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

On 10/24/2014 04:47 AM, Peter Zijlstra wrote:
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.

The additional register pressure may just cause a few more register moves which should be negligible in the overall performance . The additional icache pressure, however, may have some impact on performance. I was trying to balance the performance of the pv and non-pv versions so that we won't penalize the pv code too much for a bit more performance in the non-pv code. Doing it your way will add a lot of function call and register saving/restoring to the pv code.

Another alternative that I can think of is to generate 2 versions of the slowpath code - one pv and one non-pv out of the same source code. The non-pv code will call into the pv code once if pv is enabled. In this way, it won't increase the icache and register pressure of the non-pv code. However, this may make the source code a bit harder to read.

Please let me know your thought on this alternate approach.


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