From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Scott Wood <swood@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Joel Fernandes <joel@joelfernandes.org>,
linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Clark Williams <williams@redhat.com>
Subject: Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep
Date: Mon, 26 Aug 2019 11:12:47 -0700 [thread overview]
Message-ID: <20190826181247.GG28441@linux.ibm.com> (raw)
In-Reply-To: <72c2da8695f622b8962ac43e3571107382969555.camel@redhat.com>
On Mon, Aug 26, 2019 at 12:49:22PM -0500, Scott Wood wrote:
> On Mon, 2019-08-26 at 09:29 -0700, Paul E. McKenney wrote:
> > On Mon, Aug 26, 2019 at 05:25:23PM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2019-08-23 23:10:14 [-0400], Joel Fernandes wrote:
> > > > On Fri, Aug 23, 2019 at 02:28:46PM -0500, Scott Wood wrote:
> > > > > On Fri, 2019-08-23 at 18:20 +0200, Sebastian Andrzej Siewior wrote:
> > > > > > this looks like an ugly hack. This sleeping_lock_inc() is used
> > > > > > where we
> > > > > > actually hold a sleeping lock and schedule() which is okay. But
> > > > > > this
> > > > > > would mean we hold a RCU lock and schedule() anyway. Is that okay?
> > > > >
> > > > > Perhaps the name should be changed, but the concept is the same --
> > > > > RT-
> > > > > specific sleeping which should be considered involuntary for the
> > > > > purpose of
> > > > > debug checks. Voluntary sleeping is not allowed in an RCU critical
> > > > > section
> > > > > because it will break the critical section on certain flavors of
> > > > > RCU, but
> > > > > that doesn't apply to the flavor used on RT. Sleeping for a long
> > > > > time in an
> > > > > RCU critical section would also be a bad thing, but that also
> > > > > doesn't apply
> > > > > here.
> > > >
> > > > I think the name should definitely be changed. At best, it is super
> > > > confusing to
> > > > call it "sleeping_lock" for this scenario. In fact here, you are not
> > > > even
> > > > blocking on a lock.
> > > >
> > > > Maybe "sleeping_allowed" or some such.
> > >
> > > The mechanism that is used here may change in future. I just wanted to
> > > make sure that from RCU's side it is okay to schedule here.
> >
> > Good point.
> >
> > The effect from RCU's viewpoint will be to split any non-rcu_read_lock()
> > RCU read-side critical section at this point. This alrady happens in a
> > few places, for example, rcu_note_context_switch() constitutes an RCU
> > quiescent state despite being invoked with interrupts disabled (as is
> > required!). The __schedule() function just needs to understand (and does
> > understand) that the RCU read-side critical section that would otherwise
> > span that call to rcu_node_context_switch() is split in two by that call.
> >
> > However, if this was instead an rcu_read_lock() critical section within
> > a PREEMPT=y kernel, then if a schedule() occured within stop_one_task(),
> > RCU would consider that critical section to be preempted. This means
> > that any RCU grace period that is blocked by this RCU read-side critical
> > section would remain blocked until stop_one_cpu() resumed, returned,
> > and so on until the matching rcu_read_unlock() was reached. In other
> > words, RCU would consider that RCU read-side critical section to span
> > the call to stop_one_cpu() even if stop_one_cpu() invoked schedule().
> >
> > On the other hand, within a PREEMPT=n kernel, the call to schedule()
> > would split even an rcu_read_lock() critical section. Which is why I
> > asked earlier if sleeping_lock_inc() and sleeping_lock_dec() are no-ops
> > in !PREEMPT_RT_BASE kernels. We would after all want the usual lockdep
> > complaints in that case.
>
> migrate_enable() is PREEMPT_RT_BASE-specific -- this code won't execute at
> all with PREEMPT=n.
Understood! And yes, that was your answer to my question. Me, I was
just answering Sebastian's question. ;-)
Thanx, Paul
next prev parent reply other threads:[~2019-08-26 18:13 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-21 23:19 [PATCH RT v2 0/3] RCU fixes Scott Wood
2019-08-21 23:19 ` [PATCH RT v2 1/3] rcu: Acquire RCU lock when disabling BHs Scott Wood
2019-08-21 23:33 ` Paul E. McKenney
2019-08-22 13:39 ` Joel Fernandes
2019-08-22 15:27 ` Paul E. McKenney
2019-08-23 1:50 ` Joel Fernandes
2019-08-23 2:11 ` Paul E. McKenney
2019-08-23 3:23 ` Scott Wood
2019-08-23 12:30 ` Paul E. McKenney
2019-08-23 16:17 ` Sebastian Andrzej Siewior
2019-08-23 19:46 ` Scott Wood
2019-08-26 15:59 ` Sebastian Andrzej Siewior
2019-08-26 23:21 ` Scott Wood
2019-08-23 2:36 ` Scott Wood
2019-08-23 2:54 ` Paul E. McKenney
2019-08-21 23:19 ` [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep Scott Wood
2019-08-21 23:35 ` Paul E. McKenney
2019-08-23 1:21 ` Scott Wood
2019-08-23 16:20 ` Sebastian Andrzej Siewior
2019-08-23 19:28 ` Scott Wood
2019-08-24 3:10 ` Joel Fernandes
2019-08-26 15:25 ` Sebastian Andrzej Siewior
2019-08-26 16:29 ` Paul E. McKenney
2019-08-26 17:49 ` Scott Wood
2019-08-26 18:12 ` Paul E. McKenney [this message]
2019-08-27 9:23 ` Sebastian Andrzej Siewior
2019-08-27 13:08 ` Joel Fernandes
2019-08-27 15:58 ` Paul E. McKenney
2019-08-27 16:06 ` Joel Fernandes
2019-08-27 15:53 ` Paul E. McKenney
2019-08-28 9:27 ` Sebastian Andrzej Siewior
2019-08-28 12:54 ` Paul E. McKenney
2019-08-28 13:14 ` Sebastian Andrzej Siewior
2019-08-28 13:59 ` Joel Fernandes
2019-08-28 15:51 ` Paul E. McKenney
2019-08-28 15:50 ` Paul E. McKenney
2019-08-21 23:19 ` [PATCH RT v2 3/3] rcu: Disable use_softirq on PREEMPT_RT Scott Wood
2019-08-21 23:40 ` Paul E. McKenney
2019-08-23 16:32 ` Sebastian Andrzej Siewior
2019-08-22 13:59 ` Joel Fernandes
2019-08-22 15:29 ` Paul E. McKenney
2019-08-22 19:31 ` Scott Wood
2019-08-23 0:52 ` Joel Fernandes
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=20190826181247.GG28441@linux.ibm.com \
--to=paulmck@linux.ibm.com \
--cc=bigeasy@linutronix.de \
--cc=joel@joelfernandes.org \
--cc=juri.lelli@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-rt-users@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=swood@redhat.com \
--cc=tglx@linutronix.de \
--cc=williams@redhat.com \
/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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).