On Mon, Mar 30, 2015 at 11:57:08AM -0400, John Baldwin wrote:
> On Sunday, March 22, 2015 11:32:51 AM Konstantin Belousov wrote:
> > On Sun, Mar 22, 2015 at 09:41:53AM +1100, Bruce Evans wrote:
> > > Always using new API would lose the micro-optimizations given by the
> > > runtime
> > > decision for default CFLAGS (used by distributions for portability). To
> > > keep them, it seems best to keep the inline asm but replace
> > > popcnt_pc_map_elem(elem) by __bitcount64(elem). -mno-popcount can then
> > > be used to work around slowness in the software (that is actually
> > > hardware) case.
> >
> > So anybody has to compile his own kernel to get popcnt optimization ?
> > We do care about trivial things that improve time.
>
> That is not what Bruce said. He suggested using bitcount64() for the fallback
> if the cpuid check fails. He did not say to remove the runtime check to use
> popcnt if it is available:
>
> "Always using [bitcount64] would lose the micro-optimization... [to] keep
> [it], it seems best to keep the inline asm but replace
> popcnt_pc_map_elem(elem)
> by [bitcount64(elem)]."
Ok, thank you for the clarification.
I updated the pmap patch, see the end of the message.
>
> > BTW, I have the following WIP change, which popcnt xorl is a piece of.
> > It emulates the ifuncs with some preprocessing mess. It is much better
> > than runtime patching, and is a prerequisite to properly support more
> > things, like SMAP. I did not published it earlier, since I wanted to
> > convert TLB flush code to this.
>
> This looks fine to me. It seems to be manually converting certain symbols
> to use a dynamic lookup that must be explicitly resolved before first
> use?
I am not sure what do you mean by dynamic lookup, but possibly it
was mentioned. I can emulate the ifuncs more sincerely, by requiring
a resolver function, which is called on the first real function
invocation. I did not see it as very useful, but it is definitely
doable.
diff --git a/sys/amd64/amd64/pmap.c b/sys/amd64/amd64/pmap.c
index 6a4077c..fcfba56 100644
--- a/sys/amd64/amd64/pmap.c
+++ b/sys/amd64/amd64/pmap.c
@@ -412,7 +416,7 @@ static caddr_t crashdumpmap;
static void free_pv_chunk(struct pv_chunk *pc);
static void free_pv_entry(pmap_t pmap, pv_entry_t pv);
static pv_entry_t get_pv_entry(pmap_t pmap, struct rwlock **lockp);
-static int popcnt_pc_map_elem(uint64_t elem);
+static int popcnt_pc_map_elem_pq(uint64_t elem);
static vm_page_t reclaim_pv_chunk(pmap_t locked_pmap, struct rwlock **lockp);
static void reserve_pv_entries(pmap_t pmap, int needed,
struct rwlock **lockp);
@@ -2980,20 +3002,27 @@ retry:
/*
* Returns the number of one bits within the given PV chunk map element.
+ *
+ * The erratas for Intel processors state that "POPCNT Instruction May
+ * Take Longer to Execute Than Expected". It is believed that the
+ * issue is the spurious dependency on the destination register.
+ * Provide a hint to the register rename logic that the destination
+ * value is overwritten, by clearing it, as suggested in the
+ * optimization manual. It should be cheap for unaffected processors
+ * as well.
+ *
+ * Reference numbers for erratas are
+ * 4th Gen Core: HSD146
+ * 5th Gen Core: BDM85
*/
static int
-popcnt_pc_map_elem(uint64_t elem)
+popcnt_pc_map_elem_pq(uint64_t elem)
{
- int count;
+ u_long result;
- /*
- * This simple method of counting the one bits performs well because
- * the given element typically contains more zero bits than one bits.
- */
- count = 0;
- for (; elem != 0; elem &= elem - 1)
- count++;
- return (count);
+ __asm __volatile("xorl %k0,%k0;popcntq %1,%0"
+ : "=&r" (result) : "rm" (elem));
+ return (result);
}
/*
@@ -3025,13 +3054,13 @@ retry:
avail = 0;
TAILQ_FOREACH(pc, &pmap->pm_pvchunk, pc_list) {
if ((cpu_feature2 & CPUID2_POPCNT) == 0) {
- free = popcnt_pc_map_elem(pc->pc_map[0]);
- free += popcnt_pc_map_elem(pc->pc_map[1]);
- free += popcnt_pc_map_elem(pc->pc_map[2]);
+ free = bitcount64(pc->pc_map[0]);
+ free += bitcount64(pc->pc_map[1]);
+ free += bitcount64(pc->pc_map[2]);
} else {
- free = popcntq(pc->pc_map[0]);
- free += popcntq(pc->pc_map[1]);
- free += popcntq(pc->pc_map[2]);
+ free = popcnt_pc_map_elem_pq(pc->pc_map[0]);
+ free += popcnt_pc_map_elem_pq(pc->pc_map[1]);
+ free += popcnt_pc_map_elem_pq(pc->pc_map[2]);
}
if (free == 0)
break;
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