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From: Peter Oskolkov <>
To: Linux Kernel Mailing List <>,
	Thomas Gleixner <>,
	Ingo Molnar <>, Ingo Molnar <>,
	Peter Zijlstra <>,
	Darren Hart <>,
	Vincent Guittot <>
Cc: Peter Oskolkov <>,
	Andrei Vagin <>, Paul Turner <>,
	Ben Segall <>, Aaron Lu <>
Subject: [PATCH for 5.9 0/3] FUTEX_SWAP (tip/locking/core)
Date: Wed, 22 Jul 2020 16:45:35 -0700
Message-ID: <> (raw)

From: Peter Oskolkov <>

As Paul Turner presented at LPC in 2013 ...
- pdf:
- video:

... 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 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

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

 include/linux/sched.h                         |   1 +
 include/uapi/linux/futex.h                    |   2 +
 kernel/futex.c                                |  89 ++++++--
 kernel/sched/core.c                           |   5 +
 kernel/sched/fair.c                           |   3 +
 kernel/sched/sched.h                          |   1 +
 .../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, 315 insertions(+), 16 deletions(-)
 create mode 100644 tools/testing/selftests/futex/functional/futex_swap.c


             reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-22 23:45 Peter Oskolkov [this message]
2020-07-22 23:45 ` [PATCH for 5.9 1/3] futex: introduce FUTEX_SWAP operation Peter Oskolkov
2020-07-23 11:27   ` Peter Zijlstra
2020-07-24  0:25     ` Peter Oskolkov
2020-07-24  3:00       ` Waiman Long
2020-07-24  3:22         ` Peter Oskolkov
2020-07-27  9:51       ` peterz
2020-07-28  0:01         ` Peter Oskolkov
2020-07-22 23:45 ` [PATCH for 5.9 2/3] futex/sched: add wake_up_process_prefer_current_cpu, use in FUTEX_SWAP Peter Oskolkov
2020-07-22 23:45 ` [PATCH for 5.9 3/3] selftests/futex: add futex_swap selftest Peter Oskolkov

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