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

[Frederic Weisbecker <frederic@kernel.org>]

On Tue, Sep 20, 2022 at 03:26:28PM +0800, Pingfan Liu wrote:
> On Fri, Sep 16, 2022 at 03:42:58PM +0200, Frederic Weisbecker wrote:
> > Note this is only locking the rdp's node, not the root node.
> > Therefore if CPU 0 and CPU 256 are going off at the same time and they
> > don't belong to the same node, the above won't protect against concurrent
> > TICK_DEP_BIT_RCU set/clear.
> > 
> 
> Nice, thanks for the careful thoughts. How about moving the counting
> place to the root node?

You could but then you'd need to lock the root node.

> 
> > My suspicion is that we don't need this TICK_DEP_BIT_RCU tick dependency
> > anymore. I believe it was there because of issues that were fixed with:
> > 
> > 	53e87e3cdc15 (timers/nohz: Last resort update jiffies on nohz_full IRQ entry)
> > and:
> > 
> > 	a1ff03cd6fb9 (tick: Detect and fix jiffies update stall)
> > 
> > It's unfortunately just suspicion because the reason for that tick dependency
> > is unclear but I believe it should be safe to remove now.
> > 
> 
> I have gone through this tick dependency again, but got less.
> 
> I think at least from the RCU's viewpoint, it is useless since
> multi_cpu_stop()->rcu_momentary_dyntick_idle() has eliminate the
> requirement for tick interrupt.

Partly yes.

> Is there a way to have a convincing test so that these code can be removed?
> Or this code will be got along with?

Hmm, Paul might remember which rcutorture scenario would trigger it?