Re: [PATCH] use unfair spinlock when running on hypervisor.

Eric Dumazet
2010-06-03T04:51:59Z

Le jeudi 03 juin 2010 à 09:50 +0530, Srivatsa Vaddagiri a écrit :
> On Wed, Jun 02, 2010 at 12:00:27PM +0300, Avi Kivity wrote:
> >
> > There are two separate problems: the more general problem is that
> > the hypervisor can put a vcpu to sleep while holding a lock, causing
> > other vcpus to spin until the end of their time slice. This can
> > only be addressed with hypervisor help.
>
> Fyi - I have a early patch ready to address this issue. Basically I am using
> host-kernel memory (mmap'ed into guest as io-memory via ivshmem driver) to hint
> host whenever guest is in spin-lock'ed section, which is read by host scheduler
> to defer preemption.
>
> Guest side:
>
> static inline void spin_lock(spinlock_t *lock)
> {
> raw_spin_lock(&lock->rlock);
> + __get_cpu_var(gh_vcpu_ptr)->defer_preempt++;

1) __this_cpu_inc() should be faster

2) Isnt a bit late to do this increment _after_
raw_spin_lock(&lock->rlock);

> }
>
> static inline void spin_unlock(spinlock_t *lock)
> {
> + __get_cpu_var(gh_vcpu_ptr)->defer_preempt--;
> raw_spin_unlock(&lock->rlock);
> }