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Date: Sun, 05 Jan 2014 16:59:27 -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 Subject: Re: [PATCH v5 0/4] futex: Wakeup optimizations Folks, unless there's a reason not do so, could we get this patchset in for 3.14? We're already at -rc7 and we could benefit from more testing in -next, until 3.13 is out. Thanks, Davidlohr On Thu, 2014-01-02 at 07:05 -0800, Davidlohr Bueso wrote: > 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. > > 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 > > kernel/futex.c | 197 ++++++++++++++++++++++++++++++++++++++++++++++----------- > 1 file changed, 159 insertions(+), 38 deletions(-) > -- 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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