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Subject: [PATCH v2 0/1] Add FUTEX_SPIN operation
Date: Thu, 23 May 2024 17:07:03 -0300
Message-ID: <20240523200704.281514-1-andrealmeid@igalia.com>
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Hi,

In the last LPC, Mathieu Desnoyers and I presented[0] a proposal to extend the
rseq interface to be able to implement spin locks in userspace correctly. Thomas
Gleixner agreed that this is something that Linux could improve, but asked for
an alternative proposal first: a futex operation that allows to spin a user
lock inside the kernel. This patchset implements a prototype of this idea for
further discussion.

With FUTEX2_SPIN flag set during a futex_wait(), the futex value is expected to
be the TID of the lock owner. Then, the kernel gets the task_struct of the
corresponding TID, and checks if it's running. It spins until the futex
is awaken, the task is scheduled out or if a timeout happens.  If the lock owner
is scheduled out at any time, then the syscall follows the normal path of
sleeping as usual. The user input is masked with FUTEX_TID_MASK so we have some
bits to play.

If the futex is awaken and we are spinning, we can return to userspace quickly,
avoid the scheduling out and in again to wake from a futex_wait(), thus
speeding up the wait operation. The user input is masked with FUTEX_TID_MASK so
we have some bits to play.

Christian Brauner suggested using pidfd to avoid race conditions, and I will
implement that in the next patch iteration. I benchmarked the implementation
measuring the time required to wait for a futex for a simple loop using the code
at [2]. In my setup, the total wait time for 1000 futexes using the spin method
was almost 10% lower than just using the normal futex wait:

	Testing with FUTEX2_SPIN | FUTEX_WAIT
	Total wait time: 8650089 usecs

	Testing with FUTEX_WAIT
	Total wait time: 9447291 usecs

However, as I played with how long the lock owner would be busy, the
benchmark results of spinning vs no spinning would match, showing that the
spinning will be effective for some specific scheduling scenarios, but depending
on the wait time, there's no big difference either spinning or not.

[0] https://lpc.events/event/17/contributions/1481/

You can find a small snippet to play with this interface here:

[1] https://gist.github.com/andrealmeid/f0b8c93a3c7a5c50458247c47f7078e1

Changelog:

v1: - s/PID/TID
    - masked user input with FUTEX_TID_MASK
    - add benchmark tool to the cover letter
    - dropped debug prints
    - added missing put_task_struct()

André Almeida (1):
  futex: Add FUTEX_SPIN operation

 include/uapi/linux/futex.h |  2 +-
 kernel/futex/futex.h       |  6 ++-
 kernel/futex/waitwake.c    | 78 +++++++++++++++++++++++++++++++++++++-
 3 files changed, 82 insertions(+), 4 deletions(-)

-- 
2.45.1