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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Feb 10, 2009 at 07:57:01PM -0500, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > On Tue, Feb 10, 2009 at 04:28:33PM -0500, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > > On Tue, Feb 10, 2009 at 02:17:31PM -0500, Mathieu Desnoyers wrote:
> > > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > > > > On Mon, Feb 09, 2009 at 02:03:17AM -0500, Mathieu Desnoyers wrote:
> > > > > > 
> > > > > > [ . . . ]
> > > > > > 
> > > > > > > I just added modified rcutorture.h and api.h from your git tree
> > > > > > > specifically for an urcutorture program to the repository. Some results :
> > > > > > > 
> > > > > > > 8-way x86_64
> > > > > > > E5405 @2 GHZ
> > > > > > > 
> > > > > > > ./urcutorture 8 perf
> > > > > > > n_reads: 1937650000  n_updates: 3  nreaders: 8  nupdaters: 1 duration: 1
> > > > > > > ns/read: 4.12871  ns/update: 3.33333e+08
> > > > > > > 
> > > > > > > ./urcutorture 8 uperf
> > > > > > > n_reads: 0  n_updates: 4413892  nreaders: 0  nupdaters: 8 duration: 1
> > > > > > > ns/read: nan  ns/update: 1812.46
> > > > > > > 
> > > > > > > n_reads: 98844204  n_updates: 10  n_mberror: 0
> > > > > > > rcu_stress_count: 98844171 33 0 0 0 0 0 0 0 0 0
> > > > > > > 
> > > > > > > However, I've tried removing the second switch_qparity() call, and the
> > > > > > > rcutorture test did not detect anything wrong. I also did a variation
> > > > > > > which calls the "sched_yield" version of the urcu, "urcutorture-yield".
> > > > > > 
> > > > > > My confusion -- I was testing my old approach where the memory barriers
> > > > > > are in rcu_read_lock() and rcu_read_unlock().  To force the failures in
> > > > > > your signal-handler-memory-barrier approach, I suspect that you are
> > > > > > going to need a bigger hammer.  In this case, one such bigger hammer
> > > > > > would be:
> > > > > > 
> > > > > > o	Just before exit from the signal handler, do a
> > > > > > 	pthread_cond_wait() under a pthread_mutex().
> > > > > > 
> > > > > > o	In force_mb_all_threads(), refrain from sending a signal to self.
> > > > > > 
> > > > > > 	Then it should be safe in force_mb_all_threads() to do a
> > > > > > 	pthread_cond_broadcast() under the same pthread_mutex().
> > > > > > 
> > > > > > This should raise the probability of seeing the failure in the case
> > > > > > where there is a single switch_qparity().
> > > > > > 
> > > > > 
> > > > > I just did a mb() version of the urcu :
> > > > > 
> > > > > (uncomment CFLAGS=+-DDEBUG_FULL_MB in the Makefile)
> > > > > 
> > > > > Time per read : 48.4086 cycles
> > > > > (about 6-7 times slower, as expected)
> > > > > 
> > > > > This will be useful especially to increase the chance to trigger races.
> > > > > 
> > > > > I tried removing the second parity switch from the writer. The rcu
> > > > > torture test did not find the problem yet (maybe I am not using the
> > > > > correct parameters ? It does not run for more than 5 seconds).
> > > > > 
> > > > > So I added a "-n" option to test_urcu, so it can make the usleep(1)
> > > > > between the writes optional. I also changed the yield for a usleep with
> > > > > random delay. I also now use a circular buffer rather than malloc so we
> > > > > are sure the memory is not quickly reused by the writer and stays longer
> > > > > in an invalid state.
> > > > > 
> > > > > So what really make the problem appear quickly is to add a delay between
> > > > > the rcu_dereference and the assertion on the data validity in thr_reader.
> > > > > 
> > > > > It now appears after just a few seconds when running
> > > > > ./test_urcu_yield 20 -r -n
> > > > > Compiled with CFLAGS=+-DDEBUG_FULL_MB
> > > > > 
> > > > > It seem to be much harder to trigger with the signal-based version. It's
> > > > > expected, because the writer takes about 50 times longer to execute than
> > > > > with the -DDEBUG_FULL_MB version.
> > > > > 
> > > > > So I'll let the ./test_urcu_yield NN -r -n run for a while on the
> > > > > correct version (with DEBUG_FULL_MB) and see what it gives.
> > > > 
> > > > Hmmm...  I had worse luck this time, took three 10-second tries to
> > > > see a failure:
> > > > 
> > > > paulmck@paulmck-laptop:~/paper/perfbook/CodeSamples/defer$ ./rcu_nest32 1 stress
> > > > n_reads: 44682055  n_updates: 9609503  n_mberror: 0
> > > > rcu_stress_count: 44679377 2678 0 0 0 0 0 0 0 0 0
> > > > paulmck@paulmck-laptop:~/paper/perfbook/CodeSamples/defer$ !!
> > > > ./rcu_nest32 1 stress
> > > > n_reads: 42281884  n_updates: 9870129  n_mberror: 0
> > > > rcu_stress_count: 42277756 4128 0 0 0 0 0 0 0 0 0
> > > > paulmck@paulmck-laptop:~/paper/perfbook/CodeSamples/defer$ !!
> > > > ./rcu_nest32 1 stress
> > > > n_reads: 41384304  n_updates: 10040805  n_mberror: 0
> > > > rcu_stress_count: 41380075 4228 1 0 0 0 0 0 0 0 0
> > > > paulmck@paulmck-laptop:~/paper/perfbook/CodeSamples/defer$
> > > > 
> > > > This is my prototype version, with read-side memory barriers, no
> > > > signals, and without your initialization-value speedup.
> > > > 
> > > 
> > > It would be interesting to re-sync our trees, or if you can point me to
> > > a current version of your prototype, I could review it.
> > 
> > Look at:
> > 
> > 	CodeSamples/defer/rcu_nest32.[hc]
> > 
> > In the git archive:
> > 
> > 	git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git
> 
> flip_counter_and_wait : yours do rcu_gp_ctr += RCU_GP_CTR_BOTTOM_BIT
> mine : rcu_gp_ctr ^= RCU_GP_CTR_BOTTOM_BIT.

Yep, this is before your optimization.

> Another major difference between our tree is the lack of smp_mb() at the
> end of flip_counter_and_wait() (in your tree).
> 
> Your code does :
> 
>   smp_mb()
>   switch parity
>   smp_mb()
>   wait for each thread ongoing old gp
>     <<<<<<< ---- missing smp_mb.
>   switch parity
>   smp_mb()
>   wait for each thread ongoing old gp
>   smp_mb()

This should be OK -- or am I missing a failure scenario?
Keep in mind that I get failures only when omitting a counter
flip, not with the above code.

> I also wonder why you have a smp_mb() after spin_unlock() in your
> synchronize_rcu() -> if you follow the Linux kernel semantics for
> spinlocks, the smp_mb() should be implied. (but I have not looked at
> your spin_lock/unlock primitives yet).

Perhaps things have changed, but last I knew, spin_lock() and
spin_unlock() were only required to keep the critical section in, not
to keep things out of the critical section.

							Thanx, Paul

> Mathieu
> 
> > 							Thanx, Paul
> > 
> > _______________________________________________
> > 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