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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