From 7b091e46de4f9227b5a943e6d78283564e8c1c72 Mon Sep 17 00:00:00 2001
From: Peter Oskolkov <[email protected]>
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
--
2.25.1
On Tue, Jun 16, 2020 at 10:22:11AM -0700, Peter Oskolkov wrote:
> From 7b091e46de4f9227b5a943e6d78283564e8c1c72 Mon Sep 17 00:00:00 2001
> From: Peter Oskolkov <[email protected]>
> 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.
schbench used futex wait/wake to do sleep/wakeup between message thread
and worker thread and when worker thread is 1 per message thread, the
message thread and worker thread is also flipcall style.
So I modified schbench to make use of futex_swap and did a comparison.
In the not overloaded case, both runs roughly the same with futex_swap
performing slightly better. In the overloaded case, futex_swap performs
better than futex wait/wake in all metrics, with 90th seeing the largest
difference: 2556us vs 6us.
I guess when the scheduler change is in place, more latency gain is
expected.
Here is the log of the schbench run(on a 16core/32cpu x86_64 machine):
overloaded case
original schbench(aka futex wait/wake)
$./schbench -m 64 -t 1 -r 30
Latency percentiles (usec)
50.0000th: 7
75.0000th: 9
90.0000th: 2556
95.0000th: 7112
*99.0000th: 14160
99.5000th: 17504
99.9000th: 22688
min=0, max=30351
with futex swap
$./schbench -m 64 -t 1 -r 30
Latency percentiles (usec)
50.0th: 4
75.0th: 5
90.0th: 6
95.0th: 4568
*99.0th: 12912
99.5th: 15152
99.9th: 20384
min=0, max=30723
not overloaded case
original schbench(aka futex wait/wake)
$./schbench -m 32 -t 1 -r 30
Latency percentiles (usec)
50.0000th: 6
75.0000th: 7
90.0000th: 8
95.0000th: 9
*99.0000th: 10
99.5000th: 12
99.9000th: 18
min=0, max=398
with futex swap
$./schbench -m 32 -t 1 -r 30
Latency percentiles (usec)
50.0th: 4
75.0th: 5
90.0th: 5
95.0th: 6
*99.0th: 8
99.5th: 9
99.9th: 12
min=0, max=245