From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Frederic Weisbecker <frederic@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Josh Triplett <josh@joshtriplett.org>
Subject: Re: [PATCH 01/10] rcu: Directly lock rdp->nocb_lock on nocb code entrypoints
Date: Tue, 26 May 2020 15:29:00 -0700 [thread overview]
Message-ID: <20200526222900.GQ2869@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200526212756.GF76276@google.com>
On Tue, May 26, 2020 at 05:27:56PM -0400, Joel Fernandes wrote:
> On Tue, May 26, 2020 at 02:09:47PM -0700, Paul E. McKenney wrote:
> [...]
> > > > > BTW, I'm really itching to give it a try to make the scheduler more deadlock
> > > > > resilient (that is, if the scheduler wake up path detects a deadlock, then it
> > > > > defers the wake up using timers, or irq_work on its own instead of passing
> > > > > the burden of doing so to the callers). Thoughts?
> > > >
> > > > I have used similar approaches within RCU, but on the other hand the
> > > > scheduler often has tighter latency constraints than RCU does. So I
> > > > think that is a better question for the scheduler maintainers than it
> > > > is for me. ;-)
> > >
> > > Ok, it definitely keeps coming up in my radar first with the
> > > rcu_read_unlock_special() stuff, and now the nocb ;-). Perhaps it could also
> > > be good for a conference discussion!
> >
> > Again, please understand that RCU has way looser latency constraints
> > than the scheduler does. Adding half a jiffy to wakeup latency might
> > not go over well, especially in the real-time application area.
>
> Yeah, agreed that the "deadlock detection" code should be pretty light weight
> if/when it is written.
In addition, to even stand a chance, you would need to use hrtimers.
The half-jiffy (at a minimum) delay from any other deferral mechanism
that I know of would be the kiss of death, especially from the viewpoint
of the real-time guys.
> > But what did the scheduler maintainers say about this idea?
>
> Last I remember when it came up during the rcu_read_unlock_special() deadlock
> discussions, there's no way to know for infra like RCU to know that it was
> invoked from the scheduler.
>
> The idea I am bringing up now (about the scheduler itself detecting a
> recursion) was never brought up (not yet) with the sched maintainers (at
> least not by me).
It might be good to bounce if off of them sooner rather than later.
Thanx, Paul
next prev parent reply other threads:[~2020-05-26 22:29 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-13 16:47 [PATCH 00/10] rcu: Allow a CPU to leave and reenter NOCB state Frederic Weisbecker
2020-05-13 16:47 ` [PATCH 01/10] rcu: Directly lock rdp->nocb_lock on nocb code entrypoints Frederic Weisbecker
2020-05-20 12:29 ` Joel Fernandes
2020-05-22 17:57 ` Paul E. McKenney
2020-05-26 15:21 ` Joel Fernandes
2020-05-26 16:29 ` Paul E. McKenney
2020-05-26 20:18 ` Joel Fernandes
2020-05-26 21:09 ` Paul E. McKenney
2020-05-26 21:27 ` Joel Fernandes
2020-05-26 22:29 ` Paul E. McKenney [this message]
2020-05-27 0:45 ` Joel Fernandes
2020-05-27 0:58 ` Paul E. McKenney
2020-06-04 11:41 ` Frederic Weisbecker
2020-06-04 16:36 ` Paul E. McKenney
2020-06-08 12:57 ` Frederic Weisbecker
2020-06-09 18:02 ` Paul E. McKenney
2020-06-10 13:12 ` Frederic Weisbecker
2020-06-10 14:02 ` Paul E. McKenney
2020-06-10 22:12 ` Frederic Weisbecker
2020-06-10 23:21 ` Paul E. McKenney
2020-06-11 1:32 ` Joel Fernandes
2020-05-13 16:47 ` [PATCH 02/10] rcu: Use direct rdp->nocb_lock operations on local calls Frederic Weisbecker
2020-05-13 16:47 ` [PATCH 03/10] rcu: Make locking explicit in do_nocb_deferred_wakeup_common() Frederic Weisbecker
2020-05-26 19:54 ` Joel Fernandes
2020-05-26 19:59 ` Joel Fernandes
2020-05-13 16:47 ` [PATCH 04/10] rcu: Implement rcu_segcblist_is_offloaded() config dependent Frederic Weisbecker
2020-05-13 18:20 ` Paul E. McKenney
2020-05-13 23:03 ` Frederic Weisbecker
2020-05-14 15:47 ` Paul E. McKenney
2020-05-13 16:47 ` [PATCH 05/10] rcu: Remove useless conditional nocb unlock Frederic Weisbecker
2020-05-13 16:47 ` [PATCH 06/10] rcu: Make nocb_cb kthread parkable Frederic Weisbecker
2020-06-11 1:34 ` Joel Fernandes
2020-05-13 16:47 ` [PATCH 07/10] rcu: Temporarily assume that nohz full CPUs might not be NOCB Frederic Weisbecker
2020-05-13 18:25 ` Paul E. McKenney
2020-05-13 23:08 ` Frederic Weisbecker
2020-05-14 15:50 ` Paul E. McKenney
2020-05-14 22:49 ` Frederic Weisbecker
2020-05-13 16:47 ` [PATCH 08/10] rcu: Allow to deactivate nocb on a CPU Frederic Weisbecker
2020-05-13 18:38 ` Paul E. McKenney
2020-05-13 22:45 ` Frederic Weisbecker
2020-05-14 15:47 ` Paul E. McKenney
2020-05-14 22:30 ` Frederic Weisbecker
2020-05-14 22:47 ` Paul E. McKenney
2020-05-14 22:55 ` Frederic Weisbecker
2020-05-26 21:20 ` Joel Fernandes
2020-05-26 22:49 ` Joel Fernandes
2020-06-04 13:10 ` Frederic Weisbecker
2020-06-11 1:32 ` Joel Fernandes
2020-06-11 17:03 ` Paul E. McKenney
2020-06-04 13:14 ` Frederic Weisbecker
2020-05-13 16:47 ` [PATCH 09/10] rcu: Allow to re-offload a CPU that used to be nocb Frederic Weisbecker
2020-05-13 18:41 ` Paul E. McKenney
2020-05-13 16:47 ` [PATCH 10/10] rcu: Nocb (de)activate through sysfs Frederic Weisbecker
2020-05-13 18:42 ` Paul E. McKenney
2020-05-13 23:23 ` Frederic Weisbecker
2020-05-14 15:51 ` Paul E. McKenney
2020-05-13 18:15 ` [PATCH 00/10] rcu: Allow a CPU to leave and reenter NOCB state Paul E. McKenney
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