On 22 February 2013 13:32, Frederic Weisbecker <fweis...@gmail.com> wrote:
> 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.
I wanted to avoid having to use the sd pointer for testing NOHZ_IDLE
flag because it occurs each time we go into idle but it seems to be
not easily feasible.
Another solution could be to add a synchronization step between
rcu_assign_pointer(dom 1, NULL) and create new domain to ensure that
all pending access to old sd values, has finished but this will imply
a potential delay in the rebuild of sched_domain and i'm not sure
that it's acceptable
Vincent
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