On Sat, Apr 15, 2017 at 06:45:36PM -0400, Sowmini Varadhan wrote: > On (04/15/17 21:40), Andrew Lunn wrote: > > > > In my case, lan3 is up and idle, there are no packets flying around to > > be captured. So i would expect pcap_next_ex() to exit once a second, > > with a return value of 0. But it is not, it blocks and stays blocked. > : > > Looking at the libpcap source, the 1000ms timeout is being used as > > part of the setsockopt(3, SOL_PACKET, PACKET_RX_RING, 0xbe9445c0, 28) > > call, req.tp_retire_blk_tov is set to the timeoutval. > > right, aiui, the retire_blk_tov will only kick in if we have at > least one frame in a block, but the block is not filled up yet, > before the req.tp_retire_blk_tov (1s in your case) expires. > > If there are 0 frames pending, we should not be waking up the app, > so everything seems to be behaving as it should?
Hi Sowmini Humm, i can see the logic of that, it puts an upper bound on the latency for delivering a frame to user space, but does not wake user space when there is nothing in the queue. Yet i'm debugging an application which expects a timeout even when there are 0 packets. The Ostinator drone. It is a multi thread process, with a thread performing capture, and another thread doing control stuff. When the control thread wants to stop the capturing, it is setting a variable. The next time the capture thread comes out of pcap_next_en() it checks the variable and close the capture and the thread exists. But if there is no network traffic, it never exists. This scheme has worked before, but suddenly stopped when i upgraded something. What i cannot say is if that is libpcap, or a kernel, since i upgraded both at the same time. But it does seem like a regression somewhere. Looking at libpcap, it does seem to expect a timeout to happen even when there are 0 packets available. Has there been a kernel change with respect to this behaviour? Andrew
