On 07/02/2013 02:17 PM, Michael Wang wrote:
> On 07/02/2013 04:25 PM, Srivatsa S. Bhat wrote:
>> Hi Michael,
>>
>> On 07/02/2013 11:02 AM, Michael Wang wrote:
>>> Hi, Srivatsa
>>>
>>> On 06/28/2013 03:54 AM, Srivatsa S. Bhat wrote:
>>> [snip]
>>>> @@ -625,8 +632,9 @@ EXPORT_SYMBOL(on_each_cpu_mask);
>>>> * The function might sleep if the GFP flags indicates a non
>>>> * atomic allocation is allowed.
>>>> *
>>>> - * Preemption is disabled to protect against CPUs going offline but not
>>>> online.
>>>> - * CPUs going online during the call will not be seen or sent an IPI.
>>>> + * We use get/put_online_cpus_atomic() to protect against CPUs going
>>>> + * offline but not online. CPUs going online during the call will
>>>> + * not be seen or sent an IPI.
>>>
>>> I was a little confused about this comment, if the offline-cpu still
>>> have chances to become online, then there is chances that we will pick
>>> it from for_each_online_cpu(), isn't it? Did I miss some point?
>>>
>>
>> Whether or not the newly onlined CPU is observed in our for_each_online_cpu()
>> loop, is dependent on timing. On top of that, there are 2 paths in the code:
>> one which uses a temporary cpumask and the other which doesn't. In the former
>> case, if a CPU comes online _after_ we populate the temporary cpumask, then
>> we won't send an IPI to that cpu, since the temporary cpumask doesn't contain
>> that CPU. Whereas, if we observe the newly onlined CPU in the
>> for_each_online_cpu()
>> loop itself (either in the former or the latter case), then yes, we will send
>> the IPI to that CPU.
>
> So it is not 'during the call' but 'during the call of
> on_each_cpu_mask()', correct?
>
Well, as I said, its timing dependent. We might miss the newly onlined CPU in
the for_each_online_cpu() loop itself, based on when exactly the CPU was added
to the cpu_online_mask. So you can't exactly pin-point the places where you'll
miss the CPU and where you won't. Besides, is it _that_ important? It is after
all unpredictable..
> The comment position seems like it declaim that during the call of this
> func, online-cpu won't be seem and send IPI...
>
Doesn't matter, AFAICS. The key take-away from that whole comment is: nothing is
done to prevent CPUs from coming online while the function is running, whereas
the online CPUs are guaranteed to remain online throughout the function. In
other
words, its a weaker form of get_online_cpus()/put_online_cpus(), providing a
one-way synchronization (CPU offline).
As long as that idea is conveyed properly, the purpose of that comment is
served,
IMHO.
Regards,
Srivatsa S. Bhat
>>>> *
>>>> * You must not call this function with disabled interrupts or
>>>> * from a hardware interrupt handler or from a bottom half handler.
>>>> @@ -641,26 +649,26 @@ void on_each_cpu_cond(bool (*cond_func)(int cpu,
>>>> void *info),
>>>> might_sleep_if(gfp_flags & __GFP_WAIT);
>>>>
>>>> if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) {
>>>> - preempt_disable();
>>>> + get_online_cpus_atomic();
>>>> for_each_online_cpu(cpu)
>>>> if (cond_func(cpu, info))
>>>> cpumask_set_cpu(cpu, cpus);
>>>> on_each_cpu_mask(cpus, func, info, wait);
>>>> - preempt_enable();
>>>> + put_online_cpus_atomic();
>>>> free_cpumask_var(cpus);
>>>> } else {
>>>> /*
>>>> * No free cpumask, bother. No matter, we'll
>>>> * just have to IPI them one by one.
>>>> */
>>>> - preempt_disable();
>>>> + get_online_cpus_atomic();
>>>> for_each_online_cpu(cpu)
>>>> if (cond_func(cpu, info)) {
>>>> ret = smp_call_function_single(cpu, func,
>>>> info, wait);
>>>> WARN_ON_ONCE(!ret);
>>>> }
>>>> - preempt_enable();
>>>> + put_online_cpus_atomic();
>>>> }
>>>> }
>>>> EXPORT_SYMBOL(on_each_cpu_cond);
>>>>
>>>
>>
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