[Xen-devel] Re: [PATCH RFC 0/8] jump-label: allow early jump_label_enable()

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 09/30/2011 08:28 AM, Steven Rostedt wrote:
> On Thu, 2011-09-29 at 21:40 -0700, Jeremy Fitzhardinge wrote:
>> On 09/29/2011 05:52 PM, Steven Rostedt wrote:
>>> On Thu, 2011-09-29 at 16:26 -0700, Jeremy Fitzhardinge wrote:
>>>> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
>>>>
>>>> One big question which arises is whether the _early() function is
>>>> necessary at all.  All the stop_machine/mutex/etc stuff that
>>>> arch_jump_label_transform() ends up doing is redundant pre-SMP, but it
>>>> shouldn't hurt.  Maybe we can just drop the _early function?  It works
>>>> on x86, at least, because jump_label_enable() works, which uses the full
>>>> form.  And dropping it would reduce this to a very much smaller series.
>>> It does slow down the boot process, which is not a good thing when
>>> everyone is pushing for the fastest restarts.
>> Would it really though?  stop_machine() doesn't do very much when there
>> are no other cpus.
>>
>> Not that I measured or anything, but there was no obvious big lag at boot.
> Just bringing up the point, but without measurements, its all hand
> waving. It may not be a big deal, and simpler code is always better if
> it doesn't harm anything else.

I think the simplest thing is to make stop_machine() well-defined in a
pre-smp environment, where it just directly calls the callback:

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index ba5070c..b6ad9b3 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -485,6 +485,11 @@ int __stop_machine(int (*fn)(void *), void *data, const 
struct cpumask *cpus)
                                            .num_threads = num_online_cpus(),
                                            .active_cpus = cpus };
 
+       if (smdata.num_threads == 1) {
+               (*fn)(data);
+               return 0;
+       }
+
        /* Set the initial state and stop all online cpus. */
        set_state(&smdata, STOPMACHINE_PREPARE);
        return stop_cpus(cpu_online_mask, stop_machine_cpu_stop, &smdata);

so that its guaranteed safe to use at any point.

    J

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