[RFC/WIP PATCH 0/2] dynamic calls

Edward Cree
2018-12-12T17:48:04Z

A fix to the static_calls series (on which this series depends), and a really
hacky proof-of-concept of runtime-patched branch trees of static_calls to
avoid indirect calls / retpolines in the hot-path. Rather than any generally
applicable machinery, the patch just open-codes it for one call site (the
pt_prev->func() call in deliver_skb and __netif_receive_skb_one_core()); it
should however be possible to make a macro that takes a 'name' parameter and
expands to the whole thing. Also the _update() function could be shared and
get something useful from its work_struct, rather than needing a separate
copy of the function for every indirect call site.

Performance testing so far has been somewhat inconclusive; I applied this on
net-next, hacked up my Kconfig to use out-of-line static calls on x86-64, and
ran some 1-byte UDP stream tests with the DUT receiving.
On a single stream test, I saw packet rate go up by 7%, from 470Kpps to
504Kpps, with a considerable reduction in variance; however, CPU usage
increased by a larger factor: (packet rate / RX cpu) is a much lower-variance
measurement and went down by 13%. This however may be because it often got
into a state where, while patching the calls (and thus sending all callers
down the slow path) we continue to gather stats and see enough calls to
trigger another update; as there's no code to detect and skip an update that
doesn't change anything, we get into a tight loop of redoing updates. I am
working on this & plan to change it to not collect any stats while an update
is actually in progress.
On a 4-stream test, the variance I saw was too high to draw any conclusions;
the packet rate went down about 2½% but this was not statistically
significant (and the fastest run I saw was with dynamic calls present).

Edward Cree (2):
static_call: fix out-of-line static call implementation
net: core: rather hacky PoC implementation of dynamic calls

include/linux/static_call.h | 6 +-
net/core/dev.c | 222 +++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 221 insertions(+), 7 deletions(-)