Re: [Qemu-devel] [PATCH 05/16] io: make qio_channel_yield aware of AioContexts

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jan 16, 2017 at 12:55:59PM +0000, Daniel P. Berrange wrote:
> On Mon, Jan 16, 2017 at 07:38:24PM +0800, Fam Zheng wrote:
> > On Fri, 01/13 14:17, Paolo Bonzini wrote:
> > > Support separate coroutines for reading and writing, and place the
> > > read/write handlers on the AioContext that the QIOChannel is registered
> > > with.
> > > 
> > > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > > ---
> > >  include/io/channel.h   | 37 ++++++++++++++++++----
> > >  io/channel.c           | 86 ++++++++++++++++++++++++++++++++++++++------------
> > >  tests/Makefile.include |  2 +-
> > >  3 files changed, 96 insertions(+), 29 deletions(-)
> > > 
> > > diff --git a/include/io/channel.h b/include/io/channel.h
> > > index 665edd7..d7bad94 100644
> > > --- a/include/io/channel.h
> > > +++ b/include/io/channel.h
> > > @@ -23,6 +23,7 @@
> > >  
> > >  #include "qemu-common.h"
> > >  #include "qom/object.h"
> > > +#include "qemu/coroutine.h"
> > >  #include "block/aio.h"
> > >  
> > >  #define TYPE_QIO_CHANNEL "qio-channel"
> > > @@ -59,8 +60,6 @@ typedef gboolean (*QIOChannelFunc)(QIOChannel *ioc,
> > >                                     GIOCondition condition,
> > >                                     gpointer data);
> > >  
> > > -typedef struct QIOChannelRestart QIOChannelRestart;
> > > -
> > >  /**
> > >   * QIOChannel:
> > >   *
> > > @@ -84,8 +83,8 @@ struct QIOChannel {
> > >      unsigned int features; /* bitmask of QIOChannelFeatures */
> > >      char *name;
> > >      AioContext *ctx;
> > > -    QIOChannelRestart *read_coroutine;
> > > -    QIOChannelRestart *write_coroutine;
> > > +    Coroutine *read_coroutine;
> > > +    Coroutine *write_coroutine;
> > >  #ifdef _WIN32
> > >      HANDLE event; /* For use with GSource on Win32 */
> > >  #endif
> > > @@ -508,13 +507,37 @@ guint qio_channel_add_watch(QIOChannel *ioc,
> > >  
> > >  
> > >  /**
> > > + * qio_channel_set_aio_context:
> > > + * @ioc: the channel object
> > > + * @ctx: the #AioContext to set the handlers on
> > > + *
> > > + * Request that qio_channel_yield() sets I/O handlers on
> > > + * the given #AioContext.  If @ctx is %NULL, qio_channel_yield()
> > > + * uses QEMU's main thread event loop.
> > > + */
> > > +void qio_channel_set_aio_context(QIOChannel *ioc,
> > > +                                 AioContext *ctx);
> > > +
> > > +/**
> > > + * qio_channel_detach_aio_context:
> > > + * @ioc: the channel object
> > > + *
> > > + * Disable any I/O handlers set by qio_channel_yield().  With the
> > > + * help of aio_co_schedule(), this allows moving a coroutine that was
> > > + * paused by qio_channel_yield() to another context.
> > > + */
> > > +void qio_channel_detach_aio_context(QIOChannel *ioc);
> > > +
> > > +/**
> > >   * qio_channel_yield:
> > >   * @ioc: the channel object
> > >   * @condition: the I/O condition to wait for
> > >   *
> > > - * Yields execution from the current coroutine until
> > > - * the condition indicated by @condition becomes
> > > - * available.
> > > + * Yields execution from the current coroutine until the condition
> > > + * indicated by @condition becomes available.  @condition must
> > > + * be either %G_IO_IN or %G_IO_OUT; it cannot contain both.  In
> > > + * addition, no two coroutine can be waiting on the same condition
> > > + * and channel at the same time.
> > >   *
> > >   * This must only be called from coroutine context
> > >   */
> > > diff --git a/io/channel.c b/io/channel.c
> > > index ce470d7..1e043bf 100644
> > > --- a/io/channel.c
> > > +++ b/io/channel.c
> > > @@ -21,7 +21,7 @@
> > >  #include "qemu/osdep.h"
> > >  #include "io/channel.h"
> > >  #include "qapi/error.h"
> > > -#include "qemu/coroutine.h"
> > > +#include "qemu/main-loop.h"
> > >  
> > >  bool qio_channel_has_feature(QIOChannel *ioc,
> > >                               QIOChannelFeature feature)
> > > @@ -238,36 +238,80 @@ off_t qio_channel_io_seek(QIOChannel *ioc,
> > >  }
> > >  
> > >  
> > > -typedef struct QIOChannelYieldData QIOChannelYieldData;
> > > -struct QIOChannelYieldData {
> > > -    QIOChannel *ioc;
> > > -    Coroutine *co;
> > > -};
> > > +static void qio_channel_set_aio_fd_handlers(QIOChannel *ioc);
> > > +
> > > +static void qio_channel_restart_read(void *opaque)
> > > +{
> > > +    QIOChannel *ioc = opaque;
> > > +    Coroutine *co = ioc->read_coroutine;
> > >  
> > > +    ioc->read_coroutine = NULL;
> > > +    qio_channel_set_aio_fd_handlers(ioc);
> > > +    aio_co_wake(co);
> > > +}
> > >  
> > > -static gboolean qio_channel_yield_enter(QIOChannel *ioc,
> > > -                                        GIOCondition condition,
> > > -                                        gpointer opaque)
> > > +static void qio_channel_restart_write(void *opaque)
> > >  {
> > > -    QIOChannelYieldData *data = opaque;
> > > -    qemu_coroutine_enter(data->co);
> > > -    return FALSE;
> > > +    QIOChannel *ioc = opaque;
> > > +    Coroutine *co = ioc->write_coroutine;
> > > +
> > > +    ioc->write_coroutine = NULL;
> > > +    qio_channel_set_aio_fd_handlers(ioc);
> > > +    aio_co_wake(co);
> > >  }
> > >  
> > > +static void qio_channel_set_aio_fd_handlers(QIOChannel *ioc)
> > > +{
> > > +    IOHandler *rd_handler = NULL, *wr_handler = NULL;
> > > +    AioContext *ctx;
> > > +
> > > +    if (ioc->read_coroutine) {
> > > +	rd_handler = qio_channel_restart_read;
> > 
> > s/\t/        /
> > 
> > > +    }
> > > +    if (ioc->write_coroutine) {
> > > +	rd_handler = qio_channel_restart_write;
> > 
> > s/\t/        /
> > 
> > > +    }
> > > +
> > > +    ctx = ioc->ctx ? ioc->ctx : iohandler_get_aio_context();
> > > +    qio_channel_set_aio_fd_handler(ioc, ctx, rd_handler, wr_handler, ioc);
> > > +}
> > > +
> > > +void qio_channel_set_aio_context(QIOChannel *ioc,
> > > +                                 AioContext *ctx)
> > > +{
> > > +    AioContext *old_ctx;
> > > +    if (ioc->ctx == ctx) {
> > > +        return;
> > > +    }
> > > +
> > > +    old_ctx = ioc->ctx ? ioc->ctx : iohandler_get_aio_context();
> > > +    qio_channel_set_aio_fd_handler(ioc, old_ctx, NULL, NULL, NULL);
> > > +    ioc->ctx = ctx;
> > > +    qio_channel_set_aio_fd_handlers(ioc);
> > > +}
> > > +
> > > +void qio_channel_detach_aio_context(QIOChannel *ioc)
> > > +{
> > > +    ioc->read_coroutine = NULL;
> > > +    ioc->write_coroutine = NULL;
> > > +    qio_channel_set_aio_fd_handlers(ioc);
> > 
> > Why is qio_channel_set_aio_fd_handler not needed here?
> > 
> > > +    ioc->ctx = NULL;
> > > +}
> > >  
> > >  void coroutine_fn qio_channel_yield(QIOChannel *ioc,
> > >                                      GIOCondition condition)
> > >  {
> > > -    QIOChannelYieldData data;
> > > -
> > >      assert(qemu_in_coroutine());
> > > -    data.ioc = ioc;
> > > -    data.co = qemu_coroutine_self();
> > > -    qio_channel_add_watch(ioc,
> > > -                          condition,
> > > -                          qio_channel_yield_enter,
> > > -                          &data,
> > > -                          NULL);
> > > +    if (condition == G_IO_IN) {
> > > +        assert(!ioc->read_coroutine);
> > > +        ioc->read_coroutine = qemu_coroutine_self();
> > > +    } else if (condition == G_IO_OUT) {
> > > +        assert(!ioc->write_coroutine);
> > > +        ioc->write_coroutine = qemu_coroutine_self();
> > > +    } else {
> > > +        abort();
> > > +    }
> > > +    qio_channel_set_aio_fd_handlers(ioc);
> > >      qemu_coroutine_yield();
> > >  }
> > >  
> > > diff --git a/tests/Makefile.include b/tests/Makefile.include
> > > index 3b8ed9d..7d11bbb 100644
> > > --- a/tests/Makefile.include
> > > +++ b/tests/Makefile.include
> > > @@ -493,7 +493,7 @@ tests/check-qjson$(EXESUF): tests/check-qjson.o $(test-util-obj-y)
> > >  tests/check-qom-interface$(EXESUF): tests/check-qom-interface.o $(test-qom-obj-y)
> > >  tests/check-qom-proplist$(EXESUF): tests/check-qom-proplist.o $(test-qom-obj-y)
> > >  
> > > -tests/test-char$(EXESUF): tests/test-char.o qemu-char.o qemu-timer.o $(test-util-obj-y) $(qtest-obj-y) $(test-io-obj-y)
> > > +tests/test-char$(EXESUF): tests/test-char.o qemu-char.o qemu-timer.o $(test-util-obj-y) $(qtest-obj-y) $(test-io-obj-y) $(test-block-obj-y)
> > 
> > I guess this is a hint for moving coroutine code into a lower level library like
> > util.
> 
> The coroutine code is already in util/, so I'm assuming this is actually
> for the AioContext stuff. Yes, though, AioContext ought to be moved into
> util/ as part of this series IMHO, since the io/ channel code shouldn't
> have a dependancy on block/ layer.

That makes sense to me.  I think the move is overdue since coroutines
are widely useful outside the block layer and aren't specific to block
I/O.

Stefan

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