Re: rcu-bh design

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, May 04, 2018 at 05:15:11PM +0000, Joel Fernandes wrote:
> Hi Steven,
> Just for a warning/disclaimer, I am new to RCU-land and trying to make
> sense ;-) So forgive me if something sounds too outlandish.
> 
> On Fri, May 4, 2018 at 9:30 AM Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > On Fri, 04 May 2018 16:20:11 +0000
> > Joel Fernandes <joelaf@google.com> wrote:
> 
> > > Hi Paul, everyone,
> > >
> > > I had some question(s) about rcu-bh design.
> > > I am trying to understand the reasoning or need of it. I see that rcu-bh
> > > will disable softirqs across read-side sections. But I am wondering why
> > > this is needed. __do_softirq already disables softirq when a softirq
> > > handler is running. The only reason I can see is, rcu-bh helps in
> > > situations where - a softirq interrupts a preemptible RCU read-section
> and
> > > prevents that read section from completing. But this problem would
> happen
> > > if anyone where to use rcu-preempt - then does rcu-preempt even make
> sense
> > > to use and shouldn't everyone be using rcu-bh?
> 
> > I thought rcu-bh uses softirqs as a quiescent state. Thus, blocking
> > softirqs from happening makes sense. I don't think an
> > rcu_read_lock_bh() makes sense in a softirq.
> 
> Ok.
> 
> > > The other usecase for rcu-bh seems to be if context-switch is used as a
> > > quiescent state, then softirq flood can prevent that from happening and
> > > cause rcu grace periods from completing.
> 
> > > But preemptible RCU *does not* use context-switch as a quiescent state.
> > It doesn't?
> 
> I thought that's what preemptible rcu is about. You can get preempted but
> you shouldn't block in a read-section. Is that not true?

Almost.  All context switches in an RCU-preempt read-side critical section
must be subject to priority boosting.  Preemption is one example, because
boosting the priority of the preempted task will make it runnable.
The priority-inheritance -rt "spinlock" is another example, because
boosting the priority of the task holding the lock will eventually make
runnable the task acquiring the lock within the RCU-preempt read-side
critical section.

> > > So in that case rcu-bh would make
> > > sense only in a configuration where we're not using preemptible-rcu at
> all
> > > and are getting flooded by softirqs. Is that the reason rcu-bh needs to
> > > exist?
> 
> > Maybe I'm confused by what you are asking.
> 
> Sorry for any confusion. I was going through the below link for motivation
> of rcu-bh and why it was created:
> https://www.kernel.org/doc/Documentation/RCU/Design/Requirements/Requirements.html#Bottom-Half%20Flavor
> 
> I was asking why rcu-bh is needed in the kernel, like why can't we just use
> rcu-preempt. As per above link, the motivation of rcu-bh was to prevent
> denial of service during heavy softirq load. I was trying to understand
> that usecase. In my mind, such denial of service / out of memory is then
> even possible with preemptible rcu which is used in many places in the
> kernel, then why not just use rcu-bh for everything? I was just studying
> this RCU flavor (and all other RCU flavors) and so this question popped up.

Because RCU-bh is not preemptible.

And the non-DoS nature of RCU-bh is one challenge in my current quest to
fold all three flavors (RCU-bh,  RCU-preempt, and RCU-sched) into one
flavor to rule them all.  ;-)

							Thanx, Paul