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.
-Longman
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