Messages in this thread | | | Date | Wed, 11 Feb 2009 03:58:52 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost) |
| |
* Lai Jiangshan (laijs@cn.fujitsu.com) wrote: > Mathieu Desnoyers wrote: > > > > 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) > > > > I had read many papers of Paul. > (http://www.rdrop.com/users/paulmck/RCU/) > and I know Paul did his endeavor to remove memory barrier in > RCU read site in kernel. His work is of consequence. > > But, I think, > 1) Userspace RCU's read site can pay for the latency of > memory barrier(include atomic operator). > Userspace does not access to shared data so frequently as kernel. > and userspace's read site is not so fast as kernel. > > 2) Userspace uses RCU is for RCU's excellence, not saving a little cpu cycles > (http://lwn.net/Articles/263130/) > One of the most important excellence is lock-free. > > > If my thinking is right, the following opinion has some meaning too. > > Use All-SYSTEM 's RCU for Userspace RCU. > > All-SYSTEM 's RCU is QRCU which is implemented by Paul. > http://lwn.net/Articles/223752/ > > Any system which has mechanisms equivalent to atomic_op, > __wait_event, wake_up, mutex, This system can also implement QRCU. > So most system can implement QRCU, and I say QRCU is All-SYSTEM 's RCU. > > Obviously, we can implement a portable QRCU highly simply in NPTL. > and read lock is: > for (;;) { > int idx = qp->completed & 0x1; > if (likely(atomic_inc_not_zero(qp->ctr + idx))) > return idx; > } > "atomic_inc_not_zero" is called once likely, it's fast enough. >
Hi Lai,
There are a few reasons why we need rcu in userspace for tracing :
- We need very fast per-cpu read-side synchronization for data structure handling. Updates are rare (enabling/disabling tracing). Therefore, your argument about userspace not needing "fast" rcu does not hold in this case. Note that LTTng has the performance it has today in the kernel because I made sure to use no memory barriers when unnecessary and because I used the minimal amount of atomic operations required. Those represent costly synchronization primitives on quite a few architectures. - Being lock-free (atomic). To trace code executed in signal handlers, we need to be able to nest over any user code. With the solution you propose above, the busy-loop in the read-lock does not seems to be signal-safe : if it nests over a writer, it could busy-loop forever.
Mathieu
> Lai. > > >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
| |