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SubjectRe: [PATCH] RFC: x86/smp: use printk_deferred in native_smp_send_reschedule
On (05/08/19 16:44), Sergey Senozhatsky wrote:
> [..]
> > static void native_smp_send_reschedule(int cpu)
> > {
> > if (unlikely(cpu_is_offline(cpu))) {
> > - WARN(1, "sched: Unexpected reschedule of offline CPU#%d!\n", cpu);
> > + printk_deferred(KERN_WARNING
> > + "sched: Unexpected reschedule of offline CPU#%d!\n", cpu);
> > return;
> > }
> > apic->send_IPI(cpu, RESCHEDULE_VECTOR);
>
> Hmm,
> One thing to notice here is that the CPU in question is offline-ed,
> and printk_deferred() is a per-CPU type of deferred printk(). So the
> following thing
>
> __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
>
> might not print anything at all. In this particular case we always
> need another CPU to do console_unlock(), since this_cpu() is not
> really expected to do wake_up_klogd_work_func()->console_unlock().

D'oh... It's remote CPU which is offline, not this_cpu().
Sorry, my bad!

Any printk-related patch in this area will make PeterZ really-really
angry :)

printk_deferred(), just like prinkt_safe(), depends on IRQ work;
printk_safe(), however, can redirect multiple lines, unlike
printk_deferred(). So if you want to keep the backtrace, you may
do something like

if (unlikely(cpu_is_offline(cpu))) {
printk_safe_enter(...);
WARN(1, "sched: Unexpected reschedule of offline CPU#%d!\n",
cpu);
printk_safe_exit(...);
return;
}

I think, in this case John's reworked-printk can do better than
printk_safe/printk_deferred.

-ss

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