Re: [Xen-devel] [PATCH v12 09/11] pvqspinlock, x86: Add para-virtualization support

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Thu, Oct 16, 2014 at 02:10:38PM -0400, Waiman Long wrote:
> +static inline void pv_init_node(struct mcs_spinlock *node)
> +{
> +     struct pv_qnode *pn = (struct pv_qnode *)node;
> +
> +     BUILD_BUG_ON(sizeof(struct pv_qnode) > 5*sizeof(struct mcs_spinlock));
> +
> +     if (!pv_enabled())
> +             return;
> +
> +     pn->cpustate = PV_CPU_ACTIVE;
> +     pn->mayhalt  = false;
> +     pn->mycpu    = smp_processor_id();
> +     pn->head     = PV_INVALID_HEAD;
> +}


> @@ -333,6 +393,7 @@ queue:
>       node += idx;
>       node->locked = 0;
>       node->next = NULL;
> +     pv_init_node(node);
>  
>       /*
>        * We touched a (possibly) cold cacheline in the per-cpu queue node;


So even if !pv_enabled() the compiler will still have to emit the code
for that inline, which will generate additional register pressure,
icache pressure and lovely stuff like that.

The patch I had used pv-ops for these things that would turn into NOPs
in the regular case and callee-saved function calls for the PV case.

That still does not entirely eliminate cost, but does reduce it
significant. Please consider using that.

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