Re: fix bogus hotplug cpu warning

Re: fix bogus hotplug cpu warning

[Ingo Molnar]

this commit:

 commit d243769d3f83b318813a04a9592bb7cfedc6c280
 Author: Hugh Dickins <hugh@veritas.com>
 Date:   Mon Aug 27 16:06:19 2007 +0100

    fix bogus hotplug cpu warning

    Fix bogus DEBUG_PREEMPT warning on x86_64, when cpu brought online 
    after bootup: current_is_keventd is right to note its use of 
    smp_processor_id is preempt-safe, but should use 
    raw_smp_processor_id to avoid the warning.

-       int cpu = smp_processor_id();   /* preempt-safe: keventd is per-cpu */
+       int cpu = raw_smp_processor_id(); /* preempt-safe: keventd is per-cpu */

is incorrect. smp_processor_id() will _not_ warn about true per-cpu 
tasks:

        /*
         * Kernel threads bound to a single CPU can safely use
         * smp_processor_id():
         */
        this_mask = cpumask_of_cpu(this_cpu);

        if (cpus_equal(current->cpus_allowed, this_mask))
                goto out;

so if this check triggered for the cpu-offline code it's because keventd 
was _not_ per-cpu (perhaps because the cpu offline code broke its 
affinity?). So your patch hides a real bug.

	Ingo

Re: fix bogus hotplug cpu warning

[Hugh Dickins]

On Tue, 28 Aug 2007, Ingo Molnar wrote:
> this commit:
> 
>  commit d243769d3f83b318813a04a9592bb7cfedc6c280
>  Author: Hugh Dickins <hugh@veritas.com>
>  Date:   Mon Aug 27 16:06:19 2007 +0100
> 
>     fix bogus hotplug cpu warning
> 
>     Fix bogus DEBUG_PREEMPT warning on x86_64, when cpu brought online 
>     after bootup: current_is_keventd is right to note its use of 
>     smp_processor_id is preempt-safe, but should use 
>     raw_smp_processor_id to avoid the warning.
> 
> -       int cpu = smp_processor_id();   /* preempt-safe: keventd is per-cpu */
> +       int cpu = raw_smp_processor_id(); /* preempt-safe: keventd is per-cpu */
> 
> is incorrect. smp_processor_id() will _not_ warn about true per-cpu 
> tasks:
> 
>         /*
>          * Kernel threads bound to a single CPU can safely use
>          * smp_processor_id():
>          */
>         this_mask = cpumask_of_cpu(this_cpu);
> 
>         if (cpus_equal(current->cpus_allowed, this_mask))
>                 goto out;
> 
> so if this check triggered for the cpu-offline code it's because keventd 
> was _not_ per-cpu (perhaps because the cpu offline code broke its 
> affinity?). So your patch hides a real bug.

No, keventd is properly per-cpu, but the task which is calling
current_is_keventd() is a normal user task, so smp_processor_id()
did warn about it.  It wouldn't make much sense to have a function
current_is_keventd() if you could only call it from a keventd, would
it ;-?  Take a look at it (kernel/workqueue.c), I believe what it does
is unusual (hence that comment about being "preempt-safe") but valid.

Hugh

Re: fix bogus hotplug cpu warning

[Ingo Molnar]

* Hugh Dickins <hugh@veritas.com> wrote:

> > so if this check triggered for the cpu-offline code it's because 
> > keventd was _not_ per-cpu (perhaps because the cpu offline code 
> > broke its affinity?). So your patch hides a real bug.
> 
> No, keventd is properly per-cpu, but the task which is calling 
> current_is_keventd() is a normal user task, so smp_processor_id() did 
> warn about it.  It wouldn't make much sense to have a function 
> current_is_keventd() if you could only call it from a keventd, would 
> it ;-?  Take a look at it (kernel/workqueue.c), I believe what it does 
> is unusual (hence that comment about being "preempt-safe") but valid.

ok, you are right indeed.

	Ingo