On 18 February 2013 16:40, Frederic Weisbecker <fweis...@gmail.com> wrote:
> 2013/2/18 Vincent Guittot <vincent.guit...@linaro.org>:
>> On 18 February 2013 15:38, Frederic Weisbecker <fweis...@gmail.com> wrote:
>>> I pasted the original at: http://pastebin.com/DMm5U8J8
>>
>> We can clear the idle flag only in the nohz_kick_needed which will not
>> be called if the sched_domain is NULL so the sequence will be
>>
>> = CPU 0 = = CPU 1=
>>
>> detach_and_destroy_domain {
>> rcu_assign_pointer(cpu1_dom, NULL);
>> }
>>
>> dom = new_domain(...) {
>> nr_cpus_busy = 0;
>> set_idle(CPU 1);
>> }
>> dom =
>> rcu_dereference(cpu1_dom)
>> //dom == NULL, return
>>
>> rcu_assign_pointer(cpu1_dom, dom);
>>
>> dom =
>> rcu_dereference(cpu1_dom)
>> //dom != NULL,
>> nohz_kick_needed {
>>
>> set_idle(CPU 1)
>> dom
>> = rcu_dereference(cpu1_dom)
>>
>> //dec nr_cpus_busy,
>> }
>>
>> Vincent
>
> Ok but CPU 0 can assign NULL to the domain of cpu1 while CPU 1 is
> already in the middle of nohz_kick_needed().
Yes nothing prevents the sequence below to occur
= CPU 0 = = CPU 1=
dom =
rcu_dereference(cpu1_dom)
//dom != NULL
detach_and_destroy_domain {
rcu_assign_pointer(cpu1_dom, NULL);
}
dom = new_domain(...) {
nr_cpus_busy = 0;
//nr_cpus_busy in the new_dom
set_idle(CPU 1);
}
nohz_kick_needed {
clear_idle(CPU 1)
dom =
rcu_dereference(cpu1_dom)
//cpu1_dom == old_dom
inc nr_cpus_busy,
//nr_cpus_busy in the old_dom
}
rcu_assign_pointer(cpu1_dom, dom);
//cpu1_dom == new_dom
I'm not sure that this can happen in practice because CPU1 is in
interrupt handler but we don't have any mechanism to prevent the
sequence.
The NULL sched_domain can be used to detect this situation and the
set_cpu_sd_state_busy function can be modified like below
inline void set_cpu_sd_state_busy
{
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) {
atomic_inc(&sd->groups->sgp->nr_busy_cpus);
+ clear = 1;
}
rcu_read_unlock();
+
+ if (likely(clear))
+ clear_bit(NOHZ_IDLE, nohz_flags(cpu));
}
The NOHZ_IDLE flag will not be clear if we have a NULL sched_domain
attached to the CPU.
With this implementation, we still don't need to get the sched_domain
for testing the NOHZ_IDLE flag which occurs each time CPU becomes idle
The patch 2 become useless
Vincent
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