From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754087AbZBIFRz (ORCPT ); Mon, 9 Feb 2009 00:17:55 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751505AbZBIFRq (ORCPT ); Mon, 9 Feb 2009 00:17:46 -0500 Received: from tomts20-srv.bellnexxia.net ([209.226.175.74]:42722 "EHLO tomts20-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751182AbZBIFRp (ORCPT ); Mon, 9 Feb 2009 00:17:45 -0500 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AhoFAA9Ij0lMQWt2/2dsb2JhbACBbssmhBoG Date: Mon, 9 Feb 2009 00:17:37 -0500 From: Mathieu Desnoyers To: "Paul E. McKenney" Cc: ltt-dev@lists.casi.polymtl.ca, linux-kernel@vger.kernel.org Subject: Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost) Message-ID: <20090209051737.GA29254@Krystal> References: <20090206030543.GB8560@Krystal> <20090206045841.GA12995@Krystal> <20090206130640.GB10918@linux.vnet.ibm.com> <20090206163432.GF10918@linux.vnet.ibm.com> <20090208224419.GA19512@Krystal> <20090209041153.GR7120@linux.vnet.ibm.com> <20090209045352.GA28653@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <20090209045352.GA28653@Krystal> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 00:16:59 up 39 days, 5:15, 4 users, load average: 0.16, 0.37, 0.39 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Mathieu Desnoyers (compudj@krystal.dyndns.org) wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > On Sun, Feb 08, 2009 at 05:44:19PM -0500, Mathieu Desnoyers wrote: > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > On Fri, Feb 06, 2009 at 05:06:40AM -0800, Paul E. McKenney wrote: > > > > > On Thu, Feb 05, 2009 at 11:58:41PM -0500, Mathieu Desnoyers wrote: > > > > > > (sorry for repost, I got the ltt-dev email wrong in the previous one) > > > > > > > > > > > > Hi Paul, > > > > > > > > > > > > I figured out I needed some userspace RCU for the userspace tracing part > > > > > > of LTTng (for quick read access to the control variables) to trace > > > > > > userspace pthread applications. So I've done a quick-and-dirty userspace > > > > > > RCU implementation. > > > > > > > > > > > > It works so far, but I have not gone through any formal verification > > > > > > phase. It seems to work on paper, and the tests are also OK (so far), > > > > > > but I offer no guarantee for this 300-lines-ish 1-day hack. :-) If you > > > > > > want to comment on it, it would be welcome. It's a userland-only > > > > > > library. It's also currently x86-only, but only a few basic definitions > > > > > > must be adapted in urcu.h to port it. > > > > > > > > > > > > Here is the link to my git tree : > > > > > > > > > > > > git://lttng.org/userspace-rcu.git > > > > > > > > > > > > http://lttng.org/cgi-bin/gitweb.cgi?p=userspace-rcu.git;a=summary > > > > > > > > > > Very cool!!! I will take a look! > > > > > > > > > > I will also point you at a few that I have put together: > > > > > > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git > > > > > > > > > > (In the CodeSamples/defer directory.) > > > > > > > > Interesting approach, using the signal to force memory-barrier execution! > > > > > > > > o One possible optimization would be to avoid sending a signal to > > > > a blocked thread, as the context switch leading to blocking > > > > will have implied a memory barrier -- otherwise it would not > > > > be safe to resume the thread on some other CPU. That said, > > > > not sure whether checking to see whether a thread is blocked is > > > > any faster than sending it a signal and forcing it to wake up. > > > > > > I'm not sure it will be any faster, and it could be racy too. How would > > > you envision querying the execution state of another thread ? > > > > For my 64-bit implementation (or the old slow 32-bit version), the trick > > would be to observe that the thread didn't do an RCU read-side critical > > section during the past grace period. This observation would be by > > comparing counters. > > > > For the new 32-bit implementation, the only way I know of is to grovel > > through /proc, which would probably be slower than just sending the > > signal. > > > > Yes, I guess the signal is not so bad. > > > > > Of course, this approach does require that the enclosing > > > > application be willing to give up a signal. I suspect that most > > > > applications would be OK with this, though some might not. > > > > > > If we want to make this transparent to the application, we'll have to > > > investigate further in sigaction() and signal() library override I > > > guess. > > > > Certainly seems like it is worth a try! > > > > > > Of course, I cannot resist pointing to an old LKML thread: > > > > > > > > http://lkml.org/lkml/2001/10/8/189 > > > > > > > > But I think that the time is now right. ;-) > > > > > > > > o I don't understand the purpose of rcu_write_lock() and > > > > rcu_write_unlock(). I am concerned that it will lead people > > > > to decide that a single global lock must protect RCU updates, > > > > which is of course absolutely not the case. I strongly > > > > suggest making these internal to the urcu.c file. Yes, > > > > uses of urcu_publish_content() would then hit two locks (the > > > > internal-to-urcu.c one and whatever they are using to protect > > > > their data structure), but let's face it, if you are sending a > > > > signal to each and every thread, the additional overhead of the > > > > extra lock is the least of your worries. > > > > > > > > > > Ok, just changed it. > > > > Thank you!!! > > > > > > If you really want to heavily optimize this, I would suggest > > > > setting up a state machine that permits multiple concurrent > > > > calls to urcu_publish_content() to share the same set of signal > > > > invocations. That way, if the caller has partitioned the > > > > data structure, global locking might be avoided completely > > > > (or at least greatly restricted in scope). > > > > > > > > > > That brings an interesting question about urcu_publish_content : > > > > > > void *urcu_publish_content(void **ptr, void *new) > > > { > > > void *oldptr; > > > > > > internal_urcu_lock(); > > > oldptr = *ptr; > > > *ptr = new; > > > > > > switch_qparity(); > > > switch_qparity(); > > > internal_urcu_unlock(); > > > > > > return oldptr; > > > } > > > > > > Given that we take a global lock around the pointer assignment, we can > > > safely assume, from the caller's perspective, that the update will > > > happen as an "xchg" operation. So if the caller does not have to copy > > > the old data, it can simply publish the new data without taking any > > > lock itself. > > > > > > So the question that arises if we want to remove global locking is : > > > should we change this > > > > > > oldptr = *ptr; > > > *ptr = new; > > > > > > for an atomic xchg ? > > > > Makes sense to me! > > > > > > Of course, if updates are rare, the optimization would not > > > > help, but in that case, acquiring two locks would be even less > > > > of a problem. > > > > > > I plan updates to be quite rare, but it's always good to foresee how > > > that kind of infrastructure could be misused. :-) > > > > ;-) ;-) ;-) > > > > > > o Is urcu_qparity relying on initialization to zero? Or on the > > > > fact that, for all x, 1-x!=x mod 2^32? Ah, given that this is > > > > used to index urcu_active_readers[], you must be relying on > > > > initialization to zero. > > > > > > Yes, starts at 0. > > > > Whew! ;-) > > > > > > o In rcu_read_lock(), why is a non-atomic increment of the > > > > urcu_active_readers[urcu_parity] element safe? Are you > > > > relying on the compiler generating an x86 add-to-memory > > > > instruction? > > > > > > > > Ditto for rcu_read_unlock(). > > > > > > > > Ah, never mind!!! I now see the __thread specification, > > > > and the keeping of references to it in the reader_data list. > > > > > > Exactly :) > > > > Getting old and blind, what can I say? > > > > > > o Combining the equivalent of rcu_assign_pointer() and > > > > synchronize_rcu() into urcu_publish_content() is an interesting > > > > approach. Not yet sure whether or not it is a good idea. I > > > > guess trying it out on several applications would be the way > > > > to find out. ;-) > > > > > > > > That said, I suspect that it would be very convenient in a > > > > number of situations. > > > > > > I thought so. It seemed to be a natural way to express it to me. Usage > > > will tell. > > > > ;-) > > > > > > o It would be good to avoid having to pass the return value > > > > of rcu_read_lock() into rcu_read_unlock(). It should be > > > > possible to avoid this via counter value tricks, though this > > > > would add a bit more code in rcu_read_lock() on 32-bit machines. > > > > (64-bit machines don't have to worry about counter overflow.) > > > > > > > > See the recently updated version of CodeSamples/defer/rcu_nest.[ch] > > > > in the aforementioned git archive for a way to do this. > > > > (And perhaps I should apply this change to SRCU...) > > > > > > See my other mail about this. > > > > And likewise! > > > > > > o Your test looks a bit strange, not sure why you test all the > > > > different variables. It would be nice to take a test duration > > > > as an argument and run the test for that time. > > > > > > I made a smaller version which only reads a single variable. I agree > > > that the initial test was a bit strange on that aspect. > > > > > > I'll do a version which takes a duration as parameter. > > > > I strongly recommend taking a look at my CodeSamples/defer/rcutorture.h > > file in my git archive: > > > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git > > > > This torture test detects the missing second flip 15 times during a > > 10-second test on a two-processor machine. > > > > The first part of the rcutorture.h file is performance tests -- search > > for the string "Stress test" to find the torture test. > > > > I will. > > > > > I killed the test after better part of an hour on my laptop, > > > > will retry on a larger machine (after noting the 18 threads > > > > created!). (And yes, I first tried Power, which objected > > > > strenously to the "mfence" and "lock; incl" instructions, > > > > so getting an x86 machine to try on.) > > > > > > That should be easy enough to fix. A bit of primitive cut'n'paste would > > > do. > > > > Yep. Actually, I was considering porting your code into my environment, > > which already has the Power primitives. Any objections? (This would > > have the side effect of making a version available via perfbook.git. > > I would of course add comments referencing your git archive as the > > official version.) > > > > Yes, no objection. I am currently looking at your last patch, cleaning > it up and making the 32 and 64-bit code the same. Also trying to save a > few instructions. I'll keep you posted when it's ready and committed. > The new version is pushed into the repository. I changed you patch a bit. Flaming is welcome. :) Mathieu > Mathieu > > > > > Again, looks interesting! Looks plausible, although I have not 100% > > > > convinced myself that it is perfectly bug-free. But I do maintain > > > > a healthy skepticism of purported RCU algorithms, especially ones that > > > > I have written. ;-) > > > > > > > > > > That's always good. I also tend to always be very skeptical about what I > > > write and review. > > > > > > Thanks for the thorough review. > > > > No problem -- it has been quite fun! ;-) > > > > Thanx, Paul > > > > -- > Mathieu Desnoyers > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 > > _______________________________________________ > ltt-dev mailing list > ltt-dev@lists.casi.polymtl.ca > http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev > -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68