Messages in this thread | | | Date | Fri, 27 Sep 2013 20:46:41 -0400 | From | Waiman Long <> | Subject | Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path |
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On 09/27/2013 03:32 PM, Peter Hurley wrote: > On 09/27/2013 03:00 PM, Waiman Long wrote: >> With the 3.12-rc2 kernel, there is sizable spinlock contention on >> the rwsem wakeup code path when running AIM7's high_systime workload >> on a 8-socket 80-core DL980 (HT off) as reported by perf: >> >> 7.64% reaim [kernel.kallsyms] [k] _raw_spin_lock_irqsave >> |--41.77%-- rwsem_wake >> 1.61% reaim [kernel.kallsyms] [k] _raw_spin_lock_irq >> |--92.37%-- rwsem_down_write_failed >> >> That was 4.7% of recorded CPU cycles. >> >> On a large NUMA machine, it is entirely possible that a fairly large >> number of threads are queuing up in the ticket spinlock queue to do >> the wakeup operation. In fact, only one will be needed. This patch >> tries to reduce spinlock contention by doing just that. >> >> A new wakeup field is added to the rwsem structure. This field is >> set on entry to rwsem_wake() and __rwsem_do_wake() to mark that a >> thread is pending to do the wakeup call. It is cleared on exit from >> those functions. >> >> By checking if the wakeup flag is set, a thread can exit rwsem_wake() >> immediately if another thread is pending to do the wakeup instead of >> waiting to get the spinlock and find out that nothing need to be done. > > This will leave readers stranded if a former writer is in __rwsem_do_wake > to wake up the readers and another writer steals the lock, but before > the former writer exits without having woken up the readers, the locking > stealing writer drops the lock and sees the wakeup flag is set, so > doesn't bother to wake the readers. > > Regards, > Peter Hurley >
Yes, you are right. That can be a problem. Thank for pointing this out. The workloads that I used doesn't seem to exercise the readers. I will modify the patch to add code handle this failure case by resetting the wakeup flag, pushing it out and then retrying one more time to get the read lock. I think that should address the problem.
Regards, Longman
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