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Subject: Re: [PATCH] use unfair spinlock when running on hypervisor.
From: Eric Dumazet <eric.dumazet@gmail.com>
To: vatsa@in.ibm.com
Cc: Avi Kivity <avi@redhat.com>, Andi Kleen <andi@firstfloor.org>,
       Gleb Natapov <gleb@redhat.com>, linux-kernel@vger.kernel.org,
       kvm@vger.kernel.org, hpa@zytor.com, mingo@elte.hu, npiggin@suse.de,
       tglx@linutronix.de, mtosatti@redhat.com
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Date: Thu, 03 Jun 2010 06:51:51 +0200
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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);
> }