Re: [Qemu-devel] [PATCH v6 11/11] spapr: CPU hot unplug support

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Jan 08, 2016 at 12:25:19PM +0530, Bharata B Rao wrote:
> Remove the CPU core device by removing the underlying CPU thread devices.
> Support hot removal of CPU for sPAPR guests by sending the hot unplug
> notification to the guest via EPOW interrupt. Release the vCPU object
> after CPU hot unplug so that vCPU fd can be parked and reused.
> 
> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> ---
>  hw/ppc/spapr.c         | 113 +++++++++++++++++++++++++++++++++++++++++++++++++
>  include/hw/ppc/spapr.h |   6 +++
>  2 files changed, 119 insertions(+)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index c2af9ca..43cb726 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -93,6 +93,9 @@
>  
>  #define HTAB_SIZE(spapr)        (1ULL << ((spapr)->htab_shift))
>  
> +static QLIST_HEAD(cpu_unplug_list, sPAPRCPUUnplug) cpu_unplug_list
> +                  = QLIST_HEAD_INITIALIZER(cpu_unplug_list);
> +
>  static XICSState *try_create_xics(const char *type, int nr_servers,
>                                    int nr_irqs, Error **errp)
>  {
> @@ -2432,11 +2435,121 @@ static void spapr_machine_device_plug(HotplugHandler *hotplug_dev,
>      }
>  }
>  
> +static void spapr_cpu_destroy(PowerPCCPU *cpu)
> +{
> +    sPAPRMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
> +
> +    xics_cpu_destroy(spapr->icp, cpu);
> +    qemu_unregister_reset(spapr_cpu_reset, cpu);
> +}
> +
> +static void spapr_cpu_core_cleanup(void)
> +{
> +    sPAPRCPUUnplug *unplug, *next;
> +    Object *cpu;
> +
> +    QLIST_FOREACH_SAFE(unplug, &cpu_unplug_list, node, next) {
> +        cpu = unplug->cpu;
> +        object_unparent(cpu);
> +        QLIST_REMOVE(unplug, node);
> +        g_free(unplug);
> +    }
> +}
> +
> +static void spapr_add_cpu_to_unplug_list(Object *cpu)
> +{
> +    sPAPRCPUUnplug *unplug = g_malloc(sizeof(*unplug));
> +
> +    unplug->cpu = cpu;
> +    QLIST_INSERT_HEAD(&cpu_unplug_list, unplug, node);
> +}
> +
> +static int spapr_cpu_release(Object *obj, void *opaque)
> +{
> +    DeviceState *dev = DEVICE(obj);
> +    CPUState *cs = CPU(dev);
> +    PowerPCCPU *cpu = POWERPC_CPU(cs);
> +
> +    spapr_cpu_destroy(cpu);
> +    cpu_remove_sync(cs);
> +
> +    /*
> +     * We are still walking the core object's children list, and
> +     * hence can't cleanup this CPU thread object just yet. Put
> +     * it on a list for later removal.
> +     */
> +    spapr_add_cpu_to_unplug_list(obj);
> +    return 0;
> +}
> +
> +static void spapr_core_release(DeviceState *dev, void *opaque)
> +{
> +    object_child_foreach(OBJECT(dev), spapr_cpu_release, opaque);
> +    spapr_cpu_core_cleanup();
> +    object_unparent(OBJECT(dev));

I'd prefer to see the unplug_list as a local to this function (passed
to spapr_cpu_release via opaque) rather than using a new global.

> +}
> +
> +static int spapr_core_detach(Object *obj, void *opaque)
> +{
> +    sPAPRCoreState *core = opaque;
> +    DeviceState *dev = DEVICE(obj);
> +    CPUState *cs = CPU(dev);
> +    PowerPCCPU *cpu = POWERPC_CPU(cs);
> +    int id = ppc_get_vcpu_dt_id(cpu);
> +    int smt = kvmppc_smt_threads();
> +    sPAPRDRConnector *drc =
> +        spapr_dr_connector_by_id(SPAPR_DR_CONNECTOR_TYPE_CPU, id);
> +    sPAPRDRConnectorClass *drck;
> +    Error *local_err = NULL;
> +
> +    /*
> +     * SMT threads return from here, only main thread (thread 0) will
> +     * continue and signal hot unplug event to the guest.
> +     */

This seems silly.  spapr_core_unplug() iterates through all the
children only to have all of them except thread 0 ignore the call..

Can't you just grab the first thread and do this logic directly from
spapr_core_unplug()?

Same question for the plug side, though I didn't think of it when I
was looking at that.

> +    if ((id % smt) != 0) {
> +        return 0;
> +    }
> +    g_assert(drc);
> +
> +    drck = SPAPR_DR_CONNECTOR_GET_CLASS(drc);
> +    drck->detach(drc, core->dev, spapr_core_release, NULL, &local_err);
> +    if (local_err) {
> +        error_propagate(core->errp, local_err);
> +        return 1;
> +    }
> +
> +    /*
> +     * In addition to hotplugged CPUs, send the hot-unplug notification
> +     * interrupt to the guest for coldplugged CPUs started via -device
> +     * option too.
> +     */
> +    spapr_hotplug_req_remove_by_index(drc);
> +    return 0;
> +}
> +
> +static void spapr_core_unplug(HotplugHandler *hotplug_dev, DeviceState *dev,
> +                             Error **errp)
> +{
> +    sPAPRCoreState core;
> +
> +    core.dev = dev;
> +    core.errp = errp;
> +    object_child_foreach(OBJECT(dev), spapr_core_detach, &core);
> +}
> +
>  static void spapr_machine_device_unplug(HotplugHandler *hotplug_dev,
>                                        DeviceState *dev, Error **errp)
>  {
> +    sPAPRMachineClass *smc = SPAPR_MACHINE_GET_CLASS(qdev_get_machine());
> +
>      if (object_dynamic_cast(OBJECT(dev), TYPE_PC_DIMM)) {
>          error_setg(errp, "Memory hot unplug not supported by sPAPR");
> +    } else if (object_dynamic_cast(OBJECT(dev), TYPE_POWERPC_CPU_CORE)) {
> +        if (!smc->dr_cpu_enabled) {
> +            error_setg(errp, "CPU hot unplug not supported on this machine");
> +            return;
> +        }
> +        spapr_core_unplug(hotplug_dev, dev, errp);
>      }
>  }
>  
> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> index 68d51d6..4d89016 100644
> --- a/include/hw/ppc/spapr.h
> +++ b/include/hw/ppc/spapr.h
> @@ -88,6 +88,12 @@ typedef struct sPAPRCoreState {
>      Error **errp;
>  } sPAPRCoreState;
>  
> +/* List to store unplugged CPU objects for cleanup during unplug */
> +typedef struct sPAPRCPUUnplug {
> +    Object *cpu;
> +    QLIST_ENTRY(sPAPRCPUUnplug) node;
> +} sPAPRCPUUnplug;
> +
>  #define H_SUCCESS         0
>  #define H_BUSY            1        /* Hardware busy -- retry later */
>  #define H_CLOSED          2        /* Resource closed */

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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