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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 09/10/2012 20:26, Anthony Liguori ha scritto:
> 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.

Hmm, yeah, perhaps we need to bite the bullet and use a recursive lock.
 The lock would be taken by:

- sync I/O ops

- monitor commands that currently call bdrv_drain_all

- aio_poll when calling bottom halves or handlers

The rest of the proposal however would stand (especially with reference
to block jobs).

I think we can proceed incrementally.  The first obvious step is to
s/qemu_bh_new/aio_bh_new/ in the whole block layer (including the
CoQueue stuff), which would also help fixing the qemu-char bug that Jan
reported.

>> 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?

NBD uses coroutines; curl can use the non-unlocked
bdrv_aio_readv/writev.  In both cases they would execute in the
dataplane thread.  qcow2-over-raw would also execute its read/write code
entirely from the dataplane thread, for example.

Paolo