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    SubjectRe: rcu-bh design
    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

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