Re: [PATCH] rcu-tasks: Delay rcu_tasks_verify_self_tests() to avoid missed callbacks

[Waiman Long]

On 6/13/22 13:56, Paul E. McKenney wrote:
> On Mon, Jun 13, 2022 at 12:01:24PM -0400, Waiman Long wrote:
> > On 6/10/22 16:58, Paul E. McKenney wrote:
> > > On Fri, Jun 10, 2022 at 02:42:12PM -0400, Waiman Long wrote:
> > > > Even though rcu_tasks selftest is initiated early in the boot process,
> > > > the verification done at late initcall time may not be late enough to
> > > > catch all the callbacks especially on systems with just a few cpus and
> > > > small memory.
> > > >
> > > > After 12 bootup's On a s390x system, 1 of them had failed rcu_tasks
> > > > verification test.
> > > >
> > > > [    8.183013] call_rcu_tasks() has been failed.
> > > > [    8.183041] WARNING: CPU: 0 PID: 1 at kernel/rcu/tasks.h:1696 rcu_tasks_verify_self_tests+0x64/0xd0
> > > > [    8.203246] Callback from call_rcu_tasks() invoked.
> > > >
> > > > In this particular case, the callback missed the check by about
> > > > 20ms. Similar rcu_tasks selftest failures are also seen in ppc64le
> > > > systems.
> > > >
> > > > [    0.313391] call_rcu_tasks() has been failed.
> > > > [    0.313407] WARNING: CPU: 0 PID: 1 at kernel/rcu/tasks.h:1696 rcu_tasks_verify_self_tests+0x5c/0xa0
> > > > [    0.335569] Callback from call_rcu_tasks() invoked.
> > > >
> > > > Avoid this missed callback by delaying the verification using
> > > > delayed_work. The delay is set to be about 0.1s which hopefully will
> > > > be long enough to catch all the callbacks on systems with few cpus and
> > > > small memory.
> > > >
> > > > Fixes: bfba7ed084f8 ("rcu-tasks: Add RCU-tasks self tests")
> > > > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > > Good catch, thank you!
> > >
> > > A few days ago, I queued this:
> > >
> > > 2585014188d5 ("rcu-tasks: Be more patient for RCU Tasks boot-time testing")
> > >
> > > This is shown in full at the end of this email.  Does this fix this
> > > problem for you?
> > I think your patch should fix the false positive warning and it give plenty
> > of time for this to happen.
> >
> > I do have one question though. rcu_tasks_verify_selft_tests() is called from
> > do_initcalls(). Since it may not be the last late initcall, does that mean
> > other late initcalls queued after that may be delayed by a second or more?
> Indeed.  Which is why I would welcome the workqueues portion of your
> patch on top of the above patch in -rcu.  ;-)

Sure. I will work on such a follow-up patch.

Cheers,
Longman