Messages in this thread | | | Date | Mon, 08 Nov 2010 14:58:06 -0800 | From | Darren Hart <> | Subject | futex requeue_pi breakage |
| |
Steven,
As we've discussed, the following changes break the futex_requeue_pi testcase I use to validate the futex_requeue_pi feature introduced in 2.6.32(ish).
commit 43fa5460fe60dea5c610490a1d263415419c60f6 sched: Try not to migrate higher priority RT tasks
commit b3bc211cfe7d5fe94b310480d78e00bea96fbf2a sched: Give CPU bound RT tasks preference
The source to this test is here:
http://git.kernel.org/?p=linux/kernel/git/dvhart/futextest.git;a=summary branch: futex-lock file: futextest/functional/futex_requeue_pi.c
The futex_requeue_pi test main loop runs as sched_other while all the child threads are spawned as sched_fifo 1. This scenario results in the parent thread spinning in futex_wait_queue_me (most likely on the hash bucket spinlock) while attempting a pthread_join() on the rt child threads. However, if I run with chrt, forcing the parent thread to fifo 1, it completes reliably:
$ sudo chrt -f 1 ./futex_requeue_pi futex_requeue_pi: Test requeue functionality Arguments: broadcast=0 locked=0 owner=0 timeout=0ns Result: PASS
Whereas as sched_other: $ sudo ./futex_requeue_pi futex_requeue_pi: Test requeue functionality Arguments: broadcast=0 locked=0 owner=0 timeout=0ns <hang>
$ ps -eLo tid,rtprio,wchan:20,comm | grep futex 1987 - futex_wait_queue_me auditd 1988 - futex_wait_queue_me audispd 5533 - futex_wait_queue_me rsyslogd 5547 - futex_wait_queue_me futex_requeue_p 5549 1 futex_wait_queue_me futex_requeue_p 5550 1 futex_wait_queue_me futex_requeue_p 5551 1 futex_wait_queue_me futex_requeue_p 5552 1 futex_wait_queue_me futex_requeue_p 5553 1 futex_wait_queue_me futex_requeue_p 5554 1 futex_wait_queue_me futex_requeue_p 5555 1 futex_wait_queue_me futex_requeue_p 5556 1 futex_wait_queue_me futex_requeue_p 5557 1 futex_wait_queue_me futex_requeue_p 5558 1 ? futex_requeue_p 5559 1 ? futex_requeue_p
I'm still thinking through how your changes would impact this, and if there could be some assumption in the futex code that fails with those changes. I wanted to get the information out there in case it triggers any insight from you.
This suggests that somehow an rt task blocked while holding a spinlock (without issuing a warning to the console) or that somehow a SCHED_OTHER task preempted an RT task which is not getting scheduled on another CPU. Neither of these seem likely.
Thanks,
-- Darren Hart Embedded Linux Kernel
| |