On Tue, Feb 3, 2015 at 11:46 PM, Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
wrote:
-----Original Message-----
From: Jason Wang [mailto:jasowang@xxxxxxxxxx]
Sent: Monday, February 2, 2015 1:49 AM
>> btw, I find during netvsc_start_xmit(), ret was change to
-ENOSPC
>> when
>> queue_sends[q_idx] < 1. But non of the caller check -ENOSPC in
fact?
>
> In this case, we don't request re-send, so set ret to a value
other
> than
> -EAGAIN.
Why not? We have available slots for it to be sent now. Dropping the
packet in this case may cause out of order sending.
The EAGAIN error doesn't normally happen, because we set the hi water
mark
to stop send queue.
This is not true since only txq was stopped which means only network
stack stop sending packets but not for control path e.g
rndis_filter_send_request() or other callers who call
vmbus_sendpacket() directly (e.g recv completion).
For control path, user may meet several errors when they want to change
mac address under heavy load.
What's more serious is netvsc_send_recv_completion(), it can not even
recover from more than 3 times of EAGAIN.
I must say mixing data packets with control packets with the same
channel sounds really scary. Since control packets could be blocked or
even dropped because of data packets already queued during heavy load,
and you need to synchronize two paths carefully (e.g I didn't see any
tx lock were held if rndis_filter_send_request() call netsc_send()
which may stop or start a queue).
If in really rare case, the ring buffer is full and there
is no outstanding sends, we can't stop queue here because there will
be no
send-completion msg to wake it up.
Confused, I believe only txq is stopped but we may still get completion
interrupt in this case.
And, the ring buffer is likely to be
occupied by other special msg, e.g. receive-completion msg (not a
normal case),
so we can't assume there are available slots.
Then why not checking hv_ringbuf_avail_percent() instead? And there's
no need to check queue_sends since it does not count recv completion.
We don't request retry from
the upper layer in this case to avoid possible busy retry.
Can't we just do this by stopping txq and depending on tx interrupt to
wake it?
Thanks
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