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