Re: [PATCH v4] sched: fix init NOHZ_IDLE flag

[Vincent Guittot]

On 22 February 2013 13:32, Frederic Weisbecker <fweisbec@xxxxxxxxx> 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.guittot@xxxxxxxxxx>
>> ---
>>  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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