On Mon, Jun 27, 2016 at 10:09:59AM +0200, Peter Zijlstra wrote: [snip] > > No, this is entirely insane, also broken. > > No vectors, no actual function calls, nothing like that. You want the > below to completely compile away and generate the exact 100% same code > it does today. >
Point taken. As Xinhui also posted something similar, which worked better on not effecting the generated code if not enabled. I think I'd better to drop this workload to him ;-) > > +++ b/kernel/locking/osq_lock.c > > @@ -1,6 +1,7 @@ > > #include <linux/percpu.h> > > #include <linux/sched.h> > > #include <linux/osq_lock.h> > > +#include <linux/vcpu_preempt.h> > > > > /* > > * An MCS like lock especially tailored for optimistic spinning for > > sleeping > > @@ -87,6 +88,8 @@ bool osq_lock(struct optimistic_spin_queue *lock) > > struct optimistic_spin_node *prev, *next; > > int curr = encode_cpu(smp_processor_id()); > > int old; > > + int loops; > > + long vpc; > > > > node->locked = 0; > > node->next = NULL; > > @@ -106,6 +109,9 @@ bool osq_lock(struct optimistic_spin_queue *lock) > > node->prev = prev; > > WRITE_ONCE(prev->next, node); > > > > + old = old - 1; > > That's just nasty, and could result in an unconditional decrement being > issues, even though its never used. > Right, better to calculate this decrement at the argument of vcpu_is_preempted() callsite, and define the primitive of host code as #define vcpu_is_preempted(cpu) false , which could probably be optimized out. Regards, Boqun > > + vpc = vcpu_preempt_count(); > > + > > /* > > * Normally @prev is untouchable after the above store; because at that > > * moment unlock can proceed and wipe the node element from stack. > > @@ -118,8 +124,14 @@ bool osq_lock(struct optimistic_spin_queue *lock) > > while (!READ_ONCE(node->locked)) { > > /* > > * If we need to reschedule bail... so we can block. > > + * An over-committed guest with more vCPUs than pCPUs > > + * might fall in this loop and cause a huge overload. > > + * This is because vCPU A(prev) hold the osq lock and yield out, > > + * vCPU B(node) wait ->locked to be set, IOW, wait till > > + * vCPU A run and unlock the osq lock. > > + * NOTE that vCPU A and vCPU B might run on same physical cpu. > > */ > > - if (need_resched()) > > + if (need_resched() || vcpu_is_preempted(old) || > > vcpu_has_preempted(vpc)) > > goto unqueue; > > > > cpu_relax_lowlatency(); > >
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