Re: [PATCHv2 3/3] rcu: coordinate tick dependency during concurrent offlining

[Frederic Weisbecker <frederic@kernel.org>]

On Mon, Nov 07, 2022 at 08:07:26AM -0800, Paul E. McKenney wrote:
> > I ran 200 hours of TREE04 and got an RCU CPU stall warning.  I ran 2000
> > hours on v6.0, which precedes these commits, and everything passed.
> > 
> > I will run more, primarily on v6.0, but that is what I have thus far.
> > At the moment, I have some concerns about this change.
> 
> OK, so I have run a total of 8000 hours on v6.0 without failure.  I have
> run 4200 hours on rcu#revert_tick_dep with 15 failures.  The ones I
> looked at were RCU CPU stall warnings with timer failures.
> 
> This data suggests that the kernel is not yet ready for that commit
> to be reverted.

But that branch has the three commits reverted:

1) tick: Detect and fix jiffies update stall
2) timers/nohz: Last resort update jiffies on nohz_full IRQ entry*
3) rcu: Make CPU-hotplug removal operations enable tick

Reverting all of them is expected to fail anyway.

What we would like to know is if reverting just 3) is fine. Because
1) and 2) are supposed to fix the underlying issue.

I personally didn't manage to trigger failures with just reverting 3)
after thousands hours. But it failed with reverting all of them.

Has someone managed to trigger a failure with only 3) reverted?

Thanks.