Re: [PATCH v2] softlockup: serialized softlockup's log

[Doug Anderson]

Hi,

On Wed, Nov 22, 2023 at 2:05 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 22 Nov 2023 18:02:12 +0800 lizhe.67@xxxxxxxxxxxxx wrote:
>
> > From: Li Zhe <lizhe.67@xxxxxxxxxxxxx>
> >
> > If multiple CPUs trigger softlockup at the same time with
> > 'softlockup_all_cpu_backtrace=0', the softlockup's logs will appear
> > staggeredly in dmesg, which will affect the viewing of the logs for
> > developer. Since the code path for outputting softlockup logs is not
> > a kernel hotspot and the performance requirements for the code are
> > not strict, locks are used to serialize the softlockup log output to
> > improve the readability of the logs.
>
> Seems reasonable, but...
>
> > --- a/kernel/watchdog.c
> > +++ b/kernel/watchdog.c
> > @@ -28,6 +28,8 @@
> >  #include <linux/kvm_para.h>
> >
> >  static DEFINE_MUTEX(watchdog_mutex);
> > +/* This lock is used to prevent concurrent actions of softlockup output logs */
> > +static DEFINE_SPINLOCK(watchdog_output_lock);
>
> It would be a little neater to reduce the scope of this - move the
> definition into that little code block in watchdog_timer_fn() where it
> is actually used.

That's where Li Zhe had it in v1. I actually requested it move to a
larger scope since I think we'll want a future patch that also uses
the same lock for hardlockup and I was thinking of sending that out
after Li Zhe's patch lands. Specifically, I've seen cases on arm64
when pseudo-NMI isn't enabled that we can end up with a soft lockup
triggering while we're waiting for a backtrace of a hardlockup (since
we wait 10 seconds if a CPU isn't responding to the request to
backtrace it).

I've also been testing hardlockup (with pseudo-NMI enabled) and I also
notice similar problems with hardlockup itself. Specifically, I'm
looking at a an output currently that looks like this:

[  102.619905] lkdtm: Performing direct entry HARDLOCKUP
[  112.037900] Watchdog detected hard LOCKUP on cpu 1
[  112.038031] Watchdog detected hard LOCKUP on cpu 7
[  112.042872] Modules linked in:
[  112.047807] Modules linked in:
...

...but I'd also be OK with limiting the scope until I send my later
patch, so up to you.


> >  #if defined(CONFIG_HARDLOCKUP_DETECTOR) || defined(CONFIG_HARDLOCKUP_DETECTOR_SPARC64)
> >  # define WATCHDOG_HARDLOCKUP_DEFAULT 1
> > @@ -514,6 +516,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
> >               /* Start period for the next softlockup warning. */
> >               update_report_ts();
> >
> > +             spin_lock(&watchdog_output_lock);
>
> The hrtimer callout function is called from [soft]irq context, yes?
> Doesn't lockdep get upset when we take a spinlock in such a context?

I don't think so, but I could certainly be wrong. As far as I knew:

* spin_lock_irqsave / spin_unlock_irqrestore: Always safe to call,
whether from normal context or IRQ context. Saves whether IRQs were
disabled or not, then disables them, then restores them to how they
were before.

* spin_lock_irq, spin_unlock_irq: Safe to call as long as you're _not_
in IRQ context (because spin_lock_irq disables IRQs and
spin_unlock_irq unconditionally re-enables them)

* spin_lock / spin_unlock: Safe to call as long as you _are_ in IRQ
context. Being in IRQ context means IRQs are already disabled so the
function doesn't need to disable/reenable them. Also safe to call if
no users of this spinlock ever run in IRQ context.

The reason for all that complexity is to specifically to avoid:

1. spin_lock(&my_lock);
2. IRQ fires off
3. In IRQ context: spin_lock(&my_lock);

...because that would make us spin forever.


Just to be sure, I just tried with lockdep and things seemed OK.
Specifically, I did:

sysctl -w kernel.softlockup_panic=0
sysctl -w kernel.softlockup_all_cpu_backtrace=0
for i in $(seq 3); do
  echo SOFTLOCKUP > /sys/kernel/debug/provoke-crash/DIRECT&
done

I got no lockdep splats and things seemed synchronized. Of course, the
system was in pretty bad shape because the "provoke-crash" module
never gets out of its soft lockup, but that's part for the course and
why on our systems we run with kernel.softlockup_panic=1. I guess that
also means I'd be happy with this now:

Tested-by: Douglas Anderson <dianders@xxxxxxxxxxxx>