From: "Paul E. McKenney" <paulmck@us.ibm.com>
To: Esben Nielsen <simlo@phys.au.dk>
Cc: Ingo Molnar <mingo@elte.hu>,
dipankar@in.ibm.com, shemminger@osdl.org, akpm@osdl.org,
torvalds@osdl.org, rusty@au1.ibm.com, tgall@us.ibm.com,
jim.houston@comcast.net, manfred@colorfullife.com, gh@us.ibm.com,
linux-kernel@vger.kernel.org
Subject: Re: Real-Time Preemption and RCU
Date: Wed, 23 Mar 2005 23:02:07 -0800 [thread overview]
Message-ID: <20050324070207.GI1298@us.ibm.com> (raw)
In-Reply-To: <Pine.OSF.4.05.10503231232070.15818-100000@da410.phys.au.dk>
On Wed, Mar 23, 2005 at 12:44:52PM +0100, Esben Nielsen wrote:
> On Tue, 22 Mar 2005, Paul E. McKenney wrote:
>
> > On Tue, Mar 22, 2005 at 09:55:26AM +0100, Esben Nielsen wrote:
> > > On Mon, 21 Mar 2005, Paul E. McKenney wrote:
> > [ . . . ]
> > > > On Mon, Mar 21, 2005 at 12:23:22AM +0100, Esben Nielsen wrote:
> > > > This is in some ways similar to the K42 approach to RCU (which they call
> > > > "generations"). Dipankar put together a similar patch for Linux, but
> > > > the problem was that grace periods could be deferred for an extremely
> > > > long time. Which I suspect is what you were calling out as causing
> > > > RCU batches never to run.
> > >
> > > That is where the preempt_by_nonrt_disable/enable() is supposed to help:
> > > Then it can't take longer than the normal kernel in the situation where
> > > there is no RT tasks running. RT tasks will prolong the grace periods if
> > > they go into RCU regions, but they are supposed to be relatively small -
> > > and deterministic!
> >
> > The part that I am missing is how this helps in the case where a non-RT
> > task gets preempted in the middle of an RCU read-side critical section
> > indefinitely. Or are you boosting the priority of any task that
> > enters an RCU read-side critical section?
>
> Yes in effect: I set the priority to MAX_RT_PRIO. But actually I am
> playing around (when I get time for it that is :-( ) with cheaper
> solution:
> I assume you enter these regions where you don't want to be
> preempted by non-RT tasks are relatively short. Therefore the risc of
> getting preempted is small. Moving the priority is expensive since you
> need to lock the runqueue. I only want to do the movement when
> there is an preemption. Therefore I added code in schedule() to take care
> of it: If a task is in a rcu-read section, is non-RT and is preempted it's
> priority is set to MAX_RT_PRIO for the time being. It will keep that
> priority until the priority is recalculated, but that shouldn't hurt
> anyone.
> I am not happy about adding code to schedule() but setting the
> priority in there is very cheap because it already has the lock
> on the runqueue. Furthermore, I assume it only happens very rarely. In the
> execution of schedule() my code only takes a single test on wether the
> previous task was in a rcu-section or not. That is not very much code.
Interesting approach -- could come in handy.
> I have not yet tested it (no time :-( )
Well, being as I haven't got the lock-based scheme fully running yet,
I can't give you too much trouble about that. :-/
Thanx, Paul
> > [...]
> > > > Yes, but this is true of every other lock in the system as well, not?
> > >
> > > Other locks are not globaly used but only used for a specific subsystem.
> > > On a real-time system you are supposed to know which subsystems you can
> > > call into and still have a low enough latency as each subsystem has it's
> > > own bound. But with a global RCU locking mechanism all RCU using code is
> > > to be regarded as _one_ such subsystem.
> >
> > Yep. As would the things protected by the dcache lock, task list lock,
> > and so on, right?
>
> Yep
>
> >
> > Thanx, Paul
> >
> Esben
>
>
next prev parent reply other threads:[~2005-03-24 7:05 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-03-18 0:20 Real-Time Preemption and RCU Paul E. McKenney
2005-03-18 7:49 ` Ingo Molnar
2005-03-18 16:43 ` Paul E. McKenney
2005-03-18 17:11 ` Ingo Molnar
2005-03-18 17:29 ` Paul E. McKenney
2005-03-18 20:35 ` Paul E. McKenney
2005-03-18 22:22 ` Paul E. McKenney
2005-03-19 0:48 ` Paul E. McKenney
2005-03-18 8:44 ` Ingo Molnar
2005-03-18 9:04 ` Ingo Molnar
2005-03-18 9:38 ` Ingo Molnar
2005-03-18 9:13 ` Ingo Molnar
2005-03-18 9:28 ` Ingo Molnar
2005-03-18 9:53 ` Ingo Molnar
2005-03-18 15:33 ` Paul E. McKenney
2005-03-19 5:03 ` Manfred Spraul
2005-03-19 16:26 ` Ingo Molnar
2005-03-20 6:36 ` Manfred Spraul
2005-03-20 9:25 ` Thomas Gleixner
2005-03-20 16:57 ` Manfred Spraul
2005-03-20 21:38 ` Bill Huey
2005-03-20 21:59 ` Bill Huey
2005-03-18 10:03 ` Ingo Molnar
2005-03-18 11:30 ` Ingo Molnar
2005-03-18 16:48 ` Esben Nielsen
2005-03-18 17:19 ` Ingo Molnar
2005-03-20 13:29 ` Esben Nielsen
2005-03-20 22:38 ` Paul E. McKenney
2005-03-20 23:23 ` Esben Nielsen
2005-03-22 5:53 ` Paul E. McKenney
2005-03-22 8:55 ` Esben Nielsen
2005-03-22 9:20 ` Ingo Molnar
2005-03-22 10:19 ` Esben Nielsen
2005-03-23 5:40 ` Paul E. McKenney
2005-03-23 11:44 ` Esben Nielsen
2005-03-24 7:02 ` Paul E. McKenney [this message]
2005-03-22 10:56 ` Ingo Molnar
2005-03-22 11:39 ` Esben Nielsen
2005-03-22 13:10 ` Ingo Molnar
2005-03-22 15:08 ` Esben Nielsen
2005-03-18 15:48 ` Paul E. McKenney
2005-03-18 11:38 ` Ingo Molnar
2005-03-18 12:56 ` Bill Huey
2005-03-18 13:17 ` Bill Huey
2005-03-18 15:57 ` Paul E. McKenney
2005-03-18 16:02 ` Ingo Molnar
2005-03-18 16:55 ` Esben Nielsen
2005-03-22 10:04 ` Bill Huey
2005-03-22 10:17 ` Bill Huey
2005-03-22 10:34 ` Bill Huey
2005-03-22 10:38 ` Esben Nielsen
2005-03-18 22:26 ` Herbert Xu
2005-03-19 16:31 ` Ingo Molnar
2005-03-20 8:01 ` Kyle Moffett
2005-03-22 8:08 ` Ingo Molnar
2005-03-18 15:54 ` Paul E. McKenney
2005-03-18 15:58 ` Ingo Molnar
2009-06-11 22:57 real-time preemption " James Huang
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