Re: [net-next PATCH 2/3] net: reduce cycles spend on ICMP replies that gets rate limited

On 01/09/2017 04:04 PM, Jesper Dangaard Brouer wrote:

This patch split the global and per (inet)peer ICMP-reply limiter
code, and moves the global limit check to earlier in the packet
processing path.  Thus, avoid spending cycles on ICMP replies that
gets limited/suppressed anyhow.

The global ICMP rate limiter icmp_global_allow() is a good solution,
it just happens too late in the process.  The kernel goes through the
full route lookup (return path) for the ICMP message, before taking
the rate limit decision of not sending the ICMP reply.

Details: The kernels global rate limiter for ICMP messages got added
in commit 4cdf507d5452 ("icmp: add a global rate limitation").  It is
a token bucket limiter with a global lock.  It brilliantly avoids
locking congestion by only updating when 20ms (HZ/50) were elapsed. It
can then avoids taking lock when credit is exhausted (when under
pressure) and time constraint for refill is not yet meet.

This patch removed the rate limit bypass for localhost.  As a result, it
is impossible to write deterministic UDP client tests tests which
exercise failover behavior in response to unreachable servers.

H.J. Lu noted that a glibc test started failing on kernel 4.11 and
identified the regression:

  https://sourceware.org/ml/libc-alpha/2017-06/msg00167.html

(I have more tests which are afflicted by this, but are not yet in glibc
upstream.)

This is particularly annoying because we already run such tests in a
network namespace for isolation, but the rate limit counter is global,
so that doesn't help here.

I'm attaching a self-contained test case.  It fails for me with:

localhost-icmp: iteration 50: no ICMP message (poll timeout)

On kernel 4.10, it passes and runs within just a few milliseconds.

Would you please fix this in some way?  Thanks.

Florian