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net: report accept queue overruns at most every few seconds - #2195
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accept_tcp_from_lwip() prints one line for every connection it refuses because the queue of the listening socket is full. The console is synchronous: console_write() takes a global spinlock and the serial driver waits on the transmitter for each character. Under a burst of connections that is thousands of serial writes on the network input path, from every CPU, at the moment the system is already overloaded, and it turns an overload into a stall. Count the refusals and print at most one line every five seconds, with the number refused since the previous one.
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accept_tcp_from_lwip()prints one line for every connection it refuses because the queue of the listening socket is full. The console is synchronous:console_write()takes a global spinlock and the PL011 driver waits on the transmitter for every character. Under a burst of connections that means thousands of serial writes on the network input path, from every CPU, while the system is already overloaded, and the overload turns into a stall.This counts the refusals and prints at most one line every five seconds, with the number refused since the previous one.
On an OCI aarch64 instance (2 vCPU) serving a Java application at 1,000-2,000 connections, consoles held 18,000-21,000 of these lines, and some 15-s load passes served almost nothing. With the report rate-limited (together with #2193), no pass stalled in 192 passes up to 2,000 connections. Details in #2194.