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    SubjectRe: [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


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