From: Steven Rostedt <rostedt@goodmis.org>
To: Joel Fernandes <joelaf@google.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: rcu-bh design
Date: Fri, 4 May 2018 12:30:50 -0400 [thread overview]
Message-ID: <20180504123050.2841f80d@gandalf.local.home> (raw)
In-Reply-To: <CAJWu+oqCun1Ae6GqPxnS+eCDi3jadGPp+MO8TjOWgs+AiAh79A@mail.gmail.com>
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.
>
> 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
next prev parent reply other threads:[~2018-05-04 16:30 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-04 16:20 rcu-bh design Joel Fernandes
2018-05-04 16:30 ` Steven Rostedt [this message]
2018-05-04 17:15 ` Joel Fernandes
2018-05-04 17:43 ` Paul E. McKenney
2018-05-04 18:34 ` Joel Fernandes
2018-05-04 18:49 ` Paul E. McKenney
2018-05-04 19:57 ` Joel Fernandes
2018-05-04 20:11 ` Paul E. McKenney
2018-05-04 20:33 ` Joel Fernandes
2018-05-04 22:49 ` Paul E. McKenney
2018-05-04 23:20 ` Joel Fernandes
2018-05-04 23:43 ` Paul E. McKenney
2018-05-05 0:39 ` Joel Fernandes
2018-05-04 17:32 ` Paul E. McKenney
2018-05-04 17:37 ` Steven Rostedt
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