From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262469AbVCVF6T (ORCPT ); Tue, 22 Mar 2005 00:58:19 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262491AbVCVFzi (ORCPT ); Tue, 22 Mar 2005 00:55:38 -0500 Received: from e35.co.us.ibm.com ([32.97.110.133]:23798 "EHLO e35.co.us.ibm.com") by vger.kernel.org with ESMTP id S262469AbVCVFx3 (ORCPT ); Tue, 22 Mar 2005 00:53:29 -0500 Date: Mon, 21 Mar 2005 21:53:27 -0800 From: "Paul E. McKenney" To: Esben Nielsen Cc: Ingo Molnar , 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 Message-ID: <20050322055327.GB1295@us.ibm.com> Reply-To: paulmck@us.ibm.com References: <20050320223832.GA1298@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 21, 2005 at 12:23:22AM +0100, Esben Nielsen wrote: > On Sun, 20 Mar 2005, Paul E. McKenney wrote: > > > On Sun, Mar 20, 2005 at 02:29:17PM +0100, Esben Nielsen wrote: > > > On Fri, 18 Mar 2005, Ingo Molnar wrote: > > > > > > > [...] > > > > > > I think it can be deterministic (on the long timescale of memory management) > > > anyway: Boost any non-RT task entering an RCU region to the lowest RT priority. > > > This way only all the RT tasks + one non-RT task can be within those > > > regions. The RT-tasks are supposed to have some kind of upper bound to > > > their CPU-usage. The non-RT task will also finish "soon" as it is > > > boosted. If the RCU batches are also at the lowest RT-priority they can be > > > run immediately after the non-RT task is done. > > > > Hmmm... Sort of a preemptive-first-strike priority boost. Cute! ;-) > > > Well, I was actually thinking of an API like > preempt_by_nonrt_disable() > preempt_by_nonrt_enable() > working like the old preempt_disable()/preempt_enable() but still > allowing RT tasks (as well as priority inheriting non-RT tasks) to be > scheduled. That would kind of help "split" the kernel into two halfs: the > RT part and the non-RT part. The non-RT part would in many ways work as it > has always done. Does sound in some ways similar to the migration approach -- there, the RT/non-RT split is made across CPUs. But if RT is allowed to preempt, then you still have to deal with preemption for locking correctness, right? > > > > clearly the simplest solution is to go with the single-reader locks for > > > > now - a separate experiment could be done with a new type of rwlock that > > > > can only be used by the RCU code. (I'm not quite sure whether we could > > > > guarantee a minimum rate of RCU callback processing under such a scheme > > > > though. It's an eventual memory DoS otherwise.) > > > > > > > > > > Why are a lock needed at all? If it is doable without locking for an > > > non-preemptable SMP kernel it must be doable for an preemptable kernel as > > > well.I am convinced some kind of per-CPU rcu_read_count as I specified in > > > my previous mail can work some way or the other. call_rcu() might need to > > > do more complicated stuff and thus use CPU but call_rcu() is supposed to > > > be an relative rare event not worth optimizing for. Such an > > > implementation will work for any preemptable kernel, not only PREEMPT_RT. > > > For performance is considered it is important not to acquire any locks in > > > the rcu-read regions. > > > > You definitely don't need a lock -- you can just suppress preemption > > on the read side instead. But that potentially makes for long scheduling > > latencies. > > Well, in my patch I do not leave preemption off - only while doing the > simple ++/--. In effect, I let rcu_qsctr_inc know that some RCU reader > might be active, i.e. preempted, on the current CPU such that this isn't > and quiescent point after all. > (To others: Paul nicely unfolded my attachment below - I left it in > the mail such you can read it.) > The problem with this approach is ofcourse that user space programs might > preempt an RCU reader for a very long time such that RCU batches are never > really run. The boosting of non-RT tasks mentioned above would help a > lot. > A plus(?) in it: You can actually sleep while having the rcu_read_lock !! 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. > > The counter approach might work, and is also what the implementation #5 > > does -- check out rcu_read_lock() in Ingo's most recent patch. > > > > Do you refer to your original mail with implementing it in 5 steps? > In #5 in that one (-V0.7.41-00, right?) you use a lock and as you say that > forces syncronization between the CPUs - bad for scaling. It does make the > RCU batches somewhat deterministic, as the RCU task can boost the readers > to the rcu-task's priority. > The problem about this approach is that everybody calling into RCU code > have a worst case behaviour of the systemwide worst case RCU reader > section - which can be pretty large (in principle infinite if somebody.) > So if somebody uses a call to a function in the code containing a RCU read > area the worst case behavious would be the same as teh worst case latency > in the simple world where preempt_disable()/preempt_enable() was used. I missed something here -- readers would see the worst-case -writer- latency rather than the worst-case -reader- latency, right? Or are you concerned about the case where some reader blocks the write-side acquisitions in _synchronize_kernel(), but not before the writer has grabbed a lock, blocking any readers on the corresponding CPU? Yes, but this is true of every other lock in the system as well, not? In a separate conversation a few weeks ago, Steve Hemminger suggested placing checks in long RCU read-side critical sections -- this could be used to keep the worst-case reader latency within the desired bounds. Not pretty, but, then again, bounding lock latency will require a fair number of similar changes, I suspect. Thanx, Paul > > > I tried this approach. My UP labtop did boot on it, but I haven't testet > > > it further. I have included the very small patch as an attachment. > > > > > > > Ingo > > > > > > I have not yet looked at -V0.7.41-00... > > > > > > Esben > > > > > > > > diff -Naur --exclude-from diff_exclude linux-2.6.11-final-V0.7.40-00/include/linux/rcupdate.h linux-2.6.11-final-V0.7.40-00-RCU/include/linux/rcupdate.h > > > --- linux-2.6.11-final-V0.7.40-00/include/linux/rcupdate.h 2005-03-11 23:40:13.000000000 +0100 > > > +++ linux-2.6.11-final-V0.7.40-00-RCU/include/linux/rcupdate.h 2005-03-19 12:47:09.000000000 +0100 > > > @@ -85,6 +85,7 @@ > > > * curlist - current batch for which quiescent cycle started if any > > > */ > > > struct rcu_data { > > > + long active_readers; > > > /* 1) quiescent state handling : */ > > > long quiescbatch; /* Batch # for grace period */ > > > int passed_quiesc; /* User-mode/idle loop etc. */ > > > @@ -115,12 +116,14 @@ > > > static inline void rcu_qsctr_inc(int cpu) > > > { > > > struct rcu_data *rdp = &per_cpu(rcu_data, cpu); > > > - rdp->passed_quiesc = 1; > > > + if(rdp->active_readers==0) > > > + rdp->passed_quiesc = 1; > > > } > > > static inline void rcu_bh_qsctr_inc(int cpu) > > > { > > > struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu); > > > - rdp->passed_quiesc = 1; > > > + if(rdp->active_readers==0) > > > + rdp->passed_quiesc = 1; > > > } > > > > > > static inline int __rcu_pending(struct rcu_ctrlblk *rcp, > > > @@ -183,29 +186,27 @@ > > > * > > > * It is illegal to block while in an RCU read-side critical section. > > > */ > > > -#define rcu_read_lock() preempt_disable() > > > +static inline void rcu_read_lock(void) > > > +{ > > > + preempt_disable(); > > > + __get_cpu_var(rcu_data).active_readers++; > > > + preempt_enable(); > > > +} > > > > > > /** > > > * rcu_read_unlock - marks the end of an RCU read-side critical section. > > > * > > > * See rcu_read_lock() for more information. > > > */ > > > -#define rcu_read_unlock() preempt_enable() > > > +static inline void rcu_read_unlock(void) > > > +{ > > > + preempt_disable(); > > > + __get_cpu_var(rcu_data).active_readers--; > > > + preempt_enable(); > > > +} > > > > > > #define IGNORE_LOCK(op, lock) do { (void)(lock); op(); } while (0) > > > > > > -#ifdef CONFIG_PREEMPT_RT > > > -# define rcu_read_lock_spin(lock) spin_lock(lock) > > > -# define rcu_read_unlock_spin(lock) spin_unlock(lock) > > > -# define rcu_read_lock_read(lock) read_lock(lock) > > > -# define rcu_read_unlock_read(lock) read_unlock(lock) > > > -# define rcu_read_lock_bh_read(lock) read_lock_bh(lock) > > > -# define rcu_read_unlock_bh_read(lock) read_unlock_bh(lock) > > > -# define rcu_read_lock_down_read(rwsem) down_read(rwsem) > > > -# define rcu_read_unlock_up_read(rwsem) up_read(rwsem) > > > -# define rcu_read_lock_nort() do { } while (0) > > > -# define rcu_read_unlock_nort() do { } while (0) > > > -#else > > > # define rcu_read_lock_spin(lock) IGNORE_LOCK(rcu_read_lock, lock) > > > # define rcu_read_unlock_spin(lock) IGNORE_LOCK(rcu_read_unlock, lock) > > > # define rcu_read_lock_read(lock) IGNORE_LOCK(rcu_read_lock, lock) > > > @@ -216,15 +217,10 @@ > > > # define rcu_read_unlock_nort() rcu_read_unlock() > > > # define rcu_read_lock_bh_read(lock) IGNORE_LOCK(rcu_read_lock_bh, lock) > > > # define rcu_read_unlock_bh_read(lock) IGNORE_LOCK(rcu_read_unlock_bh, lock) > > > -#endif > > > > > > -#ifdef CONFIG_PREEMPT_RT > > > -# define rcu_read_lock_sem(lock) down(lock) > > > -# define rcu_read_unlock_sem(lock) up(lock) > > > -#else > > > # define rcu_read_lock_sem(lock) IGNORE_LOCK(rcu_read_lock, lock) > > > # define rcu_read_unlock_sem(lock) IGNORE_LOCK(rcu_read_unlock, lock) > > > -#endif > > > + > > > /* > > > * So where is rcu_write_lock()? It does not exist, as there is no > > > * way for writers to lock out RCU readers. This is a feature, not > > > diff -Naur --exclude-from diff_exclude linux-2.6.11-final-V0.7.40-00/Makefile linux-2.6.11-final-V0.7.40-00-RCU/Makefile > > > --- linux-2.6.11-final-V0.7.40-00/Makefile 2005-03-11 23:40:13.000000000 +0100 > > > +++ linux-2.6.11-final-V0.7.40-00-RCU/Makefile 2005-03-19 12:41:09.000000000 +0100 > > > @@ -1,7 +1,7 @@ > > > VERSION = 2 > > > PATCHLEVEL = 6 > > > SUBLEVEL = 11 > > > -EXTRAVERSION = -RT-V0.7.40-00 > > > +EXTRAVERSION = -RT-V0.7.40-00-RCU > > > NAME=Woozy Numbat > > > > > > # *DOCUMENTATION* > > > > - > > 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/ > > > >