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SubjectRe: [PATCH RFC] kvm: optimize out smp_mb using srcu_read_unlock
On Wed, Oct 30, 2013 at 09:09:29PM +0200, Michael S. Tsirkin wrote:
> I noticed that srcu_read_lock/unlock both have a memory barrier,
> so just by moving srcu_read_unlock earlier we can get rid of
> one call to smp_mb().
>
> Unsurprisingly, the gain is small but measureable using the unit test
> microbenchmark:
> before
> vmcall 1407
> after
> vmcall 1357
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>
> --
>
> I didn't stress test this yet, sending out for early review/flames.
>
> Paul, could you review this patch please?
> Documentation/memory-barriers.txt says that unlock has a weaker
> uni-directional barrier, but in practice srcu_read_unlock calls
> smp_mb().
>
> Is it OK to rely on this? If not, can I add
> smp_mb__after_srcu_read_unlock (making it an empty macro for now)
> so we can avoid an actual extra smp_mb()?

Please use smp_mb__after_srcu_read_unlock(). After all, it was not
that long ago that srcu_read_unlock() contained no memory barriers,
and perhaps some day it won't need to once again.

Thanx, Paul

> Thanks.
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8617c9d..a48fb36 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5949,8 +5949,10 @@ restore:
>
> /* We should set ->mode before check ->requests,
> * see the comment in make_all_cpus_request.
> + *
> + * srcu_read_unlock below acts as a memory barrier.
> */
> - smp_mb();
> + srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>
> local_irq_disable();
>
> @@ -5960,12 +5962,11 @@ restore:
> smp_wmb();
> local_irq_enable();
> preempt_enable();
> + vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
> r = 1;
> goto cancel_injection;
> }
>
> - srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
> -
> if (req_immediate_exit)
> smp_send_reschedule(vcpu->cpu);
>
>
> --
> MST
>



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