Re: [PATCH v8 4/8] smp: add func to IPI cpus based on parameter func

[Srivatsa S. Bhat]

On 02/05/2012 09:30 PM, Srivatsa S. Bhat wrote:

> On 02/05/2012 09:16 PM, Gilad Ben-Yossef wrote:
> 
>> On Sun, Feb 5, 2012 at 5:36 PM, Srivatsa S. Bhat
>> <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
>>> On 02/05/2012 07:18 PM, Gilad Ben-Yossef wrote:
>>>
>>>> Add the on_each_cpu_cond() function that wraps on_each_cpu_mask()
>>>> and calculates the cpumask of cpus to IPI by calling a function supplied
>>>> as a parameter in order to determine whether to IPI each specific cpu.
>>>>
>>>> The function works around allocation failure of cpumask variable in
>>>> CONFIG_CPUMASK_OFFSTACK=y by itereating over cpus sending an IPI a
>>>> time via smp_call_function_single().
>>>>
>>>> The function is useful since it allows to seperate the specific
>>>> code that decided in each case whether to IPI a specific cpu for
>>>> a specific request from the common boilerplate code of handling
>>>> creating the mask, handling failures etc.
>>>>
>>>> Signed-off-by: Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx>
>>> ...
>>>> diff --git a/include/linux/smp.h b/include/linux/smp.h
>>>> index d0adb78..da4d034 100644
>>>> --- a/include/linux/smp.h
>>>> +++ b/include/linux/smp.h
>>>> @@ -109,6 +109,15 @@ void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
>>>>               void *info, bool wait);
>>>>
>>>>  /*
>>>> + * Call a function on each processor for which the supplied function
>>>> + * cond_func returns a positive value. This may include the local
>>>> + * processor.
>>>> + */
>>>> +void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info),
>>>> +             smp_call_func_t func, void *info, bool wait,
>>>> +             gfp_t gfp_flags);
>>>> +
>>>> +/*
>>>>   * Mark the boot cpu "online" so that it can call console drivers in
>>>>   * printk() and can access its per-cpu storage.
>>>>   */
>>>> @@ -153,6 +162,21 @@ static inline int up_smp_call_function(smp_call_func_t func, void *info)
>>>>                       local_irq_enable();             \
>>>>               }                                       \
>>>>       } while (0)
>>>> +/*
>>>> + * Preemption is disabled here to make sure the
>>>> + * cond_func is called under the same condtions in UP
>>>> + * and SMP.
>>>> + */
>>>> +#define on_each_cpu_cond(cond_func, func, info, wait, gfp_flags) \
>>>> +     do {                                            \
>>>> +             preempt_disable();                      \
>>>> +             if (cond_func(0, info)) {               \
>>>> +                     local_irq_disable();            \
>>>> +                     (func)(info);                   \
>>>> +                     local_irq_enable();             \
>>>> +             }                                       \
>>>> +             preempt_enable();                       \
>>>> +     } while (0)
>>>>
>>>>  static inline void smp_send_reschedule(int cpu) { }
>>>>  #define num_booting_cpus()                   1
>>>> diff --git a/kernel/smp.c b/kernel/smp.c
>>>> index a081e6c..28cbcc5 100644
>>>> --- a/kernel/smp.c
>>>> +++ b/kernel/smp.c
>>>> @@ -730,3 +730,63 @@ void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
>>>>       put_cpu();
>>>>  }
>>>>  EXPORT_SYMBOL(on_each_cpu_mask);
>>>> +
>>>> +/*
>>>> + * on_each_cpu_cond(): Call a function on each processor for which
>>>> + * the supplied function cond_func returns true, optionally waiting
>>>> + * for all the required CPUs to finish. This may include the local
>>>> + * processor.
>>>> + * @cond_func:       A callback function that is passed a cpu id and
>>>> + *           the the info parameter. The function is called
>>>> + *           with preemption disabled. The function should
>>>> + *           return a blooean value indicating whether to IPI
>>>> + *           the specified CPU.
>>>> + * @func:    The function to run on all applicable CPUs.
>>>> + *           This must be fast and non-blocking.
>>>> + * @info:    An arbitrary pointer to pass to both functions.
>>>> + * @wait:    If true, wait (atomically) until function has
>>>> + *           completed on other CPUs.
>>>> + * @gfp_flags:       GFP flags to use when allocating the cpumask
>>>> + *           used internally by the function.
>>>> + *
>>>> + * The function might sleep if the GFP flags indicates a non
>>>> + * atomic allocation is allowed.
>>>> + *
>>>> + * Preemption is disabled to protect against a hotplug event.
>>>
>>>
>>> Well, disabling preemption protects us only against CPU offline right?
>>> (because we use the stop_machine thing during cpu offline).
>>>
>>> What about CPU online?
>>>
>>> Just to cross-check my understanding of the code with the existing
>>> documentation on CPU hotplug, I looked up Documentation/cpu-hotplug.txt
>>> and this is what I found:
>>>
>>> "If you merely need to avoid cpus going away, you could also use
>>> preempt_disable() and preempt_enable() for those sections....
>>> ...The preempt_disable() will work as long as stop_machine_run() is used
>>> to take a cpu down."
>>>
>>> So even this only talks about using preempt_disable() to prevent CPU offline,
>>> not CPU online. Or, am I missing something?
>>
>> You are not missing anything, this is simply a bad choice of words on my part.
>> Thank you for pointing this out.
>>
>> I should write:
>>
>> " Preemption is disabled to protect against CPU going offline but not online.
>>   CPUs going online during the call will not be seen or sent an IPI."
>>
> 
> 
> Yeah, that sounds better.
> 
>> Protecting against CPU going online during the function is useless
>> since they might
>> as well go online right after the call is finished, so the caller has
>> to take care of it, if they
>> cares.
>>
> 
> 
> Ah, makes sense, thanks!
> 


Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>

Regards,
Srivatsa S. Bhat

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