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Re: [Xen-devel] [PATCH RFC v6 09/11] pvqspinlock, x86: Add qspinlock para-virtualization support

On 03/13/2014 09:57 AM, Paolo Bonzini wrote:
Il 13/03/2014 12:21, David Vrabel ha scritto:
On 12/03/14 18:54, Waiman Long wrote:
This patch adds para-virtualization support to the queue spinlock in
the same way as was done in the PV ticket lock code. In essence, the
lock waiters will spin for a specified number of times (QSPIN_THRESHOLD
= 2^14) and then halted itself. The queue head waiter will spins
2*QSPIN_THRESHOLD times before halting itself. When it has spinned
QSPIN_THRESHOLD times, the queue head will assume that the lock
holder may be scheduled out and attempt to kick the lock holder CPU
if it has the CPU number on hand.

I don't really understand the reasoning for kicking the lock holder.

I agree. If the lock holder isn't running, there's probably a good reason for that and going to sleep will not necessarily convince the scheduler to give more CPU to the lock holder. I think there are two choices:

1) use yield_to to donate part of the waiter's quantum to the lock holder? For this we probably need a new, separate hypercall interface. For KVM it would be the same as hlt in the guest but with an additional yield_to in the host.

2) do nothing, just go to sleep.

Could you get (or do you have) numbers for (2)?

I will take out the lock holder kick portion from the patch. I will also try to collect more test data.

More important, I think a barrier is missing:

    Lock holder ---------------------------------------

    // queue_spin_unlock
    ACCESS_ONCE(qlock->lock) = 0;

This is not the unlock code that is used when PV spinlock is enabled. The right unlock code is

        if (static_key_false(&paravirt_spinlocks_enabled)) {
* Need to atomically clear the lock byte to avoid racing with * queue head waiter trying to set _QSPINLOCK_LOCKED_SLOWPATH.
                if (likely(cmpxchg(&qlock->lock, _QSPINLOCK_LOCKED, 0)
                                == _QSPINLOCK_LOCKED))

        } else {

    // pv_kick_node:
    if (pv->cpustate != PV_CPU_HALTED)
    ACCESS_ONCE(pv->cpustate) = PV_CPU_KICKED;
    __queue_kick_cpu(pv->mycpu, PV_KICK_QUEUE_HEAD);

        Waiter -------------------------------------------

        // pv_head_spin_check
        ACCESS_ONCE(pv->cpustate) = PV_CPU_HALTED;
        lockval = cmpxchg(&qlock->lock,
        if (lockval == 0) {
             * Can exit now as the lock is free
            ACCESS_ONCE(pv->cpustate) = PV_CPU_ACTIVE;
            *count = 0;

Nothing protects from writing qlock->lock before pv->cpustate is read, leading to this:

    Lock holder            Waiter
    read pv->cpustate
        (it is PV_CPU_ACTIVE)
                    pv->cpustate = PV_CPU_HALTED
                    lockval = cmpxchg(...)
    qlock->lock = 0
    if (pv->cpustate != PV_CPU_HALTED)

The lock holder will read cpustate only if the lock byte has been changed to _QSPINLOCK_LOCKED_SLOWPATH. So the setting of the lock byte synchronize the 2 threads. The only thing that I am not certain is when the waiter is trying to go to sleep while, at the same time, the lock holder is trying to kick it. Will there be a missed wakeup because of this timing issue?


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