Linux Virtualization - Aug 2018 - [PATCH net-next v7 3/4] net: vhost: factor out busy polling logic to vhost_net_busy_poll()

On 2018?08?03? 10:51, Tonghao Zhang wrote:
> On Thu, Aug 2, 2018 at 5:23 PM Jason Wang <jasowang at redhat.com>
wrote:
>>
>>
>> On 2018?08?02? 16:41, Toshiaki Makita wrote:
>>> On 2018/08/02 17:18, Jason Wang wrote:
>>>> On 2018?08?01? 17:52, Tonghao Zhang wrote:
>>>>>> +static void vhost_net_busy_poll_check(struct vhost_net
*net,
>>>>>> +                                   struct
vhost_virtqueue *rvq,
>>>>>> +                                   struct
vhost_virtqueue *tvq,
>>>>>> +                                   bool rx)
>>>>>> +{
>>>>>> +     struct socket *sock = rvq->private_data;
>>>>>> +
>>>>>> +     if (rx)
>>>>>> +             vhost_net_busy_poll_try_queue(net, tvq);
>>>>>> +     else if (sock &&
sk_has_rx_data(sock->sk))
>>>>>> +             vhost_net_busy_poll_try_queue(net, rvq);
>>>>>> +     else {
>>>>>> +             /* On tx here, sock has no rx data, so we
>>>>>> +              * will wait for sock wakeup for rx, and
>>>>>> +              * vhost_enable_notify() is not needed.
*/
>>>>> A possible case is we do have rx data but guest does not
refill the rx
>>>>> queue. In this case we may lose notifications from guest.
>>>> Yes, should consider this case. thanks.
>>> I'm a bit confused. Isn't this covered by the previous
>>> "else if (sock && sk_has_rx_data(...))" block?
>> The problem is it does nothing if vhost_vq_avail_empty() is true and
>> vhost_enble_notify() is false.
>>
>>>>>>> +
>>>>>>> +             cpu_relax();
>>>>>>> +     }
>>>>>>> +
>>>>>>> +     preempt_enable();
>>>>>>> +
>>>>>>> +     if (!rx)
>>>>>>> +             vhost_net_enable_vq(net, vq);
>>>>>> No need to enable rx virtqueue, if we are sure
handle_rx() will be
>>>>>> called soon.
>>>>> If we disable rx virtqueue in handle_tx and don't send
packets from
>>>>> guest anymore(handle_tx is not called), so we can wake up
for sock rx.
>>>>> so the network is broken.
>>>> Not sure I understand here. I mean is we schedule work for
handle_rx(),
>>>> there's no need to enable it since handle_rx() will do this
for us.
>>> Looks like in the last "else" block in
vhost_net_busy_poll_check() we
>>> need to enable vq since in that case we have no rx data and
handle_rx()
>>> is not scheduled.
>>>
>> Yes.
> So we will use the vhost_has_work() to check whether or not the
> handle_rx is scheduled ?
> If we use the vhost_has_work(), the work in the dev work_list may be
> rx work, or tx work, right ?
Yes. We can add a boolean to record whether or not we've called 
vhost_poll_queue() for rvq. And avoid calling vhost_net_enable_vq() if 
it was true.

So here's the needed changes:

1) Split the wakeup disabling to another patch
2) Squash the vhost_net_busy_poll_try_queue() and 
vhost_net_busy_poll_check() into vhost_net_busy_poll() and reduce 
duplicated checks.
3) If possible, rename the boolean rx in vhost_net_busy_poll() to 
poll_rx, this makes code more readable.

Thanks
>> Thanks

On Fri, Aug 3, 2018 at 11:07 AM Jason Wang <jasowang at redhat.com>
wrote:
>
>
>
> On 2018?08?03? 10:51, Tonghao Zhang wrote:
> > On Thu, Aug 2, 2018 at 5:23 PM Jason Wang <jasowang at
redhat.com> wrote:
> >>
> >>
> >> On 2018?08?02? 16:41, Toshiaki Makita wrote:
> >>> On 2018/08/02 17:18, Jason Wang wrote:
> >>>> On 2018?08?01? 17:52, Tonghao Zhang wrote:
> >>>>>> +static void vhost_net_busy_poll_check(struct
vhost_net *net,
> >>>>>> +                                   struct
vhost_virtqueue *rvq,
> >>>>>> +                                   struct
vhost_virtqueue *tvq,
> >>>>>> +                                   bool rx)
> >>>>>> +{
> >>>>>> +     struct socket *sock = rvq->private_data;
> >>>>>> +
> >>>>>> +     if (rx)
> >>>>>> +             vhost_net_busy_poll_try_queue(net,
tvq);
> >>>>>> +     else if (sock &&
sk_has_rx_data(sock->sk))
> >>>>>> +             vhost_net_busy_poll_try_queue(net,
rvq);
> >>>>>> +     else {
> >>>>>> +             /* On tx here, sock has no rx data,
so we
> >>>>>> +              * will wait for sock wakeup for rx,
and
> >>>>>> +              * vhost_enable_notify() is not
needed. */
> >>>>> A possible case is we do have rx data but guest does
not refill the rx
> >>>>> queue. In this case we may lose notifications from
guest.
> >>>> Yes, should consider this case. thanks.
> >>> I'm a bit confused. Isn't this covered by the previous
> >>> "else if (sock && sk_has_rx_data(...))"
block?
> >> The problem is it does nothing if vhost_vq_avail_empty() is true
and
> >> vhost_enble_notify() is false.
> >>
> >>>>>>> +
> >>>>>>> +             cpu_relax();
> >>>>>>> +     }
> >>>>>>> +
> >>>>>>> +     preempt_enable();
> >>>>>>> +
> >>>>>>> +     if (!rx)
> >>>>>>> +             vhost_net_enable_vq(net, vq);
> >>>>>> No need to enable rx virtqueue, if we are sure
handle_rx() will be
> >>>>>> called soon.
> >>>>> If we disable rx virtqueue in handle_tx and don't
send packets from
> >>>>> guest anymore(handle_tx is not called), so we can wake
up for sock rx.
> >>>>> so the network is broken.
> >>>> Not sure I understand here. I mean is we schedule work for
handle_rx(),
> >>>> there's no need to enable it since handle_rx() will do
this for us.
> >>> Looks like in the last "else" block in
vhost_net_busy_poll_check() we
> >>> need to enable vq since in that case we have no rx data and
handle_rx()
> >>> is not scheduled.
> >>>
> >> Yes.
> > So we will use the vhost_has_work() to check whether or not the
> > handle_rx is scheduled ?
> > If we use the vhost_has_work(), the work in the dev work_list may be
> > rx work, or tx work, right ?
>
> Yes. We can add a boolean to record whether or not we've called
> vhost_poll_queue() for rvq. And avoid calling vhost_net_enable_vq() if
> it was true.
so, the commit be294a51a "vhost_net: Avoid rx queue wake-ups during
busypoll"
may not consider the case: work is tx work in the dev work list.
> So here's the needed changes:
>
> 1) Split the wakeup disabling to another patch
> 2) Squash the vhost_net_busy_poll_try_queue() and
> vhost_net_busy_poll_check() into vhost_net_busy_poll() and reduce
> duplicated checks.
> 3) If possible, rename the boolean rx in vhost_net_busy_poll() to
> poll_rx, this makes code more readable.
OK
> Thanks
>
> >> Thanks
>

On 2018/08/03 12:24, Tonghao Zhang wrote:
> On Fri, Aug 3, 2018 at 11:07 AM Jason Wang <jasowang at redhat.com>
wrote:
>> On 2018?08?03? 10:51, Tonghao Zhang wrote:
>>> On Thu, Aug 2, 2018 at 5:23 PM Jason Wang <jasowang at
redhat.com> wrote:
>>>> On 2018?08?02? 16:41, Toshiaki Makita wrote:
>>>>> On 2018/08/02 17:18, Jason Wang wrote:
>>>>>> On 2018?08?01? 17:52, Tonghao Zhang wrote:
>>>>>>>> +static void vhost_net_busy_poll_check(struct
vhost_net *net,
>>>>>>>> +                                   struct
vhost_virtqueue *rvq,
>>>>>>>> +                                   struct
vhost_virtqueue *tvq,
>>>>>>>> +                                   bool rx)
>>>>>>>> +{
>>>>>>>> +     struct socket *sock =
rvq->private_data;
>>>>>>>> +
>>>>>>>> +     if (rx)
>>>>>>>> +            
vhost_net_busy_poll_try_queue(net, tvq);
>>>>>>>> +     else if (sock &&
sk_has_rx_data(sock->sk))
>>>>>>>> +            
vhost_net_busy_poll_try_queue(net, rvq);
>>>>>>>> +     else {
>>>>>>>> +             /* On tx here, sock has no rx
data, so we
>>>>>>>> +              * will wait for sock wakeup for
rx, and
>>>>>>>> +              * vhost_enable_notify() is not
needed. */
>>>>>>> A possible case is we do have rx data but guest
does not refill the rx
>>>>>>> queue. In this case we may lose notifications from
guest.
>>>>>> Yes, should consider this case. thanks.
>>>>> I'm a bit confused. Isn't this covered by the
previous
>>>>> "else if (sock && sk_has_rx_data(...))"
block?
>>>> The problem is it does nothing if vhost_vq_avail_empty() is
true and
>>>> vhost_enble_notify() is false.
>>>>
>>>>>>>>> +
>>>>>>>>> +             cpu_relax();
>>>>>>>>> +     }
>>>>>>>>> +
>>>>>>>>> +     preempt_enable();
>>>>>>>>> +
>>>>>>>>> +     if (!rx)
>>>>>>>>> +             vhost_net_enable_vq(net, vq);
>>>>>>>> No need to enable rx virtqueue, if we are sure
handle_rx() will be
>>>>>>>> called soon.
>>>>>>> If we disable rx virtqueue in handle_tx and
don't send packets from
>>>>>>> guest anymore(handle_tx is not called), so we can
wake up for sock rx.
>>>>>>> so the network is broken.
>>>>>> Not sure I understand here. I mean is we schedule work
for handle_rx(),
>>>>>> there's no need to enable it since handle_rx() will
do this for us.
>>>>> Looks like in the last "else" block in
vhost_net_busy_poll_check() we
>>>>> need to enable vq since in that case we have no rx data and
handle_rx()
>>>>> is not scheduled.
>>>>>
>>>> Yes.
>>> So we will use the vhost_has_work() to check whether or not the
>>> handle_rx is scheduled ?
>>> If we use the vhost_has_work(), the work in the dev work_list may
be
>>> rx work, or tx work, right ?
>>
>> Yes. We can add a boolean to record whether or not we've called
>> vhost_poll_queue() for rvq. And avoid calling vhost_net_enable_vq() if
>> it was true.
> so, the commit be294a51a "vhost_net: Avoid rx queue wake-ups during
busypoll"
> may not consider the case: work is tx work in the dev work list.
Not sure what you are concerned but what I can say is that we need to
poll rx work if vhost_has_work() detects tx work in
vhost_net_rx_peek_head_len() since rx busypoll exits prematurely in that
case.
>> So here's the needed changes:
>>
>> 1) Split the wakeup disabling to another patch
>> 2) Squash the vhost_net_busy_poll_try_queue() and
>> vhost_net_busy_poll_check() into vhost_net_busy_poll() and reduce
>> duplicated checks.
>> 3) If possible, rename the boolean rx in vhost_net_busy_poll() to
>> poll_rx, this makes code more readable.
> OK
>> Thanks
>>
>>>> Thanks
>>
> 
> 
-- 
Toshiaki Makita

On 2018?08?03? 11:24, Tonghao Zhang wrote:
> On Fri, Aug 3, 2018 at 11:07 AM Jason Wang <jasowang at redhat.com>
wrote:
>>
>>
>> On 2018?08?03? 10:51, Tonghao Zhang wrote:
>>> On Thu, Aug 2, 2018 at 5:23 PM Jason Wang <jasowang at
redhat.com> wrote:
>>>>
>>>> On 2018?08?02? 16:41, Toshiaki Makita wrote:
>>>>> On 2018/08/02 17:18, Jason Wang wrote:
>>>>>> On 2018?08?01? 17:52, Tonghao Zhang wrote:
>>>>>>>> +static void vhost_net_busy_poll_check(struct
vhost_net *net,
>>>>>>>> +                                   struct
vhost_virtqueue *rvq,
>>>>>>>> +                                   struct
vhost_virtqueue *tvq,
>>>>>>>> +                                   bool rx)
>>>>>>>> +{
>>>>>>>> +     struct socket *sock =
rvq->private_data;
>>>>>>>> +
>>>>>>>> +     if (rx)
>>>>>>>> +            
vhost_net_busy_poll_try_queue(net, tvq);
>>>>>>>> +     else if (sock &&
sk_has_rx_data(sock->sk))
>>>>>>>> +            
vhost_net_busy_poll_try_queue(net, rvq);
>>>>>>>> +     else {
>>>>>>>> +             /* On tx here, sock has no rx
data, so we
>>>>>>>> +              * will wait for sock wakeup for
rx, and
>>>>>>>> +              * vhost_enable_notify() is not
needed. */
>>>>>>> A possible case is we do have rx data but guest
does not refill the rx
>>>>>>> queue. In this case we may lose notifications from
guest.
>>>>>> Yes, should consider this case. thanks.
>>>>> I'm a bit confused. Isn't this covered by the
previous
>>>>> "else if (sock && sk_has_rx_data(...))"
block?
>>>> The problem is it does nothing if vhost_vq_avail_empty() is
true and
>>>> vhost_enble_notify() is false.
>>>>
>>>>>>>>> +
>>>>>>>>> +             cpu_relax();
>>>>>>>>> +     }
>>>>>>>>> +
>>>>>>>>> +     preempt_enable();
>>>>>>>>> +
>>>>>>>>> +     if (!rx)
>>>>>>>>> +             vhost_net_enable_vq(net, vq);
>>>>>>>> No need to enable rx virtqueue, if we are sure
handle_rx() will be
>>>>>>>> called soon.
>>>>>>> If we disable rx virtqueue in handle_tx and
don't send packets from
>>>>>>> guest anymore(handle_tx is not called), so we can
wake up for sock rx.
>>>>>>> so the network is broken.
>>>>>> Not sure I understand here. I mean is we schedule work
for handle_rx(),
>>>>>> there's no need to enable it since handle_rx() will
do this for us.
>>>>> Looks like in the last "else" block in
vhost_net_busy_poll_check() we
>>>>> need to enable vq since in that case we have no rx data and
handle_rx()
>>>>> is not scheduled.
>>>>>
>>>> Yes.
>>> So we will use the vhost_has_work() to check whether or not the
>>> handle_rx is scheduled ?
>>> If we use the vhost_has_work(), the work in the dev work_list may
be
>>> rx work, or tx work, right ?
>> Yes. We can add a boolean to record whether or not we've called
>> vhost_poll_queue() for rvq. And avoid calling vhost_net_enable_vq() if
>> it was true.
> so, the commit be294a51a "vhost_net: Avoid rx queue wake-ups during
busypoll"
> may not consider the case: work is tx work in the dev work list.
So two kinds of work, tx kick or tx wakeup.

For tx kick, we check vhost_vq_avail_empty() and avoid unnecessary kicks 
by not enabling kick if we found something is pending on txq. For tx 
wakeup, yes, the commit does not consider it. And that's why we want to 
disable tx wakeups during busy polling.

Thanks
>
>> So here's the needed changes:
>>
>> 1) Split the wakeup disabling to another patch
>> 2) Squash the vhost_net_busy_poll_try_queue() and
>> vhost_net_busy_poll_check() into vhost_net_busy_poll() and reduce
>> duplicated checks.
>> 3) If possible, rename the boolean rx in vhost_net_busy_poll() to
>> poll_rx, this makes code more readable.
> OK
>> Thanks
>>
>>>> Thanks