Re: [RFC git tree] Userspace RCU (urcu) for Linux (repost)

[Paul E. McKenney]

On Sun, Feb 08, 2009 at 05:44:19PM -0500, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) 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.

> > 	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 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.)

> > 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
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