy state so a CPU that > enters idle during the build of the sched_domain will not corrupt the > initialization state > > Change since V1: > - remove the patch for SCHED softirq on an idle core use case as it was > a side effect of the other use cases. > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > include/linux/sched.h | 6 +++ > kernel/sched/core.c | 105 ++++++++++++++++++++++++++++++++++++++++++++----- > kernel/sched/fair.c | 35 +++++++++++------ > kernel/sched/sched.h | 24 +++++++++-- > 4 files changed, 145 insertions(+), 25 deletions(-) > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index d35d2b6..2a52188 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -959,6 +959,12 @@ struct sched_domain { > unsigned long span[0]; > }; > > +struct sched_domain_rq { > + struct sched_domain *sd;
Why not make it part of the structure content instead of pointing to it?
> + unsigned long flags; > + struct rcu_head rcu; /* used during destruction */ > +};
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