From: Mathieu Desnoyers <compudj@krystal.dyndns.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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)
Date: Mon, 9 Feb 2009 00:17:37 -0500 [thread overview]
Message-ID: <20090209051737.GA29254@Krystal> (raw)
In-Reply-To: <20090209045352.GA28653@Krystal>
* 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
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>
--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
next prev parent reply other threads:[~2009-02-09 5:17 UTC|newest]
Thread overview: 116+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-02-06 3:05 [RFC git tree] Userspace RCU (urcu) for Linux Mathieu Desnoyers
2009-02-06 4:58 ` [RFC git tree] Userspace RCU (urcu) for Linux (repost) Mathieu Desnoyers
2009-02-06 13:06 ` Paul E. McKenney
2009-02-06 16:34 ` Paul E. McKenney
2009-02-07 15:10 ` Paul E. McKenney
2009-02-07 22:16 ` Paul E. McKenney
2009-02-08 0:19 ` Mathieu Desnoyers
2009-02-07 23:38 ` Mathieu Desnoyers
2009-02-08 0:44 ` Paul E. McKenney
2009-02-08 21:46 ` Mathieu Desnoyers
2009-02-08 22:36 ` Paul E. McKenney
2009-02-09 0:24 ` Paul E. McKenney
2009-02-09 0:54 ` Mathieu Desnoyers
2009-02-09 1:08 ` [ltt-dev] " Mathieu Desnoyers
2009-02-09 3:47 ` Paul E. McKenney
2009-02-09 3:42 ` Paul E. McKenney
2009-02-09 0:40 ` [ltt-dev] " Mathieu Desnoyers
2009-02-08 22:44 ` Mathieu Desnoyers
2009-02-09 4:11 ` Paul E. McKenney
2009-02-09 4:53 ` Mathieu Desnoyers
2009-02-09 5:17 ` Mathieu Desnoyers [this message]
2009-02-09 7:03 ` [ltt-dev] " Mathieu Desnoyers
2009-02-09 15:33 ` Paul E. McKenney
2009-02-10 19:17 ` Mathieu Desnoyers
2009-02-10 21:16 ` Paul E. McKenney
2009-02-10 21:28 ` Mathieu Desnoyers
2009-02-10 22:21 ` Paul E. McKenney
2009-02-10 22:58 ` Paul E. McKenney
2009-02-10 23:01 ` Paul E. McKenney
2009-02-11 0:57 ` Mathieu Desnoyers
2009-02-11 5:28 ` Paul E. McKenney
2009-02-11 6:35 ` Mathieu Desnoyers
2009-02-11 15:32 ` Paul E. McKenney
2009-02-11 18:52 ` Mathieu Desnoyers
2009-02-11 20:09 ` Paul E. McKenney
2009-02-11 21:42 ` Mathieu Desnoyers
2009-02-11 22:08 ` Mathieu Desnoyers
[not found] ` <20090212003549.GU6694@linux.vnet.ibm.com>
2009-02-12 2:33 ` Paul E. McKenney
2009-02-12 2:37 ` Paul E. McKenney
2009-02-12 4:10 ` Mathieu Desnoyers
2009-02-12 5:09 ` Paul E. McKenney
2009-02-12 5:47 ` Mathieu Desnoyers
2009-02-12 16:18 ` Paul E. McKenney
2009-02-12 18:40 ` Mathieu Desnoyers
2009-02-12 20:28 ` Paul E. McKenney
2009-02-12 21:27 ` Mathieu Desnoyers
2009-02-12 23:26 ` Paul E. McKenney
2009-02-13 13:12 ` Mathieu Desnoyers
2009-02-12 4:08 ` Mathieu Desnoyers
2009-02-12 5:01 ` Paul E. McKenney
2009-02-12 7:05 ` Mathieu Desnoyers
2009-02-12 16:46 ` Paul E. McKenney
2009-02-12 19:29 ` Mathieu Desnoyers
2009-02-12 20:02 ` Paul E. McKenney
2009-02-12 20:09 ` Mathieu Desnoyers
2009-02-12 20:35 ` Paul E. McKenney
2009-02-12 21:15 ` Mathieu Desnoyers
2009-02-12 20:13 ` Linus Torvalds
2009-02-12 20:39 ` Paul E. McKenney
2009-02-12 21:15 ` Linus Torvalds
2009-02-12 21:59 ` Paul E. McKenney
2009-02-13 13:50 ` Nick Piggin
2009-02-13 14:56 ` Paul E. McKenney
2009-02-13 15:10 ` Mathieu Desnoyers
2009-02-13 15:55 ` Mathieu Desnoyers
2009-02-13 16:18 ` Linus Torvalds
2009-02-13 17:33 ` Mathieu Desnoyers
2009-02-13 17:53 ` Linus Torvalds
2009-02-13 18:09 ` Linus Torvalds
2009-02-13 18:54 ` Mathieu Desnoyers
2009-02-13 19:36 ` Paul E. McKenney
2009-02-14 5:07 ` Mike Frysinger
2009-02-14 5:20 ` Paul E. McKenney
2009-02-14 5:46 ` Mike Frysinger
2009-02-14 15:06 ` Paul E. McKenney
2009-02-14 17:37 ` Mike Frysinger
2009-02-22 14:23 ` Pavel Machek
2009-02-22 18:28 ` Mike Frysinger
2009-02-14 6:42 ` Mathieu Desnoyers
2009-02-14 3:15 ` [Uclinux-dist-devel] " Mike Frysinger
2009-02-13 18:40 ` Mathieu Desnoyers
2009-02-13 16:05 ` Linus Torvalds
2009-02-14 3:11 ` [Uclinux-dist-devel] " Mike Frysinger
2009-02-14 4:58 ` Robin Getz
2009-02-12 19:38 ` Mathieu Desnoyers
2009-02-12 20:17 ` Paul E. McKenney
2009-02-12 21:53 ` Mathieu Desnoyers
2009-02-12 23:04 ` Paul E. McKenney
2009-02-13 12:49 ` Mathieu Desnoyers
2009-02-11 5:08 ` Lai Jiangshan
2009-02-11 8:58 ` Mathieu Desnoyers
2009-02-09 13:23 ` Paul E. McKenney
2009-02-09 17:28 ` Mathieu Desnoyers
2009-02-09 17:47 ` Paul E. McKenney
2009-02-09 18:13 ` Mathieu Desnoyers
2009-02-09 18:19 ` Mathieu Desnoyers
2009-02-09 18:37 ` Paul E. McKenney
2009-02-09 18:49 ` Paul E. McKenney
2009-02-09 19:05 ` Mathieu Desnoyers
2009-02-09 19:15 ` Mathieu Desnoyers
2009-02-09 19:35 ` Paul E. McKenney
2009-02-09 19:23 ` Paul E. McKenney
2009-02-09 13:16 ` Paul E. McKenney
2009-02-09 17:19 ` Bert Wesarg
2009-02-09 17:34 ` Paul E. McKenney
2009-02-09 17:35 ` Bert Wesarg
2009-02-09 17:40 ` Paul E. McKenney
2009-02-09 17:42 ` Mathieu Desnoyers
2009-02-09 18:00 ` Paul E. McKenney
2009-02-09 17:45 ` Bert Wesarg
2009-02-09 17:59 ` Paul E. McKenney
2009-02-07 22:56 ` Kyle Moffett
2009-02-07 23:50 ` Mathieu Desnoyers
2009-02-08 0:13 ` Paul E. McKenney
2009-02-06 8:55 ` [RFC git tree] Userspace RCU (urcu) for Linux Bert Wesarg
2009-02-06 11:36 ` Mathieu Desnoyers
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