Re: [Qemu-devel] [PATCH 9/9] dataplane: use a QContext event loop in place of custom thread

[Paolo Bonzini <pbonzini@redhat.com>]

Il 03/05/2013 18:03, Michael Roth ha scritto:
> virtio-blk dataplane currently creates/manages it's own thread to
> offload work to a separate event loop.
> 
> This patch insteads allows us to specify a QContext-based event loop by
> adding a "context" property for virtio-blk we can use like so:
> 
>   qemu ... \
>     -object glib-qcontext,id=ctx0,threaded=yes
>     -drive file=file.raw,id=drive0,aio=native,cache=none \
>     -device virtio-blk,drive=drive0,scsi=off,x-data-plane=on,context=ctx0
> 
> virtio-blk dataplane then simply attachs/detaches it's AioContext to the
> ctx0 event loop on start/stop.
> 
> This also makes available the option to drive a virtio-blk dataplane via
> the default main loop:
> 
>   qemu ... \
>     -drive file=file.raw,id=drive0,aio=native,cache=none \
>     -device virtio-blk,drive=drive0,scsi=off,x-data-plane=on,context=main
> 
> This doesn't do much in and of itself, but helps to demonstrate how we
> might model a general mechanism to offload device workloads to separate
> threads.
> 
> Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> ---
>  hw/block/dataplane/virtio-blk.c |   46 ++++++++++++---------------------------
>  include/hw/virtio/virtio-blk.h  |    7 ++++--
>  2 files changed, 19 insertions(+), 34 deletions(-)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 0356665..08ea10f 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -24,6 +24,8 @@
>  #include "virtio-blk.h"
>  #include "block/aio.h"
>  #include "hw/virtio/virtio-bus.h"
> +#include "qcontext/qcontext.h"
> +#include "qcontext/glib-qcontext.h"
>  
>  enum {
>      SEG_MAX = 126,                  /* maximum number of I/O segments */
> @@ -60,6 +62,7 @@ struct VirtIOBlockDataPlane {
>       * use it).
>       */
>      AioContext *ctx;
> +    QContext *qctx;
>      EventNotifier io_notifier;      /* Linux AIO completion */
>      EventNotifier host_notifier;    /* doorbell */
>  
> @@ -375,26 +378,6 @@ static void handle_io(EventNotifier *e)
>      }
>  }
>  
> -static void *data_plane_thread(void *opaque)
> -{
> -    VirtIOBlockDataPlane *s = opaque;
> -
> -    do {
> -        aio_poll(s->ctx, true);
> -    } while (!s->stopping || s->num_reqs > 0);
> -    return NULL;
> -}
> -
> -static void start_data_plane_bh(void *opaque)
> -{
> -    VirtIOBlockDataPlane *s = opaque;
> -
> -    qemu_bh_delete(s->start_bh);
> -    s->start_bh = NULL;
> -    qemu_thread_create(&s->thread, data_plane_thread,
> -                       s, QEMU_THREAD_JOINABLE);
> -}
> -
>  bool virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *blk,
>                                    VirtIOBlockDataPlane **dataplane)
>  {
> @@ -460,6 +443,7 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
>      VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
>      VirtQueue *vq;
>      int i;
> +    Error *err = NULL;
>  
>      if (s->started) {
>          return;
> @@ -502,9 +486,16 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
>      /* Kick right away to begin processing requests already in vring */
>      event_notifier_set(virtio_queue_get_host_notifier(vq));
>  
> -    /* Spawn thread in BH so it inherits iothread cpusets */
> -    s->start_bh = qemu_bh_new(start_data_plane_bh, s);
> -    qemu_bh_schedule(s->start_bh);
> +    /* use QEMU main loop/context by default */
> +    if (!s->blk->context) {
> +        s->blk->context = g_strdup("main");
> +    }

Or rather create a device-specific context by default?

Paolo

> +    s->qctx = qcontext_find_by_name(s->blk->context, &err);
> +    if (err) {
> +        fprintf(stderr, "virtio-blk failed to start: %s", error_get_pretty(err));
> +        exit(1);
> +    }
> +    aio_context_attach(s->ctx, s->qctx);
>  }
>  
>  void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
> @@ -517,15 +508,6 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
>      s->stopping = true;
>      trace_virtio_blk_data_plane_stop(s);
>  
> -    /* Stop thread or cancel pending thread creation BH */
> -    if (s->start_bh) {
> -        qemu_bh_delete(s->start_bh);
> -        s->start_bh = NULL;
> -    } else {
> -        aio_notify(s->ctx);
> -        qemu_thread_join(&s->thread);
> -    }
> -
>      aio_set_event_notifier(s->ctx, &s->io_notifier, NULL, NULL);
>      ioq_cleanup(&s->ioqueue);
>  
> diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> index fc71853..c5514a4 100644
> --- a/include/hw/virtio/virtio-blk.h
> +++ b/include/hw/virtio/virtio-blk.h
> @@ -110,6 +110,7 @@ struct VirtIOBlkConf
>      uint32_t scsi;
>      uint32_t config_wce;
>      uint32_t data_plane;
> +    char *context;
>  };
>  
>  struct VirtIOBlockDataPlane;
> @@ -138,13 +139,15 @@ typedef struct VirtIOBlock {
>          DEFINE_BLOCK_CHS_PROPERTIES(_state, _field.conf),                     \
>          DEFINE_PROP_STRING("serial", _state, _field.serial),                  \
>          DEFINE_PROP_BIT("config-wce", _state, _field.config_wce, 0, true),    \
> -        DEFINE_PROP_BIT("scsi", _state, _field.scsi, 0, true)
> +        DEFINE_PROP_BIT("scsi", _state, _field.scsi, 0, true),                \
> +        DEFINE_PROP_STRING("context", _state, _field.context)
>  #else
>  #define DEFINE_VIRTIO_BLK_PROPERTIES(_state, _field)                          \
>          DEFINE_BLOCK_PROPERTIES(_state, _field.conf),                         \
>          DEFINE_BLOCK_CHS_PROPERTIES(_state, _field.conf),                     \
>          DEFINE_PROP_STRING("serial", _state, _field.serial),                  \
> -        DEFINE_PROP_BIT("config-wce", _state, _field.config_wce, 0, true)
> +        DEFINE_PROP_BIT("config-wce", _state, _field.config_wce, 0, true),    \
> +        DEFINE_PROP_STRING("context", _state, _field.context)
>  #endif /* __linux__ */
>  
>  void virtio_blk_set_conf(DeviceState *dev, VirtIOBlkConf *blk);
>