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

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

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@benyossef.com>
...
> 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 must not call this function with disabled interrupts or
> + * from a hardware interrupt handler or from a bottom half handler.
> + */
> +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)
> +{
> +	cpumask_var_t cpus;
> +	int cpu, ret;
> +
> +	might_sleep_if(gfp_flags & __GFP_WAIT);
> +
> +	if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) {
> +		preempt_disable();
> +		for_each_online_cpu(cpu)
> +			if (cond_func(cpu, info))
> +				cpumask_set_cpu(cpu, cpus);


IOW, what prevents a new CPU from becoming online at this point?

> +		on_each_cpu_mask(cpus, func, info, wait);
> +		preempt_enable();
> +		free_cpumask_var(cpus);
> +	} else {
> +		/*
> +		 * No free cpumask, bother. No matter, we'll
> +		 * just have to IPI them one by one.
> +		 */
> +		preempt_disable();
> +		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();
> +	}
> +}
> +EXPORT_SYMBOL(on_each_cpu_cond);



Regards,
Srivatsa S. Bhat

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