Re: [PATCH] RFC: x86/smp: use printk_deferred in native_smp_send_reschedule

[Daniel Vetter]

On Wed, May 8, 2019 at 9:53 AM Sergey Senozhatsky
<sergey.senozhatsky.work@xxxxxxxxx> wrote:
>
> 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.

[coffee slowly kicking in it seems]

Locking at __up_console_sem in printk.c, we already do this. I get a
bit a feeling that the 2nd attempt in this saga (pulling the
wake_up_process out from under semaphore.lock spinlock of the
console_lock) is all we really need, since the more direct recursion
that Petr pointed out is already handled with printk_safe_enter/exit
around the up().

https://patchwork.kernel.org/patch/10930673/ for reference that
approach, in case it's lost in your inbox.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch