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 <[email protected]> wrote: > On Fri, 04 May 2018 16:20:11 +0000 > Joel Fernandes <[email protected]> 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? > > 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. thanks, - Joel
