Messages in this thread | | | Subject | Re: [PATCH] locking/pvqspinlock: Hybrid PV queued/unfair locks | From | Paolo Bonzini <> | Date | Tue, 7 Nov 2017 12:19:52 +0100 |
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On 03/11/2017 16:35, Waiman Long wrote: > Currently, all the lock waiters entering the slowpath will do one > lock stealing attempt to acquire the lock. That helps performance, > especially in VMs with over-committed vCPUs. However, the current > pvqspinlocks still don't perform as good as unfair locks in many cases. > On the other hands, unfair locks do have the problem of lock starvation > that pvqspinlocks don't have. > > This patch combines the best attributes of an unfair lock and a > pvqspinlock into a hybrid lock with 2 modes - queued mode & unfair > mode. A lock waiter goes into the unfair mode when there are waiters > in the wait queue but the pending bit isn't set. Otherwise, it will > go into the queued mode waiting in the queue for its turn. > > On a 2-socket 36-core E5-2699 v3 system (HT off), a kernel build > (make -j<n>) was done in a VM with unpinned vCPUs 3 times with the > best time selected and <n> is the number of vCPUs available. The build > times of the original pvqspinlock, hybrid pvqspinlock and unfair lock > with various number of vCPUs are as follows: > > vCPUs pvqlock hybrid pvqlock unfair lock > ----- ------- -------------- ----------- > 30 342.1s 329.1s 329.1s > 36 314.1s 305.3s 307.3s > 45 345.0s 302.1s 306.6s > 54 365.4s 308.6s 307.8s > 72 358.9s 293.6s 303.9s > 108 343.0s 285.9s 304.2s > > The hybrid pvqspinlock performs better or comparable to the unfair > lock. > > By turning on QUEUED_LOCK_STAT, the table below showed the number > of lock acquisitions in unfair mode and queue mode after a kernel > build with various number of vCPUs. > > vCPUs queued mode unfair mode > ----- ----------- ----------- > 30 9,130,518 294,954 > 36 10,856,614 386,809 > 45 8,467,264 11,475,373 > 54 6,409,987 19,670,855 > 72 4,782,063 25,712,180 > > It can be seen that as the VM became more and more over-committed, > the ratio of locks acquired in unfair mode increases. This is all > done automatically to get the best overall performance as possible. > > The table below shows the kernel build times on a smaller 2-socket > 16-core 32-thread E5-2620 v4 system. > > vCPUs pvqlock hybrid pvqlock unfair lock > ----- ------- -------------- ----------- > 16 436.8s 433.4s 435.6s > 36 366.2s 364.8s 364.5s > 48 423.6s 376.3s 370.2s > 64 433.1s 376.6s 376.8s > > Again, the performance of the hybrid pvqspinlock was comparable to > that of the unfair lock. > > Signed-off-by: Waiman Long <longman@redhat.com> > --- > kernel/locking/qspinlock_paravirt.h | 28 +++++++++++++++++++++------- > 1 file changed, 21 insertions(+), 7 deletions(-) > > diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h > index 4355568..405a923 100644 > --- a/kernel/locking/qspinlock_paravirt.h > +++ b/kernel/locking/qspinlock_paravirt.h > @@ -60,21 +60,35 @@ struct pv_node { > #include "qspinlock_stat.h" > > /* > + * Hybrid PV queued/unfair lock > + * > * By replacing the regular queued_spin_trylock() with the function below, > * it will be called once when a lock waiter enter the PV slowpath before > - * being queued. By allowing one lock stealing attempt here when the pending > - * bit is off, it helps to reduce the performance impact of lock waiter > - * preemption without the drawback of lock starvation. > + * being queued. By allowing lock stealing attempts here when there are > + * waiters but the pending bit is off, it helps to reduce the performance > + * impact of lock waiter preemption without the drawback of lock starvation. > */ > #define queued_spin_trylock(l) pv_queued_spin_steal_lock(l) > static inline bool pv_queued_spin_steal_lock(struct qspinlock *lock) > { > struct __qspinlock *l = (void *)lock; > > - if (!(atomic_read(&lock->val) & _Q_LOCKED_PENDING_MASK) && > - (cmpxchg_acquire(&l->locked, 0, _Q_LOCKED_VAL) == 0)) { > - qstat_inc(qstat_pv_lock_stealing, true); > - return true; > + /* > + * Stay in unfair lock mode as long as waiters are present but > + * the pending bit isn't set. > + */ > + for (;;) { > + int val = atomic_read(&lock->val); > + > + if (!(val & _Q_LOCKED_PENDING_MASK) && > + (cmpxchg_acquire(&l->locked, 0, _Q_LOCKED_VAL) == 0)) { > + qstat_inc(qstat_pv_lock_stealing, true); > + return true; > + } > + if (!(val & _Q_TAIL_MASK) || (val & _Q_PENDING_MASK)) > + break; > + > + cpu_relax(); > } > > return false; >
Awesome! Thanks Waiman.
Paolo
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