LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Joel Fernandes <joelaf@google.com>
To: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: rcu-bh design
Date: Fri, 04 May 2018 16:20:11 +0000
Message-ID: <CAJWu+oqCun1Ae6GqPxnS+eCDi3jadGPp+MO8TjOWgs+AiAh79A@mail.gmail.com> (raw)

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?

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. 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?

thanks!

- Joel

             reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-04 16:20 Joel Fernandes [this message]
2018-05-04 16:30 ` Steven Rostedt
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CAJWu+oqCun1Ae6GqPxnS+eCDi3jadGPp+MO8TjOWgs+AiAh79A@mail.gmail.com \
    --to=joelaf@google.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mathieu.desnoyers@efficios.com \
    --cc=paulmck@linux.vnet.ibm.com \
    --cc=rostedt@goodmis.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git