a0969d51749a838607276fda27e44fbb43ff175ddecf1955156092eb.patch

From bc7ced87a4283a64e5565e395974fe3e669e4a6a Mon Sep 17 00:00:00 2001
From: Peter Oskolkov <posk@google.com>
Date: Wed, 22 Jul 2020 16:45:36 -0700
Subject: [PATCH] FROMLIST: futex: introduce FUTEX_SWAP operation

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 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: (this patch) 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.

Tested: see patch 3 in this patchset.

Signed-off-by: Peter Oskolkov <posk@google.com>
(am from https://lore.kernel.org/patchwork/patch/1277905/)
(also found at https://lore.kernel.org/r/20200722234538.166697-2-posk@posk.io)

BUG=b:170967073
TEST=eve-kernelnext and manually verification with selftests

Change-Id: I0f693816689477a5f7252ff361df4dd96ef4af88
Signed-off-by: Guenter Roeck <groeck@chromium.org>
---
 include/uapi/linux/futex.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/uapi/linux/futex.h b/include/uapi/linux/futex.h
--- a/include/uapi/linux/futex.h
+++ b/include/uapi/linux/futex.h
@@ -22,6 +22,7 @@
 #define FUTEX_WAIT_REQUEUE_PI	11
 #define FUTEX_CMP_REQUEUE_PI	12
 #define FUTEX_LOCK_PI2		13
+#define FUTEX_SWAP		14
 
 #define FUTEX_PRIVATE_FLAG	128
 #define FUTEX_CLOCK_REALTIME	256
@@ -42,6 +43,7 @@
 					 FUTEX_PRIVATE_FLAG)
 #define FUTEX_CMP_REQUEUE_PI_PRIVATE	(FUTEX_CMP_REQUEUE_PI | \
 					 FUTEX_PRIVATE_FLAG)
+#define FUTEX_SWAP_PRIVATE		(FUTEX_SWAP | FUTEX_PRIVATE_FLAG)
 
 /*
  * Flags to specify the bit length of the futex word for futex2 syscalls.
-- 
2.34.0.rc2.393.gf8c9666880-goog