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Jason Wang

2014-Oct-15 10:19 UTC

[RFC PATCH net-next 2/6] virtio: introduce virtio_enable_cb_avail()

On 10/15/2014 05:28 PM, Michael S. Tsirkin wrote:> On Wed, Oct 15, 2014 at 03:25:26PM +0800, Jason Wang wrote:
>> This patch introduces virtio_enable_cb_avail() to publish avail idx
>> and used event. This could be used by batched buffer submitting to
>> reduce the number of tx interrupts.
>>
>> Cc: Rusty Russell <rusty at rustcorp.com.au>
>> Cc: Michael S. Tsirkin <mst at redhat.com>
>> Signed-off-by: Jason Wang <jasowang at redhat.com>
>> ---
>>  drivers/virtio/virtio_ring.c |   22 ++++++++++++++++++++--
>>  include/linux/virtio.h       |    2 ++
>>  2 files changed, 22 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_ring.c
b/drivers/virtio/virtio_ring.c
>> index 1b3929f..d67fbf8 100644
>> --- a/drivers/virtio/virtio_ring.c
>> +++ b/drivers/virtio/virtio_ring.c
>> @@ -567,14 +567,32 @@ unsigned virtqueue_enable_cb_prepare(struct
virtqueue *_vq)
>>  	 * entry. Always do both to keep code simple. */
>>  	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
>>  	/* Make sure used event never go backwards */
>> -	if (!vring_need_event(vring_used_event(&vq->vring),
>> -			      vq->vring.avail->idx, last_used_idx))
>> +	if (vq->vring.avail->idx != vring_used_event(&vq->vring)
&&
>> +	    !vring_need_event(vring_used_event(&vq->vring),
>> +			      vq->vring.avail->idx, last_used_idx)) {
>>  		vring_used_event(&vq->vring) = last_used_idx;
>> +	}
>>  	END_USE(vq);
>>  	return last_used_idx;
>>  }
>>  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
>>
> I see you are also changing virtqueue_enable_cb_prepare, why?
This is also used to prevent it from moving the used event backwards.
This may happens when we handle tx napi after we publish avail idx as
used event (virtqueue_enable_cb_avail() was called).>
>> +bool virtqueue_enable_cb_avail(struct virtqueue *_vq)
>> +{
>> +	struct vring_virtqueue *vq = to_vvq(_vq);
>> +	bool ret;
>> +
>> +	START_USE(vq);
>> +	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
>> +	vring_used_event(&vq->vring) = vq->vring.avail->idx;
>> +	ret = vring_need_event(vq->vring.avail->idx,
>> +			       vq->last_used_idx, vq->vring.used->idx);
>> +	END_USE(vq);
>> +
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(virtqueue_enable_cb_avail);
>> +
>>  /**
>>   * virtqueue_poll - query pending used buffers
>>   * @vq: the struct virtqueue we're talking about.
> Could not figure out what this does.
> Please add documentation.
>
Sure, does something like below explain what does this function do?

/**                                                                            

 * virtqueue_enable_cb_avail - restart callbacks after
disable_cb.           
 * @vq: the struct virtqueue we're talking
about.                              
 *                                                                             

 * This re-enables callbacks but hints to the other side to
delay              
 * interrupts all of the available buffers have been processed;         
 * it returns "false" if there are at least one pending buffer in the
queue,          
 * to detect a possible race between the driver checking for more
work,        
 * and enabling
callbacks.                                                     
 *                                                                             

 * Caller must ensure we don't call this with other
virtqueue                  
 * operations at the same time (except where
noted).                           
 */
>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>> index b46671e..bfaf058 100644
>> --- a/include/linux/virtio.h
>> +++ b/include/linux/virtio.h
>> @@ -65,6 +65,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
>>  
>>  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
>>  
>> +bool virtqueue_enable_cb_avail(struct virtqueue *vq);
>> +
>>  bool virtqueue_poll(struct virtqueue *vq, unsigned);
>>  
>>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
>> -- 
>> 1.7.1
> --
> To unsubscribe from this list: send the line "unsubscribe
linux-kernel" in
> the body of a message to majordomo at vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

Michael S. Tsirkin

2014-Oct-15 10:41 UTC

head link

[RFC PATCH net-next 2/6] virtio: introduce virtio_enable_cb_avail()

On Wed, Oct 15, 2014 at 06:19:15PM +0800, Jason Wang
wrote:> On 10/15/2014 05:28 PM, Michael S. Tsirkin wrote:
> > On Wed, Oct 15, 2014 at 03:25:26PM +0800, Jason Wang wrote:
> >> This patch introduces virtio_enable_cb_avail() to publish avail
idx
> >> and used event. This could be used by batched buffer submitting to
> >> reduce the number of tx interrupts.
> >>
> >> Cc: Rusty Russell <rusty at rustcorp.com.au>
> >> Cc: Michael S. Tsirkin <mst at redhat.com>
> >> Signed-off-by: Jason Wang <jasowang at redhat.com>
> >> ---
> >>  drivers/virtio/virtio_ring.c |   22 ++++++++++++++++++++--
> >>  include/linux/virtio.h       |    2 ++
> >>  2 files changed, 22 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/virtio/virtio_ring.c
b/drivers/virtio/virtio_ring.c
> >> index 1b3929f..d67fbf8 100644
> >> --- a/drivers/virtio/virtio_ring.c
> >> +++ b/drivers/virtio/virtio_ring.c
> >> @@ -567,14 +567,32 @@ unsigned virtqueue_enable_cb_prepare(struct
virtqueue *_vq)
> >>  	 * entry. Always do both to keep code simple. */
> >>  	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> >>  	/* Make sure used event never go backwards */
> >> -	if (!vring_need_event(vring_used_event(&vq->vring),
> >> -			      vq->vring.avail->idx, last_used_idx))
> >> +	if (vq->vring.avail->idx !=
vring_used_event(&vq->vring) &&
> >> +	    !vring_need_event(vring_used_event(&vq->vring),
> >> +			      vq->vring.avail->idx, last_used_idx)) {
> >>  		vring_used_event(&vq->vring) = last_used_idx;
> >> +	}
> >>  	END_USE(vq);
> >>  	return last_used_idx;
> >>  }
> >>  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
> >>
> > I see you are also changing virtqueue_enable_cb_prepare, why?
> 
> This is also used to prevent it from moving the used event backwards.
> This may happens when we handle tx napi after we publish avail idx as
> used event (virtqueue_enable_cb_avail() was called).
So it's wrong exactly in the same way.

But also, please document this stuff, don't put
unrelated changes in a patch called "introduce
virtqueue_enable_cb_avail".

> >
> >> +bool virtqueue_enable_cb_avail(struct virtqueue *_vq)
> >> +{
> >> +	struct vring_virtqueue *vq = to_vvq(_vq);
> >> +	bool ret;
> >> +
> >> +	START_USE(vq);
> >> +	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> >> +	vring_used_event(&vq->vring) =
vq->vring.avail->idx;
> >> +	ret = vring_need_event(vq->vring.avail->idx,
> >> +			       vq->last_used_idx, vq->vring.used->idx);
> >> +	END_USE(vq);
> >> +
> >> +	return ret;
> >> +}
> >> +EXPORT_SYMBOL_GPL(virtqueue_enable_cb_avail);
> >> +
> >>  /**
> >>   * virtqueue_poll - query pending used buffers
> >>   * @vq: the struct virtqueue we're talking about.
> > Could not figure out what this does.
> > Please add documentation.
> >
> 
> Sure, does something like below explain what does this function do?
> 
> /**
> 
>  * virtqueue_enable_cb_avail - restart callbacks after
> disable_cb.           
>  * @vq: the struct virtqueue we're talking
> about.                              
>  *
> 
>  * This re-enables callbacks but hints to the other side to
> delay              
>  * interrupts all of the available buffers have been processed;         

So this is like virtqueue_enable_cb_delayed but even more
aggressive?
I think it's too agressive: it's better to wake up guest
after you are through most of buffers, but not all,
so guest and host can work in parallel.

>  * it returns "false" if there are at least one pending buffer in
the
> queue,          
>  * to detect a possible race between the driver checking for more
> work,        
>  * and enabling
> callbacks.                                                     
>  *
> 
>  * Caller must ensure we don't call this with other
> virtqueue                  
>  * operations at the same time (except where
> noted).                           
>  */
> 
> >> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> >> index b46671e..bfaf058 100644
> >> --- a/include/linux/virtio.h
> >> +++ b/include/linux/virtio.h
> >> @@ -65,6 +65,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
> >>  
> >>  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
> >>  
> >> +bool virtqueue_enable_cb_avail(struct virtqueue *vq);
> >> +
> >>  bool virtqueue_poll(struct virtqueue *vq, unsigned);
> >>  
> >>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
> >> -- 
> >> 1.7.1
> > --
> > To unsubscribe from this list: send the line "unsubscribe
linux-kernel" in
> > the body of a message to majordomo at vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/

Jason Wang

2014-Oct-15 10:58 UTC

head link

[RFC PATCH net-next 2/6] virtio: introduce virtio_enable_cb_avail()

On 10/15/2014 06:41 PM, Michael S. Tsirkin wrote:> On Wed, Oct 15, 2014 at 06:19:15PM +0800, Jason Wang wrote:
>> On 10/15/2014 05:28 PM, Michael S. Tsirkin wrote:
>>> On Wed, Oct 15, 2014 at 03:25:26PM +0800, Jason Wang wrote:
>>>> This patch introduces virtio_enable_cb_avail() to publish avail
idx
>>>> and used event. This could be used by batched buffer submitting
to
>>>> reduce the number of tx interrupts.
>>>>
>>>> Cc: Rusty Russell <rusty at rustcorp.com.au>
>>>> Cc: Michael S. Tsirkin <mst at redhat.com>
>>>> Signed-off-by: Jason Wang <jasowang at redhat.com>
>>>> ---
>>>>  drivers/virtio/virtio_ring.c |   22 ++++++++++++++++++++--
>>>>  include/linux/virtio.h       |    2 ++
>>>>  2 files changed, 22 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/virtio/virtio_ring.c
b/drivers/virtio/virtio_ring.c
>>>> index 1b3929f..d67fbf8 100644
>>>> --- a/drivers/virtio/virtio_ring.c
>>>> +++ b/drivers/virtio/virtio_ring.c
>>>> @@ -567,14 +567,32 @@ unsigned
virtqueue_enable_cb_prepare(struct virtqueue *_vq)
>>>>  	 * entry. Always do both to keep code simple. */
>>>>  	vq->vring.avail->flags &=
~VRING_AVAIL_F_NO_INTERRUPT;
>>>>  	/* Make sure used event never go backwards */
>>>> -	if (!vring_need_event(vring_used_event(&vq->vring),
>>>> -			      vq->vring.avail->idx, last_used_idx))
>>>> +	if (vq->vring.avail->idx !=
vring_used_event(&vq->vring) &&
>>>> +	    !vring_need_event(vring_used_event(&vq->vring),
>>>> +			      vq->vring.avail->idx, last_used_idx)) {
>>>>  		vring_used_event(&vq->vring) = last_used_idx;
>>>> +	}
>>>>  	END_USE(vq);
>>>>  	return last_used_idx;
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
>>>>
>>> I see you are also changing virtqueue_enable_cb_prepare, why?
>> This is also used to prevent it from moving the used event backwards.
>> This may happens when we handle tx napi after we publish avail idx as
>> used event (virtqueue_enable_cb_avail() was called).
> So it's wrong exactly in the same way.
>
> But also, please document this stuff, don't put
> unrelated changes in a patch called "introduce
> virtqueue_enable_cb_avail".
>
>
>>>> +bool virtqueue_enable_cb_avail(struct virtqueue *_vq)
>>>> +{
>>>> +	struct vring_virtqueue *vq = to_vvq(_vq);
>>>> +	bool ret;
>>>> +
>>>> +	START_USE(vq);
>>>> +	vq->vring.avail->flags &=
~VRING_AVAIL_F_NO_INTERRUPT;
>>>> +	vring_used_event(&vq->vring) =
vq->vring.avail->idx;
>>>> +	ret = vring_need_event(vq->vring.avail->idx,
>>>> +			       vq->last_used_idx, vq->vring.used->idx);
>>>> +	END_USE(vq);
>>>> +
>>>> +	return ret;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(virtqueue_enable_cb_avail);
>>>> +
>>>>  /**
>>>>   * virtqueue_poll - query pending used buffers
>>>>   * @vq: the struct virtqueue we're talking about.
>>> Could not figure out what this does.
>>> Please add documentation.
>>>
>> Sure, does something like below explain what does this function do?
>>
>> /**
>>
>>  * virtqueue_enable_cb_avail - restart callbacks after
>> disable_cb.           
>>  * @vq: the struct virtqueue we're talking
>> about.                              
>>  *
>>
>>  * This re-enables callbacks but hints to the other side to
>> delay              
>>  * interrupts all of the available buffers have been processed;
>
> So this is like virtqueue_enable_cb_delayed but even more
> aggressive?
> I think it's too agressive: it's better to wake up guest
> after you are through most of buffers, but not all,
> so guest and host can work in parallel.
Note that:

- it was only used when there are still few of free slots (which is
greater than 2 + MAX_SKB_FRAGS)
- my patch keeps the free_old_xmit_skbs() in the beginning of
start_xmit(), so the tx skb reclaiming does not depends totally on tx
interrupt. If more packets comes, we'd expect some of them were freed in
ndo_start_xmit(). If not, finally we may trigger the
virtqueue_enable_cb_delayed().

So probably not as aggressive as it looks. I will do benchmark on
this.>
>
>>  * it returns "false" if there are at least one pending
buffer in the
>> queue,          
>>  * to detect a possible race between the driver checking for more
>> work,        
>>  * and enabling
>> callbacks.                                                     
>>  *
>>
>>  * Caller must ensure we don't call this with other
>> virtqueue                  
>>  * operations at the same time (except where
>> noted).                           
>>  */
>>
>>>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>>>> index b46671e..bfaf058 100644
>>>> --- a/include/linux/virtio.h
>>>> +++ b/include/linux/virtio.h
>>>> @@ -65,6 +65,8 @@ bool virtqueue_enable_cb(struct virtqueue
*vq);
>>>>  
>>>>  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
>>>>  
>>>> +bool virtqueue_enable_cb_avail(struct virtqueue *vq);
>>>> +
>>>>  bool virtqueue_poll(struct virtqueue *vq, unsigned);
>>>>  
>>>>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
>>>> -- 
>>>> 1.7.1
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe
linux-kernel" in
>>> the body of a message to majordomo at vger.kernel.org
>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>> Please read the FAQ at  http://www.tux.org/lkml/
> --
> To unsubscribe from this list: send the line "unsubscribe
linux-kernel" in
> the body of a message to majordomo at vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

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