Messages in this thread | | | Date | Fri, 24 Oct 2014 16:53:27 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v12 09/11] pvqspinlock, x86: Add para-virtualization support |
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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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