linux-kernel - Re: rcu-bh design

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Date:   Fri, 4 May 2018 12:30:50 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: rcu-bh design

On Fri, 04 May 2018 16:20:11 +0000
Joel Fernandes <joelaf@...gle.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.

> 
> 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?

> 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.

-- Steve