On Thu, Feb 21, 2013 at 09:29:16AM +0100, 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 seems to be:
> 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.
> During the initialization of the sched_domain, we set the NOHZ_IDLE flag when
> nr_busy_cpus is initialized to 0 in order to have a coherent configuration.
> If a CPU enters idle and call set_cpu_sd_state_idle during the build of the
> new sched_domain it will not corrupt the initial state
> set_cpu_sd_state_busy is modified and clears the NOHZ_IDLE only if a non NULL
> sched_domain is attached to the CPU (which is the case during the rebuild)
>
> 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.guit...@linaro.org>
> ---
> kernel/sched/core.c | 4 +++-
> kernel/sched/fair.c | 9 +++++++--
> 2 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 26058d0..c730a4e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5884,7 +5884,9 @@ static void init_sched_groups_power(int cpu, struct
> sched_domain *sd)
> return;
>
> update_group_power(sd, cpu);
> - atomic_set(&sg->sgp->nr_busy_cpus, sg->group_weight);
> + atomic_set(&sg->sgp->nr_busy_cpus, 0);
> + set_bit(NOHZ_IDLE, nohz_flags(cpu));
> +
> }
>
> int __weak arch_sd_sibling_asym_packing(void)
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 81fa536..2701a92 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5403,15 +5403,20 @@ static inline void set_cpu_sd_state_busy(void)
> {
> struct sched_domain *sd;
> int cpu = smp_processor_id();
> + int clear = 0;
>
> if (!test_bit(NOHZ_IDLE, nohz_flags(cpu)))
> return;
> - clear_bit(NOHZ_IDLE, nohz_flags(cpu));
>
> rcu_read_lock();
> - for_each_domain(cpu, sd)
> + for_each_domain(cpu, sd) {
> atomic_inc(&sd->groups->sgp->nr_busy_cpus);
> + clear = 1;
> + }
> rcu_read_unlock();
> +
> + if (likely(clear))
> + clear_bit(NOHZ_IDLE, nohz_flags(cpu));
I fear there is still a race window:
= CPU 0 = = CPU 1 =
// NOHZ_IDLE is set
set_cpu_sd_state_busy() {
dom1 = rcu_dereference(dom1);
inc(dom1->nr_busy_cpus)
rcu_assign_pointer(dom 1, NULL)
// create new domain
init_sched_group_power() {
atomic_set(&tmp->nr_busy_cpus, 0);
set_bit(NOHZ_IDLE, nohz_flags(cpu 1));
rcu_assign_pointer(dom 1, tmp)
clear_bit(NOHZ_IDLE, nohz_flags(cpu));
}
I don't know if there is any sane way to deal with this issue other than
having nr_busy_cpus and nohz_flags in the same object sharing the same
lifecycle.
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