[PATCH v2 0/4] futex: Wakeup optimizations

Davidlohr Bueso
2013-12-03T09:45:50Z

Changes from v1 [https://lkml.org/lkml/2013/11/22/525]:
- Removed patch "futex: Check for pi futex_q only once".

- Cleaned up ifdefs for larger hash table.

- Added a doc patch from tglx that describes the futex
ordering guarantees.

- Improved the lockless plist check for the wake calls.
Based on the community feedback, the necessary abstractions
and barriers are added to maintain ordering guarantees.
Code documentation is also updated.

- Removed patch "sched,futex: Provide delayed wakeup list".
Based on feedback from PeterZ, I will look into this as
a separate issue once the other patches are settled.

We have been dealing with a customer database workload on large
12Tb, 240 core 16 socket NUMA system that exhibits high amounts
of contention on some of the locks that serialize internal futex
data structures. This workload specially suffers in the wakeup
paths, where waiting on the corresponding hb->lock can account for
up to ~60% of the time. The result of such calls can mostly be
classified as (i) nothing to wake up and (ii) wakeup large amount
of tasks.

Before these patches are applied, we can see this pathological behavior:

37.12% 826174 xxx [kernel.kallsyms] [k] _raw_spin_lock
--- _raw_spin_lock
|
|--97.14%-- futex_wake
| do_futex
| sys_futex
| system_call_fastpath
| |
| |--99.70%-- 0x7f383fbdea1f
| | yyy

43.71% 762296 xxx [kernel.kallsyms] [k] _raw_spin_lock
--- _raw_spin_lock
|
|--53.74%-- futex_wake
| do_futex
| sys_futex
| system_call_fastpath
| |
| |--99.40%-- 0x7fe7d44a4c05
| | zzz
|--45.90%-- futex_wait_setup
| futex_wait
| do_futex
| sys_futex
| system_call_fastpath
| 0x7fe7ba315789
| syscall

With these patches, contention is practically non existent:

0.10% 49 xxx [kernel.kallsyms] [k] _raw_spin_lock
--- _raw_spin_lock
|
|--76.06%-- futex_wait_setup
| futex_wait
| do_futex
| sys_futex
| system_call_fastpath
| |
| |--99.90%-- 0x7f3165e63789
| | syscall|
...
|--6.27%-- futex_wake
| do_futex
| sys_futex
| system_call_fastpath
| |
| |--54.56%-- 0x7f317fff2c05
...

Patch 1 is a cleanup.

Patch 2 addresses the well known issue of the global hash table.
By creating a larger and NUMA aware table, we can reduce the false
sharing and collisions, thus reducing the chance of different futexes
using hb->lock.

Patch 3 documents the futex ordering guarantees.

Patch 4 reduces contention on the corresponding hb->lock by not trying to
acquire it if there are no blocked tasks in the waitqueue.
This particularly deals with point (i) above, where we see that it is not
uncommon for up to 90% of wakeup calls end up returning 0, indicating that no
tasks were woken.

This patchset has also been tested on smaller systems for a variety of
benchmarks, including java workloads, kernel builds and custom bang-the-hell-out-of
hb locks programs. So far, no functional or performance regressions have been seen.
Furthermore, no issues were found when running the different tests in the futextest
suite: http://git.kernel.org/cgit/linux/kernel/git/dvhart/futextest.git/

This patchset applies on top of Linus' tree as of v3.13-rc2 (2e7babfa).

Special thanks to Scott Norton, Tom Vanden, Mark Ray and Aswin Chandramouleeswaran
for help presenting, debugging and analyzing the data.

futex: Misc cleanups
futex: Larger hash table
futex: Document ordering guarantees
futex: Avoid taking hb lock if nothing to wakeup

kernel/futex.c | 230 ++++++++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 194 insertions(+), 36 deletions(-)

--
1.8.1.4