Re: [PATCH v2 4/5] KVM: add KVM_USER_EXIT vcpu ioctl for userspace exit

[Avi Kivity]

On 08/18/2015 10:57 PM, Paolo Bonzini wrote:
> On 18/08/2015 11:30, Avi Kivity wrote:
> > > KVM_USER_EXIT in practice should be so rare (at least with in-kernel
> > > LAPIC) that I don't think this matters.  KVM_USER_EXIT is relatively
> > > uninteresting, it only exists to provide an alternative to signals that
> > > doesn't require expensive atomics on each and every KVM_RUN. :(
> > Ah, so the idea is to remove the cost of changing the signal mask?
> Yes, it's explained in the cover letter.
>
> > Yes, although it looks like a thread-local operation, it takes a
> > process-wide lock.
> IIRC the lock was only task-wide and uncontended.  Problem is, it's on
> the node that created the thread rather than the node that is running
> it, and inter-node atomics are really, really slow.

Cached inter-node atomics are (relatively) fast, but I think it really 
is a process-wide lock:

sigprocmask calls:

void __set_current_blocked(const sigset_t *newset)
{
    struct task_struct *tsk = current;

    spin_lock_irq(&tsk->sighand->siglock);
    __set_task_blocked(tsk, newset);
    spin_unlock_irq(&tsk->sighand->siglock);
}

struct sighand_struct {
    atomic_t        count;
    struct k_sigaction    action[_NSIG];
    spinlock_t        siglock;
    wait_queue_head_t    signalfd_wqh;
};

Since sigaction is usually process-wide, I conclude that so will 
tsk->sighand.



> For guests spanning >1 host NUMA nodes it's not really practical to
> ensure that the thread is created on the right node.  Even for guests
> that fit into 1 host node, if you rely on AutoNUMA the VCPUs are created
> too early for AutoNUMA to have any effect.  And newer machines have
> frighteningly small nodes (two nodes per socket, so it's something like
> 7 pCPUs if you don't have hyper-threading enabled).  True, the NUMA
> penalty within the same socket is not huge, but it still costs a few
> thousand clock cycles on vmexit.flat and this feature sweeps it away
> completely.
>
> > I expect most user wakeups are via irqfd, so indeed the performance of
> > KVM_USER_EXIT is uninteresting.
> Yup, either irqfd or KVM_SET_SIGNAL_MSI.
>
> Paolo

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