Add stats for the size of extent_avail heap

doc/jemalloc.xml.in

@@ -2597,6 +2597,17 @@ struct extent_hooks_s {
         details.</para></listitem>
       </varlistentry>
 
+      <varlistentry id="stats.arenas.i.extent_avail">
+        <term>
+          <mallctl>stats.arenas.&lt;i&gt;.extent_avail</mallctl>
+          (<type>size_t</type>)
+          <literal>r-</literal>
+          [<option>--enable-stats</option>]
+        </term>
+        <listitem><para>Number of allocated (but unused) extent structs in this
+	arena.</para></listitem>
+      </varlistentry>
+
       <varlistentry id="stats.arenas.i.base">
         <term>
           <mallctl>stats.arenas.&lt;i&gt;.base</mallctl>

include/jemalloc/internal/arena_stats.h

@@ -87,6 +87,9 @@ struct arena_stats_s {
 	 */
 	atomic_zu_t		retained; /* Derived. */
 
+	/* Number of extent_t structs allocated by base, but not being used. */
+	atomic_zu_t		extent_avail;
+
 	arena_stats_decay_t	decay_dirty;
 	arena_stats_decay_t	decay_muzzy;
 

include/jemalloc/internal/arena_structs_b.h

@@ -196,6 +196,7 @@ struct arena_s {
 	 * Synchronization: extent_avail_mtx.
 	 */
 	extent_tree_t		extent_avail;
+	atomic_zu_t		extent_avail_cnt;
 	malloc_mutex_t		extent_avail_mtx;
 
 	/*

src/arena.c

@@ -100,6 +100,10 @@ arena_stats_merge(tsdn_t *tsdn, arena_t *arena, unsigned *nthreads,
 	arena_stats_accum_zu(&astats->retained,
 	    extents_npages_get(&arena->extents_retained) << LG_PAGE);
 
+	atomic_store_zu(&astats->extent_avail,
+	    atomic_load_zu(&arena->extent_avail_cnt, ATOMIC_RELAXED),
+	    ATOMIC_RELAXED);
+
 	arena_stats_accum_u64(&astats->decay_dirty.npurge,
 	    arena_stats_read_u64(tsdn, &arena->stats,
 	    &arena->stats.decay_dirty.npurge));

src/ctl.c

@@ -190,6 +190,7 @@ CTL_PROTO(stats_arenas_i_pdirty)
 CTL_PROTO(stats_arenas_i_pmuzzy)
 CTL_PROTO(stats_arenas_i_mapped)
 CTL_PROTO(stats_arenas_i_retained)
+CTL_PROTO(stats_arenas_i_extent_avail)
 CTL_PROTO(stats_arenas_i_dirty_npurge)
 CTL_PROTO(stats_arenas_i_dirty_nmadvise)
 CTL_PROTO(stats_arenas_i_dirty_purged)
@@ -510,6 +511,7 @@ static const ctl_named_node_t stats_arenas_i_node[] = {
 	{NAME("pmuzzy"),	CTL(stats_arenas_i_pmuzzy)},
 	{NAME("mapped"),	CTL(stats_arenas_i_mapped)},
 	{NAME("retained"),	CTL(stats_arenas_i_retained)},
+	{NAME("extent_avail"),	CTL(stats_arenas_i_extent_avail)},
 	{NAME("dirty_npurge"),	CTL(stats_arenas_i_dirty_npurge)},
 	{NAME("dirty_nmadvise"), CTL(stats_arenas_i_dirty_nmadvise)},
 	{NAME("dirty_purged"),	CTL(stats_arenas_i_dirty_purged)},
@@ -804,6 +806,8 @@ ctl_arena_stats_sdmerge(ctl_arena_t *ctl_sdarena, ctl_arena_t *ctl_arena,
 			    &astats->astats.mapped);
 			accum_atomic_zu(&sdstats->astats.retained,
 			    &astats->astats.retained);
+			accum_atomic_zu(&sdstats->astats.extent_avail,
+			    &astats->astats.extent_avail);
 		}
 
 		ctl_accum_arena_stats_u64(&sdstats->astats.decay_dirty.npurge,
@@ -2764,6 +2768,10 @@ CTL_RO_CGEN(config_stats, stats_arenas_i_mapped,
 CTL_RO_CGEN(config_stats, stats_arenas_i_retained,
     atomic_load_zu(&arenas_i(mib[2])->astats->astats.retained, ATOMIC_RELAXED),
     size_t)
+CTL_RO_CGEN(config_stats, stats_arenas_i_extent_avail,
+    atomic_load_zu(&arenas_i(mib[2])->astats->astats.extent_avail,
+        ATOMIC_RELAXED),
+    size_t)
 
 CTL_RO_CGEN(config_stats, stats_arenas_i_dirty_npurge,
     ctl_arena_stats_read_u64(

src/extent.c

@@ -186,6 +186,7 @@ extent_alloc(tsdn_t *tsdn, arena_t *arena) {
 		return base_alloc_extent(tsdn, arena->base);
 	}
 	extent_avail_remove(&arena->extent_avail, extent);
+	atomic_fetch_sub_zu(&arena->extent_avail_cnt, 1, ATOMIC_RELAXED);
 	malloc_mutex_unlock(tsdn, &arena->extent_avail_mtx);
 	return extent;
 }
@@ -194,6 +195,7 @@ void
 extent_dalloc(tsdn_t *tsdn, arena_t *arena, extent_t *extent) {
 	malloc_mutex_lock(tsdn, &arena->extent_avail_mtx);
 	extent_avail_insert(&arena->extent_avail, extent);
+	atomic_fetch_add_zu(&arena->extent_avail_cnt, 1, ATOMIC_RELAXED);
 	malloc_mutex_unlock(tsdn, &arena->extent_avail_mtx);
 }
 

src/stats.c

@@ -628,7 +628,7 @@ stats_arena_print(emitter_t *emitter, unsigned i, bool bins, bool large,
 	const char *dss;
 	ssize_t dirty_decay_ms, muzzy_decay_ms;
 	size_t page, pactive, pdirty, pmuzzy, mapped, retained;
-	size_t base, internal, resident, metadata_thp;
+	size_t base, internal, resident, metadata_thp, extent_avail;
 	uint64_t dirty_npurge, dirty_nmadvise, dirty_purged;
 	uint64_t muzzy_npurge, muzzy_nmadvise, muzzy_purged;
 	size_t small_allocated;
@@ -911,6 +911,7 @@ stats_arena_print(emitter_t *emitter, unsigned i, bool bins, bool large,
 	GET_AND_EMIT_MEM_STAT(metadata_thp)
 	GET_AND_EMIT_MEM_STAT(tcache_bytes)
 	GET_AND_EMIT_MEM_STAT(resident)
+	GET_AND_EMIT_MEM_STAT(extent_avail)
 #undef GET_AND_EMIT_MEM_STAT
 
 	if (mutex) {