Re: Real-Time Preemption and RCU
From: Esben Nielsen (simlo_at_phys.au.dk)
Date: 03/23/05
- Previous message: Andrew Morton: "Re: Redirecting output"
- In reply to: Paul E. McKenney: "Re: Real-Time Preemption and RCU"
- Next in thread: Paul E. McKenney: "Re: Real-Time Preemption and RCU"
- Reply: Paul E. McKenney: "Re: Real-Time Preemption and RCU"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Date: Wed, 23 Mar 2005 12:44:52 +0100 (MET) To: "Paul E. McKenney" <paulmck@us.ibm.com>
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.
I have not yet tested it (no time :-( )
> [...]
> > > 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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
- Previous message: Andrew Morton: "Re: Redirecting output"
- In reply to: Paul E. McKenney: "Re: Real-Time Preemption and RCU"
- Next in thread: Paul E. McKenney: "Re: Real-Time Preemption and RCU"
- Reply: Paul E. McKenney: "Re: Real-Time Preemption and RCU"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Relevant Pages
|
|
