On 2024-05-23 16:07, André Almeida wrote:
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
What exactly are you trying to benchmark here ? I've looked at this toy
program, and I suspect that most of the delay you observe is due to
initial scheduling of a newly cloned thread, because this is what is
repeatedly being done in the delay you measure.
I would recommend to change this benchmark program to measure something
meaningful, e.g.:
- N threads repeatedly contending on a lock, until a "stop" flag is set,
- run for "duration" seconds, after which main() sets a "stop" flag.
- delay loop of "work_delay" us within the lock critical section,
- delay loop of "inactive_delay" us between locking attempts,
- measure the time it takes to grab the lock, report stats on this,
- measure the total number of operations done within the given
"duration".
- report statistics on the number of operations per thread to see
the impact on fairness,
The run the program with the following constraints:
- Pin one thread per core, with nb thread <= nb cores. This should
be a best case scenario for spinning.
- Pin all threads to a single core. when nb threads > nb cores, this
should be the worse scenario for spinning.
- Groups things between those two extremes to see how things evolve.
I would not be surprised that if you measure relevant delays, you will
observe much better speedups than what you currently have.
Thanks,
Mathieu
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(-)
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
