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    SubjectRe: [PATCH] use unfair spinlock when running on hypervisor.
    On Thu, Jun 03, 2010 at 09:50:51AM +0530, Srivatsa Vaddagiri wrote:
    > On Wed, Jun 02, 2010 at 12:00:27PM +0300, Avi Kivity wrote:
    > >
    > > There are two separate problems: the more general problem is that
    > > the hypervisor can put a vcpu to sleep while holding a lock, causing
    > > other vcpus to spin until the end of their time slice. This can
    > > only be addressed with hypervisor help.
    > Fyi - I have a early patch ready to address this issue. Basically I am using
    > host-kernel memory (mmap'ed into guest as io-memory via ivshmem driver) to hint
    > host whenever guest is in spin-lock'ed section, which is read by host scheduler
    > to defer preemption.

    Looks like a ni.ce simple way to handle this for the kernel.

    However I suspect user space will hit the same issue sooner
    or later. I assume your way is not easily extensable to futexes?

    > One pathological case where this may actually hurt is routines in guest like
    > flush_tlb_others_ipi() which take a spinlock and then enter a while() loop
    > waiting for other cpus to ack something. In this case, deferring preemption just
    > because guest is in critical section actually hurts! Hopefully the upper bound
    > for deferring preemtion and the fact that such routines may not be frequently
    > hit should help alleviate such situations.

    So do you defer during the whole spinlock region or just during the spin?

    I assume the the first?

    -- -- Speaking for myself only.

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