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SubjectRe: [PATCH] RFC: x86/smp: use printk_deferred in native_smp_send_reschedule
On Wed, May 8, 2019 at 9:44 AM Sergey Senozhatsky
<sergey.senozhatsky.work@gmail.com> wrote:
>
> On (05/07/19 19:33), Daniel Vetter wrote:
> [..]
> > - make the console_trylock trylock also the spinlock. This works in
> > the limited case of the console_lock use-case, but doesn't fix the
> > same semaphore.lock acquisition in the up() path in console_unlock,
> > which we can't avoid with a trylock.
> >
> > - move the wake_up_process in up() out from under the semaphore.lock
> > spinlock critical section. Again this works for the limited case of
> > the console_lock, and does fully break the cycle for this lock.
> > Unfortunately there's still plenty of scheduler related locks that
> > wake_up_process needs, so the loop is still there, just with a few
> > less locks involved.
> >
> > Hence now third attempt, trying to fix this by using printk_deferred()
> > instead of the normal printk that WARN() uses.
> > native_smp_send_reschedule is only called from scheduler related code,
> > which has to use printk_deferred due to this locking recursion, so
> > this seems consistent.
> >
> > It has the unfortunate downside that we're losing the backtrace though
> > (I didn't find a printk_deferred version of WARN, and I'm not sure
> > it's a bright idea to dump that much using printk_deferred.)
>
> I'm catching up with the emails now (was offline for almost 2 weeks),
> so I haven't seen [yet] all of the previous patches/discussions.
>
> [..]
> > 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().

Hm right, I was happy enough when Petr pointed out the printk_deferred
infrastructure that I didn't look too deeply into how it works. From a
quick loo




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

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