[RFC PATCH 0/3 v2] futex: introduce FUTEX_SWAP operation

[Peter Oskolkov <posk@posk.io>]

From 7b091e46de4f9227b5a943e6d78283564e8c1c72 Mon Sep 17 00:00:00 2001
From: Peter Oskolkov <posk@google.com>
Date: Tue, 16 Jun 2020 10:13:58 -0700
Subject: [RFC PATCH 0/3 v2] futex: introduce FUTEX_SWAP operation

This is an RFC!

As Paul Turner presented at LPC in 2013 ...
- pdf: http://pdxplumbers.osuosl.org/2013/ocw//system/presentations/1653/original/LPC%20-%20User%20Threading.pdf
- video: https://www.youtube.com/watch?v=KXuZi9aeGTw

... Google has developed an M:N userspace threading subsystem backed
by Google-private SwitchTo Linux Kernel API (page 17 in the pdf referenced
above). This subsystem provides latency-sensitive services at Google with
fine-grained user-space control/scheduling over what is running when,
and this subsystem is used widely internally (called schedulers or fibers).

This RFC patchset is the first step to open-source this work. As explained
in the linked pdf and video, SwitchTo API has three core operations: wait,
resume, and swap (=switch). So this patchset adds a FUTEX_SWAP operation
that, in addition to FUTEX_WAIT and FUTEX_WAKE, will provide a foundation
on top of which user-space threading libraries can be built.

Another common use case for FUTEX_SWAP is message passing a-la RPC
between tasks: task/thread T1 prepares a message,
wakes T2 to work on it, and waits for the results; when T2 is done, it
wakes T1 and waits for more work to arrive. Currently the simplest
way to implement this is

a. T1: futex-wake T2, futex-wait
b. T2: wakes, does what it has been woken to do
c. T2: futex-wake T1, futex-wait

With FUTEX_SWAP, steps a and c above can be reduced to one futex operation
that runs 5-10 times faster.

Patches in this patchset:

Patch 1: introduce FUTEX_SWAP futex operation that,
         internally, does wake + wait. The purpose of this patch is
         to work out the API.
Patch 2: a first rough attempt to make FUTEX_SWAP faster than
         what wake + wait can do.
Patch 3: a selftest that can also be used to benchmark FUTEX_SWAP vs
         FUTEX_WAKE + FUTEX_WAIT.

v2: fix undefined symbol error ifndef CONFIG_SMP.

Peter Oskolkov (3):
  futex: introduce FUTEX_SWAP operation
  futex, sched: add wake_up_swap, use in FUTEX_SWAP
  selftests/futex: add futex_swap selftest

 include/linux/sched.h                         |   1 +
 include/uapi/linux/futex.h                    |   2 +
 kernel/futex.c                                |  96 ++++++--
 kernel/sched/core.c                           |   5 +
 kernel/sched/fair.c                           |   3 +
 kernel/sched/sched.h                          |   3 +-
 .../selftests/futex/functional/.gitignore     |   1 +
 .../selftests/futex/functional/Makefile       |   1 +
 .../selftests/futex/functional/futex_swap.c   | 209 ++++++++++++++++++
 .../selftests/futex/include/futextest.h       |  19 ++
 10 files changed, 323 insertions(+), 17 deletions(-)
 create mode 100644 tools/testing/selftests/futex/functional/futex_swap.c

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2.25.1