On Tue, 2016-03-22 at 13:46 -0400, Waiman Long wrote: > v2->v3: > - Remove patch 4 as it is not useful. > - Allow need_resched() check for waiter & add more comments about > changes to address issues raised by PeterZ. > > v1->v2: > - Set task state to running before doing optimistic spinning. > - Add 2 more patches to handle possible missed wakeups and wasteful > spinning in try_to_wake_up() function. > > This patchset is a variant of PeterZ's "locking/mutex: Avoid spinner > vs waiter starvation" patch. The major difference is that the > waiter-spinner won't enter into the OSQ used by the spinners. Instead, > it will spin directly on the lock in parallel with the queue head > of the OSQ. So there will be a bit more cacheline contention on the > lock cacheline, but that shouldn't cause noticeable impact on system > performance. > > This patchset tries to address 2 issues with Peter's patch: > > 1) Ding Tianhong still find that hanging task could happen in some cases. > 2) Jason Low found that there was performance regression for some AIM7 > workloads. > > By making the waiter-spinner to spin directly on the mutex, it will > increase the chance for the waiter-spinner to get the lock instead > of waiting in the OSQ for its turn. > > Patch 1 modifies the mutex_optimistic_spin() function to enable it > to be called by a waiter-spinner that doesn't need to go into the OSQ. > > Patch 2 modifies the mutex locking slowpath to make the waiter call > mutex_optimistic_spin() to do spinning after being waken up.
Just in case, here was the Waiman's patch that I was referring to for addressing some of the starvation issues with mutex by allowing the waiter to do optimistic spinning after being woken up. > Patch 3 reverses the sequence of setting task state and changing > mutex count to -1 to prevent the possibility of missed wakeup. > > Patch 4 modifies the wakeup code to abandon the wakeup operation > while spinning on the on_cpu flag if the task has changed back to a > non-sleeping state. > > My own test on a 4-socket E7-4820 v3 system showed a regression of > about 4% in the high_systime workload with Peter's patch which this > new patch effectively eliminates. > > Testing on an 8-socket Westmere-EX server, however, has performance > change from -9% to than +140% on the fserver workload of AIM7 > depending on how the system was set up. > > Waiman Long (3): > locking/mutex: Add waiter parameter to mutex_optimistic_spin() > locking/mutex: Enable optimistic spinning of woken task in wait queue > locking/mutex: Avoid missed wakeup of mutex waiter > > kernel/locking/mutex.c | 126 > ++++++++++++++++++++++++++++++++++-------------- > 1 files changed, 90 insertions(+), 36 deletions(-)
