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Subject[PATCH for 5.9 0/3] FUTEX_SWAP (tip/locking/core)
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


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