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    SubjectRe: [PATCH v8 4/8] smp: add func to IPI cpus based on parameter func
    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@linux.vnet.ibm.com> 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@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 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!

    Regards,

    Srivatsa S. Bhat



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