Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts""

[Anthony Liguori <aliguori@us.ibm.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> Il 09/10/2012 17:37, Anthony Liguori ha scritto:
>>>> >> In the very short term, I can imagine an aio fastpath that was only
>>>> >> implemented in terms of the device API.  We could have a slow path that
>>>> >> acquired the BQL.
>>> >
>>> > Not sure I follow.
>> 
>> As long as the ioeventfd thread can acquire qemu_mutex in order to call
>> bdrv_* functions.  The new device-only API could do this under the
>> covers for everything but the linux-aio fast path initially.
>
> Ok, so it's about the locking.  I'm not even sure we need locking if we
> have cooperative multitasking.  For example if bdrv_aio_readv/writev
> is called from a VCPU thread, it can just schedule a bottom half for
> itself in the appropriate AioContext.  Similarly for block jobs.


Okay, let's separate out the two issues here though.  One is whether we
need a device specific block API.  The second is whether we should short
cut to a fast path in the short term and go after a fully unlocked bdrv_
layer in the long(shortish?) term.

So let's talk about your proposal...

> The only part where I'm not sure how it would work is bdrv_read/write,
> because of the strange "qemu_aio_wait() calls select with a lock taken".
> Maybe we can just forbid synchronous I/O if you set a non-default
> AioContext.

Not sure how practical that is.  The is an awful lot of sync I/O still left.

> This would be entirely hidden in the block layer.  For example the
> following does it for bdrv_aio_readv/writev:
>
> diff --git a/block.c b/block.c
> index e95f613..7165e82 100644
> --- a/block.c
> +++ b/block.c
> @@ -3712,15 +3712,6 @@ static AIOPool bdrv_em_co_aio_pool = {
>      .cancel             = bdrv_aio_co_cancel_em,
>  };
>  
> -static void bdrv_co_em_bh(void *opaque)
> -{
> -    BlockDriverAIOCBCoroutine *acb = opaque;
> -
> -    acb->common.cb(acb->common.opaque, acb->req.error);
> -    qemu_bh_delete(acb->bh);
> -    qemu_aio_release(acb);
> -}
> -
>  /* Invoke bdrv_co_do_readv/bdrv_co_do_writev */
>  static void coroutine_fn bdrv_co_do_rw(void *opaque)
>  {
> @@ -3735,8 +3726,17 @@ static void coroutine_fn bdrv_co_do_rw(void *opaque)
>              acb->req.nb_sectors, acb->req.qiov, 0);
>      }
>  
> -    acb->bh = qemu_bh_new(bdrv_co_em_bh, acb);
> -    qemu_bh_schedule(acb->bh);
> +    acb->common.cb(acb->common.opaque, acb->req.error);
> +    qemu_aio_release(acb);
> +}
> +
> +static void bdrv_co_em_bh(void *opaque)
> +{
> +    BlockDriverAIOCBCoroutine *acb = opaque;
> +
> +    qemu_bh_delete(acb->bh);
> +    co = qemu_coroutine_create(bdrv_co_do_rw);
> +    qemu_coroutine_enter(co, acb);
>  }
>  
>  static BlockDriverAIOCB *bdrv_co_aio_rw_vector(BlockDriverState *bs,
> @@ -3756,8 +3756,8 @@ static BlockDriverAIOCB *bdrv_co_aio_rw_vector(BlockDriverState *bs,
>      acb->req.qiov = qiov;
>      acb->is_write = is_write;
>  
> -    co = qemu_coroutine_create(bdrv_co_do_rw);
> -    qemu_coroutine_enter(co, acb);
> +    acb->bh = qemu_bh_new(bdrv_co_em_bh, acb);
> +    qemu_bh_schedule(acb->bh);
>  
>      return &acb->common;
>  }
>
>
> Then we can add a bdrv_aio_readv/writev_unlocked API to the protocols, which
> would run outside the bottom half and provide the desired fast path.

This works for some of the block layer I think.  How does this interact
with thread pools for AIO?

But this wouldn't work well with things like NBD or curl, right?  What's
the plan there?

Regards,

Anthony Liguori

>
> Paolo
>
>> That means that we can convert block devices to use the device-only API
>> across the board (provided we make BQL recursive).
>> 
>> It also means we get at least some of the benefits of data-plane in the
>> short term.