[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] 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. -Longman _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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