Daniel Sedlak [off-list ref] writes:

This patch is a result of our long-standing debug sessions, where it all
started as "networking is slow", and TCP network throughput suddenly
dropped from tens of Gbps to few Mbps, and we could not see anything in
the kernel log or netstat counters.

Currently, we have two memory pressure counters for TCP sockets [1],
which we manipulate only when the memory pressure is signalled through
the proto struct [2]. However, the memory pressure can also be signaled
through the cgroup memory subsystem, which we do not reflect in the
netstat counters. In the end, when the cgroup memory subsystem signals
that it is under pressure, we silently reduce the advertised TCP window
with tcp_adjust_rcv_ssthresh() to 4*advmss, which causes a significant
throughput reduction.

Keep in mind that when the cgroup memory subsystem signals the socket
memory pressure for a given cgroup, it affects all sockets used in that
cgroup, including children cgroups.

This patch exposes a new file for each cgroup in sysfs which is a
read-only single value file showing how many microseconds this cgroup
contributed to throttling the throughput of network sockets. The file is
accessible in the following path.

  /sys/fs/cgroup/**/<cgroup name>/memory.net.throttled_usec

Hi Daniel!

How this value is going to be used? In other words, do you need an
exact number or something like memory.events::net_throttled would be
enough for your case?

Thanks!