Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Aug 09, 2019 at 08:40:27PM -0700, Paul E. McKenney wrote:
[snip]
> > > In contrast, a heavy duty userspace-driven workload would transition to
> > > and from userspace for each kfree_rcu(), and that would increment the
> > > dyntick-idle count on each transition to and from userspace.  Adding the
> > > rcu_momentary_dyntick_idle() emulates a pair of such transitions.
> > 
> > But even if we're in kernel mode and not transitioning, I thought the FQS
> > loop (rcu_implicit_dynticks_qs() function) would set need_heavy_qs to true at
> > 2 * jiffies_to_sched_qs.
> > 
> > Hmm, I forgot that jiffies_to_sched_qs can be quite large I guess. You're
> > right, we could call rcu_momentary_dyntick_idle() in advance before waiting
> > for FQS loop to do the setting of need_heavy_qs.
> > 
> > Or, am I missing something with the rcu_momentary_dyntick_idle() point you
> > made?
> 
> The trick is that rcu_momentary_dyntick_idle() directly increments the
> CPU's dyntick counter, so that the next FQS loop will note that the CPU
> passed through a quiescent state.  No need for need_heavy_qs in this case.

Yes, that's what I also understand. Thanks for confirming,

 - Joel