Messages in this thread | | | Date | Fri, 4 May 2018 10:43:30 -0700 | From | "Paul E. McKenney" <> | Subject | Re: rcu-bh design |
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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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