From: "Paul E. McKenney" <paulmck@us.ibm.com>
To: Ingo Molnar <mingo@elte.hu>
Subject: Re: Real-Time Preemption and RCU
Date: Fri, 18 Mar 2005 07:33:25 -0800 [thread overview]
Message-ID: <20050318153325.GA1299@us.ibm.com> (raw)
In-Reply-To: <20050318095327.GA15190@elte.hu>
On Fri, Mar 18, 2005 at 10:53:27AM +0100, Ingo Molnar wrote:
>
> * Ingo Molnar <mingo@elte.hu> wrote:
>
> > there's one detail on PREEMPT_RT though (which i think you noticed too).
> >
> > Priority inheritance handling can be done in a pretty straightforward
> > way as long as no true read-side nesting is allowed for rwsems and
> > rwlocks - i.e. there's only one owner of a lock at a time. So
> > PREEMPT_RT restricts rwsem and rwlock concurrency: readers are
> > writers, with the only exception that they are allowed to 'self-nest'.
> > [...]
>
> this does not affect read-side RCU, because read-side RCU can never
> block a higher-prio thread. (as long as callback processing is pushed
> into a separate kernel thread.)
>
> so RCU will be pretty much the only mechanism (besides lock-free code)
> that allows reader concurrency on PREEMPT_RT.
This is a relief! I was wondering how on earth I was going to solve
the multi-task priority-inheritance problem!
But... How do we handle the following scenario?
0. A bunch of low-priority threads are preempted in the
middle of various RCU read-side critical sections.
1. High-priority thread does kmalloc(), but there is no
memory, so it blocks.
2. OOM handling notices, and decides to clean up the outstanding
RCU callbacks. It therefore invokes _synchronize_kernel()
(in implementation #5).
3. The _synchronize_kernel() function tries to acquire all of
the read-side locks, which are held by numerous preempted
low-priority threads.
4. What now???
Or does the current patch do priority inheritance across the memory
allocator? In other words, can we get away with ignoring this one? ;-)
Thanx, Paul
next prev parent reply other threads:[~2005-03-18 15:51 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 [this message]
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
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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