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    SubjectRe: [PATCH v2 0/4] implement vcpu preempted check
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    On 06/07/2016 14:08, Wanpeng Li wrote:
    > 2016-07-06 18:44 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
    >>
    >>
    >> On 06/07/2016 08:52, Peter Zijlstra wrote:
    >>> On Tue, Jun 28, 2016 at 10:43:07AM -0400, Pan Xinhui wrote:
    >>>> change fomr v1:
    >>>> a simplier definition of default vcpu_is_preempted
    >>>> skip mahcine type check on ppc, and add config. remove dedicated macro.
    >>>> add one patch to drop overload of rwsem_spin_on_owner and mutex_spin_on_owner.
    >>>> add more comments
    >>>> thanks boqun and Peter's suggestion.
    >>>>
    >>>> This patch set aims to fix lock holder preemption issues.
    >>>>
    >>>> test-case:
    >>>> perf record -a perf bench sched messaging -g 400 -p && perf report
    >>>>
    >>>> 18.09% sched-messaging [kernel.vmlinux] [k] osq_lock
    >>>> 12.28% sched-messaging [kernel.vmlinux] [k] rwsem_spin_on_owner
    >>>> 5.27% sched-messaging [kernel.vmlinux] [k] mutex_unlock
    >>>> 3.89% sched-messaging [kernel.vmlinux] [k] wait_consider_task
    >>>> 3.64% sched-messaging [kernel.vmlinux] [k] _raw_write_lock_irq
    >>>> 3.41% sched-messaging [kernel.vmlinux] [k] mutex_spin_on_owner.is
    >>>> 2.49% sched-messaging [kernel.vmlinux] [k] system_call
    >>>>
    >>>> We introduce interface bool vcpu_is_preempted(int cpu) and use it in some spin
    >>>> loops of osq_lock, rwsem_spin_on_owner and mutex_spin_on_owner.
    >>>> These spin_on_onwer variant also cause rcu stall before we apply this patch set
    >>>
    >>> Paolo, could you help out with an (x86) KVM interface for this?
    >>
    >> If it's just for spin loops, you can check if the version field in the
    >> steal time structure has changed.
    >
    > Steal time will not be updated until ahead of next vmentry except
    > wrmsr MSR_KVM_STEAL_TIME. So it can't represent it is preempted
    > currently, right?

    Hmm, you're right. We can use bit 0 of struct kvm_steal_time's flags to
    indicate that pad[0] is a "VCPU preempted" field; if pad[0] is 1, the
    VCPU has been scheduled out since the last time the guest reset the bit.
    The guest can use an xchg to test-and-clear it. The bit can be
    accessed at any time, independent of the version field.

    Paolo

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