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Paul E. McKenney [Thu, 12 Feb 2009 18:25:05 +0000 (13:25 -0500)]
Fix formal model nesting
Also, the original model I sent out has a minor bug that prevents it
from fully modeling the nested-read-side case. The patch below fixes this.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Thu, 12 Feb 2009 06:44:34 +0000 (01:44 -0500)]
Add read/write counts to test
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Thu, 12 Feb 2009 06:43:54 +0000 (01:43 -0500)]
Fix force_mb_all_threads must be called within internal local
It iterates on threads, and must therefore be called within the mutex.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Thu, 12 Feb 2009 06:00:00 +0000 (01:00 -0500)]
Add barriers
Use smp_rmb() is busy-loops to make sure we re-read the variable.
Kick the reader threads we are waiting for after a few loops.
Documents memory ordering in rcu_read_lock().
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Thu, 12 Feb 2009 04:52:31 +0000 (23:52 -0500)]
Remove debug yield statements
Just too ugly.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Thu, 12 Feb 2009 04:47:15 +0000 (23:47 -0500)]
use smp_*mb()
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Thu, 12 Feb 2009 03:49:22 +0000 (22:49 -0500)]
Add Promela model
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Wed, 11 Feb 2009 21:52:54 +0000 (16:52 -0500)]
Add missing memory barriers to ensure progress and remove an unnecessary ACCESS_ONCE
[...]
So we could have the following reader+writer sequence :
Interleaved writer Sequence 2 and reader seq. 1.
Reader:
R.1 - read urcu_active_readers
S.2 - read counter
Writer:
B - read other threads urcu_active_readers (there are none)
A.1 - flip counter
A.2 - read counter
Reader:
RS.2- write urcu_active_readers, depends on read counter and read
urcu_active_readers
Here, the reader would have updated its counter as belonging to the old
q.s. period, but the writer will later wait for the new period. But
given the writer will eventually do a second flip+wait, the reader in
the other q.s. window will be caught by the second flip.
Therefore, we could be tempted to think that those mb() could be
unnecessary, which would lead to a scheme where urcu_active_readers and
urcu_gp_ctr are done in a completely random order one vs the other.
Let's see what it gives :
synchronize_rcu()
force_mb_all_threads() /*
* Orders pointer publication and
* (urcu_active_readers/urcu_gp_ctr accesses)
*/
switch_qparity()
switch_next_urcu_qparity() [just does counter flip 0->1]
wait_for_quiescent_state()
wait for all threads in parity 0
switch_qparity()
switch_next_urcu_qparity() [Just does counter flip 1->0]
wait_for_quiescent_state()
Wait for all threads in parity 1
force_mb_all_threads() /*
* Orders
* (urcu_active_readers/urcu_gp_ctr accesses)
* and old data removal.
*/
*but* ! There is a reason why we don't want to do this. If
switch_next_urcu_qparity() [Just does counter flip 1->0]
happens before the end of the previous
Wait for all threads in parity 0
We enter in a situation where all newly coming readers will see the
parity bit as 0, although we are still waiting for that parity to end.
We end up in a state when the writer can be blocked forever (no possible
progress) if there are steadily readers subscribed for the data.
Basically, to put it differently, we could simply remove the bit
flipping from the writer and wait for *all* readers to exit their
critical section (even the ones simply interested in the new pointer).
But this shares the same problem the version above has, which is that we
end up in a situation where the writer won't progress if there are
always readers in a critical section.
The same applies to
switch_next_urcu_qparity() [Just does counter flip 0->1]
wait for all threads in parity 0
If we don't put a mb() between those two (as I mistakenly did), we can
end up waiting for readers in parity 0 while the parity bit wasn't
flipped yet. oops. Same potential no-progress situation.
The ordering of memory reads in the reader for
urcu_active_readers/urcu_gp_ctr accesses does not seem to matter because
the data contains information about which q.s. period parity it is in.
In whichever order those variables are read seems to all work fine.
In the end, it's to insure that the writer will always progress that we
have to enforce smp_mb() between *all* switch_next_urcu_qparity and wait
for threads. Mine and yours.
The code in rcu_old_gp_ongoing (in my git tree) uses ACCESS_ONCE around
urcu_active_readers and urcu_gp_ctr reads. I think the ACCESS_ONCE
around urcu_gp_crt is useless because urcu_gp_ctr is only being modified
by ourself and we hold a mutex.
However, making sure that urcu_active_readers is only read once is
clearly required.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Tue, 10 Feb 2009 18:57:53 +0000 (13:57 -0500)]
Enhance test cases
New -n (to disable sleep in writer)
Now use a random delay for -w and -r.
Add -DDEBUG_FULL_MB option to use mb() in the reader rather than the
signal-based alternative.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 19:05:19 +0000 (14:05 -0500)]
Add comment in rcu_add_lock
Comment optimization to make it clear to the reader.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 18:52:41 +0000 (13:52 -0500)]
Fix int->long and keep a reader count of 1 in the global GP variable
- Forgot a few migration from int to long
- Accelerate fast path by reading a GP variable which already has 1 reader
count. This saves an increment in the fast path.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 18:48:38 +0000 (13:48 -0500)]
Fix RCU_GP_CTR_BIT
> > You lost me on this one:
> >
> > sizeof(long) << 2 = 0x10
> >
> > I could believe the following (run on a 32-bit machine):
> >
> > 1 << (sizeof(long) * 8 - 1) = 0x80000000
> >
> > Or, if you were wanting to use a bit halfway up the word, perhaps this:
> >
> > 1 << (sizeof(long) * 4 - 1) = 0x8000
> >
> > Or am I confused?
>
> Well, I am at least partly confused. You were wanting a low-order bit,
> so you want to lose the "- 1" above. Here are some of the possibilities:
>
> sizeof(long) = 0x4
> sizeof(long) << 2 = 0x10
> 1 << (sizeof(long) * 8 - 1) = 0x80000000
> 1 << (sizeof(long) * 4) = 0x10000
> 1 << (sizeof(long) * 4 - 1) = 0x8000
> 1 << (sizeof(long) * 2) = 0x100
> 1 << (sizeof(long) * 2 - 1) = 0x80
>
> My guess is that 1 << (sizeof(long) * 4) and 1 << (sizeof(long) * 2)
> are of the most interest.
>
Exactly. I'll change it to :
I somehow thought this define was used as a bit number rather than the
bit mask.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 18:00:31 +0000 (13:00 -0500)]
Fix data type, should now be long rather than int.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 17:55:09 +0000 (12:55 -0500)]
Raise the number of nested readers limit 2^16 and 2^32 on 32 and 64-bits arch.
> I don't know of any code in the Linux kernel that nests rcu_read_lock()
> anywhere near that deep. And if someone does find such a case, it is
> pretty easy to use 15 bits rather than 8 to hold the nesting depth, just
> by changing the definition of RCU_GP_CTR_BIT.
>
You know what ? Changing RCU_GP_CTR_BIT to 16 uses a
testw %ax, %ax instead of a testb %al, %al. The trick here is that
RCU_GP_CTR_BIT must be a multiple of 8 so we can use a full 8-bits,
16-bits or 32-bits bitmask for the lower order bits.
On 64-bits, using a RCU_GP_CTR_BIT of 32 is also ok. It uses a testl.
To provide 32-bits compability and allow the deepest nesting possible, I
think it makes sense to use
/* Use the amount of bits equal to half of the architecture long size */
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 17:58:11 +0000 (12:58 -0500)]
Fix get_cycles for 32-bits x86
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 17:06:53 +0000 (12:06 -0500)]
don't __USE_GNU in urcu.h
Not required. Let the .c do it.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 17:05:45 +0000 (12:05 -0500)]
Include pthread.h in urcu.h
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 17:04:15 +0000 (12:04 -0500)]
Add branch prediction, fix xchg for -mtune=core2
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 06:48:04 +0000 (01:48 -0500)]
Add urcu-asm.c
Small file to get the assembly output of fast path.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 06:44:01 +0000 (01:44 -0500)]
Add rcutorture with yield
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 06:31:05 +0000 (01:31 -0500)]
Add rcutorture
Add a modified version of rcutorture.h and api.h.
Used bu urcutorture.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 06:27:48 +0000 (01:27 -0500)]
Fix lock -> unlock in synchronize_rcu
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 05:54:50 +0000 (00:54 -0500)]
Use xchg in publish content
Also changes the publish content parameter. Now takes the pointer itself as
first parameter rather than a pointer to the pointer.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 05:29:58 +0000 (00:29 -0500)]
Add rcu_assign_pointer
rcu_assign_pointer has a memory barrier which lets the writer make sure the data
has been properly written to memory before setting the pointer.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 04:56:15 +0000 (23:56 -0500)]
Remove parameter from rcu_read_lock()
Also makes the read fast-path twice faster :
7 cycles instead of 14 on a 8-cores x86_64.
Mathieu :
I limited the amount of nested readers to 256. Should be enough and lets us use
testb generically.
Changed the 64-bits code to make it the same as 32-bits. I prefer to have the
exact same behavior on both architectures.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 00:21:34 +0000 (19:21 -0500)]
Add randomness to yield debug test
Add randomness to allow 1/2 odds that the yield will be done.
This lets some paths go quickly and others not.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Mon, 9 Feb 2009 00:01:58 +0000 (19:01 -0500)]
Add DEBUG_YIELD, add test duration
Add some testing calling the scheduler, and add a duration parameter to
test_urcu.c.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Sun, 8 Feb 2009 22:10:22 +0000 (17:10 -0500)]
Change API
* rcu_read_lock
> A patch below to allow nested rcu_read_lock() while keeping to the Linux
> kernel API, just FYI. One can argue that the overhead of accessing the
> extra per-thread variables is offset by the fact that there no longer
> needs to be a return value from rcu_read_lock() nor an argument to
> rcu_read_unlock(), but hard to say.
>
I ran your modified version within my benchmarks :
with return value : 14.164 cycles per read
without return value : 16.4017 cycles per read
So we have a 14% performance decrease due to this. We also pollute the
branch prediction buffer and we add a cache access due to the added
variables in the TLS. Returning the value has the clear advantage of
letting the compiler keep it around in registers or on the stack, which
clearly costs less.
So I think the speed factor outweights the visual considerations. Maybe
we could switch to something like :
unsigned int qparity;
urcu_read_lock(&qparity);
...
urcu_read_unlock(&qparity);
That would be a bit like local_irq_save() in the kernel, except that we
could do it in a static inline because we pass the address. I
personnally dislike the local_irq_save() way of hiding the fact that it
writes to the variable in a "clever" macro. I'd really prefer to leave
the " & ".
* rcu_write_lock
Removed.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Sun, 8 Feb 2009 20:42:08 +0000 (15:42 -0500)]
Add timing tests
initial results :
On a 8-cores x86_64
test_rwlock_timing.c : 4339.07 cycles
test_urcu_timing.c : 14.16 cycles
Speedup : 306
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Paul E. McKenney [Sun, 8 Feb 2009 00:00:55 +0000 (19:00 -0500)]
Do two parity flip in the writer to fix race condition
On Sat, Feb 07, 2009 at 07:10:28AM -0800, Paul E. McKenney wrote:
> So, how to fix? Here are some approaches:
>
> o Make urcu_publish_content() do two parity flips rather than one.
> I use this approach in my rcu_rcpg, rcu_rcpl, and rcu_rcpls
> algorithms in CodeSamples/defer.
>
> o Use a single free-running counter, in a manner similar to rcu_nest,
> as suggested earlier. This one is interesting, as I rely on a
> read-side memory barrier to handle the long-preemption case.
> However, if you believe that any thread that waits several minutes
> between executing adjacent instructions must have been preempted
> (which the memory barriers that are required to do a context
> switch), then a compiler barrier suffices. ;-)
>
> Of course, the probability of seeing this failure during test is quite
> low, since it is unlikely that thread 0 would run just long enough to
> execute its signal handler. However, it could happen. And if you were
> to adapt this algorithm for use in a real-time application, then priority
> boosting could cause this to happen naturally.
And here is a patch, taking the first approach. It also exposes a
synchronize_rcu() API that is used by the existing urcu_publish_content()
API. This allows easier handling of structures that are referenced by
more than one pointer. And should also allow to be more easily plugged
into my rcutorture test. ;-)
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Bert Wesarg [Sat, 7 Feb 2009 23:59:29 +0000 (18:59 -0500)]
test_urcu.c: use gettid()
It's probably better to print the tid for each thread, not the pid.
Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Bert Wesarg [Fri, 6 Feb 2009 11:45:42 +0000 (06:45 -0500)]
URCU : use pthread_equal()
But you should use pthread_equal() for you equality test of pthread_t.
Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Fri, 6 Feb 2009 11:13:49 +0000 (06:13 -0500)]
Run longer tests
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Mathieu Desnoyers [Fri, 6 Feb 2009 03:25:51 +0000 (22:25 -0500)]
update Makefile, -Wall
Mathieu Desnoyers [Fri, 6 Feb 2009 02:47:01 +0000 (21:47 -0500)]
remove ugly gcc warning removal ack, simply cast the caller parameter
Mathieu Desnoyers [Fri, 6 Feb 2009 02:41:04 +0000 (21:41 -0500)]
add acknowledgements, fix gcc warnings
Mathieu Desnoyers [Fri, 6 Feb 2009 02:14:20 +0000 (21:14 -0500)]
fix wait_for_quiescent_state
Mathieu Desnoyers [Fri, 6 Feb 2009 01:49:36 +0000 (20:49 -0500)]
add licensing
Mathieu Desnoyers [Fri, 6 Feb 2009 01:46:14 +0000 (20:46 -0500)]
modify test values
Mathieu Desnoyers [Fri, 6 Feb 2009 01:04:29 +0000 (20:04 -0500)]
runs
Mathieu Desnoyers [Fri, 6 Feb 2009 01:04:03 +0000 (20:04 -0500)]
runs
Mathieu Desnoyers [Fri, 6 Feb 2009 00:42:15 +0000 (19:42 -0500)]
add make clean
Mathieu Desnoyers [Fri, 6 Feb 2009 00:41:34 +0000 (19:41 -0500)]
runs
Mathieu Desnoyers [Fri, 6 Feb 2009 00:16:25 +0000 (19:16 -0500)]
compile
Mathieu Desnoyers [Fri, 6 Feb 2009 00:06:44 +0000 (19:06 -0500)]
init version
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