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Message-Id: <1389569486-25487-1-git-send-email-davidlohr@hp.com> Date: Sun, 12 Jan 2014 15:31:21 -0800 From: Davidlohr Bueso <davidlohr@...com> To: linux-kernel@...r.kernel.org Cc: mingo@...nel.org, dvhart@...ux.intel.com, peterz@...radead.org, tglx@...utronix.de, paulmck@...ux.vnet.ibm.com, efault@....de, jeffm@...e.com, torvalds@...ux-foundation.org, jason.low2@...com, Waiman.Long@...com, tom.vaden@...com, scott.norton@...com, aswin@...com, davidlohr@...com Subject: [PATCH v6 0/5] futex: Wakeup optimizations Changes from v5 [https://lkml.org/lkml/2014/1/2/178]: - Added latest review tags - Removed redundant CONFIG_SMP ifdef in futex_get_mm() - Updated comments per Darren's suggestions. Changes from v3/v4 [http://lkml.org/lkml/2013/12/19/627]: - Almost completely redid patch 4, based on suggestions by Linus. Instead of adding an atomic counter to keep track of the plist size, couple the list's head empty call with a check to see if the hb lock is locked. This solves the race that motivated the use of the new atomic field. - Fix grammar in patch 3 - Fix SOB tags. Changes from v2 [http://lwn.net/Articles/575449/]: - Reordered SOB tags to reflect me as primary author. - Improved ordering guarantee comments for patch 4. - Rebased patch 4 against Linus' tree (this patch didn't apply after the recent futex changes/fixes). 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. Patch 5 silences a gcc warning msg. 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-rc6 (9a0bb296) 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 futex: Silence uninitialized warnings kernel/futex.c | 205 +++++++++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 163 insertions(+), 42 deletions(-) -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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