Re: [Qemu-devel] [PATCH 01/15] qemu coroutine: support bypass mode

[Paolo Bonzini <pbonzini@redhat.com>]

Il 30/07/2014 13:39, Ming Lei ha scritto:
> This patch introduces several APIs for supporting bypass qemu coroutine
> in case of being not necessary and for performance's sake.

No, this is wrong.  Dataplane *must* use the same code as non-dataplane,
anything else is a step backwards.

If you want to bypass coroutines, bdrv_aio_readv/writev must detect the
conditions that allow doing that and call the bdrv_aio_readv/writev
directly.

To begin with, have you benchmarked QEMU and can you provide a trace of
*where* the coroutine overhead lies?

Paolo

> Signed-off-by: Ming Lei <ming.lei@canonical.com>
> ---
>  include/block/coroutine.h     |    8 ++++++++
>  include/block/coroutine_int.h |    5 +++++
>  qemu-coroutine-lock.c         |    4 ++--
>  qemu-coroutine.c              |   33 +++++++++++++++++++++++++++++++++
>  4 files changed, 48 insertions(+), 2 deletions(-)
> 
> diff --git a/include/block/coroutine.h b/include/block/coroutine.h
> index b408f96..46d2642 100644
> --- a/include/block/coroutine.h
> +++ b/include/block/coroutine.h
> @@ -223,4 +223,12 @@ void coroutine_fn co_aio_sleep_ns(AioContext *ctx, QEMUClockType type,
>   * Note that this function clobbers the handlers for the file descriptor.
>   */
>  void coroutine_fn yield_until_fd_readable(int fd);
> +
> +/* qemu coroutine bypass APIs */
> +void qemu_coroutine_set_bypass(bool bypass);
> +bool qemu_coroutine_bypassed(Coroutine *self);
> +bool qemu_coroutine_self_bypassed(void);
> +void qemu_coroutine_set_var(void *var);
> +void *qemu_coroutine_get_var(void);
> +
>  #endif /* QEMU_COROUTINE_H */
> diff --git a/include/block/coroutine_int.h b/include/block/coroutine_int.h
> index f133d65..106d0b2 100644
> --- a/include/block/coroutine_int.h
> +++ b/include/block/coroutine_int.h
> @@ -39,6 +39,11 @@ struct Coroutine {
>      Coroutine *caller;
>      QSLIST_ENTRY(Coroutine) pool_next;
>  
> +    bool bypass;
> +
> +    /* only used in bypass mode */
> +    void *opaque;
> +
>      /* Coroutines that should be woken up when we yield or terminate */
>      QTAILQ_HEAD(, Coroutine) co_queue_wakeup;
>      QTAILQ_ENTRY(Coroutine) co_queue_next;
> diff --git a/qemu-coroutine-lock.c b/qemu-coroutine-lock.c
> index e4860ae..7c69ff6 100644
> --- a/qemu-coroutine-lock.c
> +++ b/qemu-coroutine-lock.c
> @@ -82,13 +82,13 @@ static bool qemu_co_queue_do_restart(CoQueue *queue, bool single)
>  
>  bool coroutine_fn qemu_co_queue_next(CoQueue *queue)
>  {
> -    assert(qemu_in_coroutine());
> +    assert(qemu_in_coroutine() || qemu_coroutine_self_bypassed());
>      return qemu_co_queue_do_restart(queue, true);
>  }
>  
>  void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue)
>  {
> -    assert(qemu_in_coroutine());
> +    assert(qemu_in_coroutine() || qemu_coroutine_self_bypassed());
>      qemu_co_queue_do_restart(queue, false);
>  }
>  
> diff --git a/qemu-coroutine.c b/qemu-coroutine.c
> index 4708521..0597ed9 100644
> --- a/qemu-coroutine.c
> +++ b/qemu-coroutine.c
> @@ -137,3 +137,36 @@ void coroutine_fn qemu_coroutine_yield(void)
>      self->caller = NULL;
>      coroutine_swap(self, to);
>  }
> +
> +void qemu_coroutine_set_bypass(bool bypass)
> +{
> +    Coroutine *self = qemu_coroutine_self();
> +
> +    self->bypass = bypass;
> +}
> +
> +bool qemu_coroutine_bypassed(Coroutine *self)
> +{
> +    return self->bypass;
> +}
> +
> +bool qemu_coroutine_self_bypassed(void)
> +{
> +    Coroutine *self = qemu_coroutine_self();
> +
> +    return qemu_coroutine_bypassed(self);
> +}
> +
> +void qemu_coroutine_set_var(void *var)
> +{
> +    Coroutine *self = qemu_coroutine_self();
> +
> +    self->opaque = var;
> +}
> +
> +void *qemu_coroutine_get_var(void)
> +{
> +    Coroutine *self = qemu_coroutine_self();
> +
> +    return self->opaque;
> +}
>