Add PAC decay/deferral unit test coverage.

Add unit tests to cover public interface of pac decay & deferral
relevant functions.  To align with the decopuling between arena and pac,
rename arena_decay to pac_decay here.

Makefile.in

@@ -204,7 +204,6 @@ C_UTIL_CPP_SRCS := $(srcroot)src/nstime.c $(srcroot)src/malloc_io.c
 endif
 TESTS_UNIT := \
 	$(srcroot)test/unit/a0.c \
-	$(srcroot)test/unit/arena_decay.c \
 	$(srcroot)test/unit/arena_reset.c \
 	$(srcroot)test/unit/arenas_management.c \
 	$(srcroot)test/unit/atomic.c \
@@ -272,6 +271,7 @@ TESTS_UNIT := \
 	$(srcroot)test/unit/oversize_threshold.c \
 	$(srcroot)test/unit/pa.c \
 	$(srcroot)test/unit/pac.c \
+	$(srcroot)test/unit/pac_decay.c \
 	$(srcroot)test/unit/pac_sec_integration.c \
 	$(srcroot)test/unit/pack.c \
 	$(srcroot)test/unit/pages.c \

test/unit/decay.c

@@ -273,9 +273,67 @@ TEST_BEGIN(test_decay_ns_until_purge) {
 }
 TEST_END
 
+/*
+ * Branch coverage for the decay-level math the early-wake path
+ * (pac_decay_should_wake_early) relies on, exercised here at the public decay
+ * seam with explicit nstime rather than via real-thread timing.
+ */
+TEST_BEGIN(test_decay_early_wake_branches) {
+	decay_t decay;
+	memset(&decay, 0, sizeof(decay));
+
+	nstime_t curtime;
+	nstime_init(&curtime, 0);
+
+	uint64_t decay_ms = 1000;
+	nstime_t decay_nstime;
+	nstime_init(&decay_nstime, decay_ms * 1000 * 1000);
+	expect_false(decay_init(&decay, &curtime, (ssize_t)decay_ms),
+	    "Failed to initialize decay");
+
+	size_t new_pages = 100;
+
+	/*
+	 * Parity trap: with zero new pages the accumulation contributes
+	 * nothing, so decay_npages_purge_in must return 0 regardless of the
+	 * remaining-sleep window.  (The early-wake THRESHOLD compare on the
+	 * EXISTING backlog still runs in pac_decay_should_wake_early; that is
+	 * deliberately kept and documented in src/pac.c.)
+	 */
+	nstime_t window;
+	nstime_copy(&window, &decay_nstime);
+	expect_u64_eq(decay_npages_purge_in(&decay, &window, 0), 0,
+	    "Zero new pages must contribute zero to the purge estimate");
+
+	/*
+	 * A remaining-sleep window at/after the full decay period caps the
+	 * estimate at the new-page count (all of them will have decayed).
+	 */
+	expect_u64_eq(decay_npages_purge_in(&decay, &window, new_pages),
+	    new_pages, "All new pages should be due within the decay window");
+
+	nstime_t beyond;
+	nstime_copy(&beyond, &decay_nstime);
+	nstime_add(&beyond, &decay_nstime);
+	expect_u64_eq(decay_npages_purge_in(&decay, &beyond, new_pages),
+	    new_pages,
+	    "Estimate must cap at the new-page count beyond the decay window");
+
+	/*
+	 * No-backlog branch: with a zero threshold, decay_ns_until_purge is
+	 * unbounded (nothing schedulable).  This mirrors the indefinite-sleep
+	 * case.
+	 */
+	expect_u64_eq(decay_ns_until_purge(&decay, 0, 0),
+	    DECAY_UNBOUNDED_TIME_TO_PURGE,
+	    "Zero threshold must yield an unbounded time-to-purge");
+}
+TEST_END
+
 int
 main(void) {
 	return test(test_decay_init, test_decay_ms_valid,
 	    test_decay_npages_purge_in, test_decay_maybe_advance_epoch,
-	    test_decay_empty, test_decay, test_decay_ns_until_purge);
+	    test_decay_empty, test_decay, test_decay_ns_until_purge,
+	    test_decay_early_wake_branches);
 }

test/unit/pac.c

@@ -2,6 +2,13 @@
 
 #include "jemalloc/internal/pa.h"
 
+/*
+ * pac_decay_deferred is JET_EXTERN (test-only; see src/pac.c) and has no public
+ * header declaration, so declare it locally for the deterministic test below.
+ */
+extern void pac_decay_deferred(tsdn_t *tsdn, pac_t *pac,
+    pac_purge_eagerness_t eagerness, pac_deferred_work_result_t *result);
+
 #define PAC_TEST_ARENA_IND 124
 
 static bool pac_test_dalloc_fail;
@@ -114,12 +121,14 @@ pac_test_data_init(bool custom_hooks, ssize_t dirty_decay_ms,
 
 	nstime_t time;
 	nstime_init(&time, 0);
-	assert_false(pa_central_init(&data->central, data->base, /* hpa */ false,
-	    &hpa_hooks_default), "Unexpected pa_central_init failure");
+	assert_false(pa_central_init(&data->central, data->base,
+	    /* hpa */ false, &hpa_hooks_default),
+	    "Unexpected pa_central_init failure");
 	assert_false(pa_shard_init(tsdn, &data->shard, &data->central,
 	    &data->emap, data->base, PAC_TEST_ARENA_IND, &data->stats,
 	    &data->stats_mtx, &time, SC_LARGE_MAXCLASS + PAGE,
-	    dirty_decay_ms, muzzy_decay_ms), "Unexpected pa_shard_init failure");
+	    dirty_decay_ms, muzzy_decay_ms),
+	    "Unexpected pa_shard_init failure");
 
 	return data;
 }
@@ -382,6 +391,45 @@ TEST_BEGIN(test_pac_large_guarded_dalloc_unguards_before_caching) {
 }
 TEST_END
 
+TEST_BEGIN(test_pac_deferred_never_does_not_purge) {
+	/*
+	 * PAC_PURGE_NEVER is the eagerness the application free path selects
+	 * when a background thread is enabled, meaning current thread does not
+	 * purge.  It is only reachable through the exposed pac_decay_deferred
+	 * primitive (the production entry pac_do_deferred_work picks eagerness
+	 * itself), so this is a deliberate internal test of that critical,
+	 * stable edge.
+	 */
+	pac_test_hooks_reset();
+	/*
+	 * Finite dirty decay so a purge is gated by eagerness, not decay_ms<=0.
+	 */
+	pac_test_data_t *data = pac_test_data_init(
+	    /* custom_hooks */ true, /* dirty_decay_ms */ 100 * 1000, -1);
+	pac_t *pac = &data->shard.pac;
+
+	edata_t *edata = pac_alloc_expect(data, PAGE, /* guarded */ false);
+	size_t ndirty_before_dalloc = ecache_npages_get(&pac->ecache_dirty);
+
+	/* Dalloc caches the extent as dirty and requests deferred work. */
+	bool deferred_work_generated = false;
+	pac_dalloc(tsdn_fetch(), pac, edata, &deferred_work_generated);
+	expect_true(deferred_work_generated,
+	    "Non-pinned dalloc should request deferred work");
+	size_t ndirty_after_dalloc = ecache_npages_get(&pac->ecache_dirty);
+	expect_zu_gt(ndirty_after_dalloc, ndirty_before_dalloc,
+	    "Dalloc should add dirty pages");
+
+	/* PAC_PURGE_NEVER must leave the dirty pages untouched. */
+	pac_deferred_work_result_t result;
+	pac_decay_deferred(tsdn_fetch(), pac, PAC_PURGE_NEVER, &result);
+	expect_zu_eq(ecache_npages_get(&pac->ecache_dirty), ndirty_after_dalloc,
+	    "PAC_PURGE_NEVER must not purge any dirty pages");
+
+	pac_test_data_destroy(data);
+}
+TEST_END
+
 int
 main(void) {
 	return test_no_reentrancy(test_pac_dirty_muzzy_alloc_priority,
@@ -391,5 +439,6 @@ main(void) {
 	    test_pac_decay_retains_when_dalloc_fails,
 	    test_pac_non_pinned_dalloc_signals_deferred_work,
 	    test_pac_non_pinned_shrink_signals_deferred_work,
-	    test_pac_large_guarded_dalloc_unguards_before_caching);
+	    test_pac_large_guarded_dalloc_unguards_before_caching,
+	    test_pac_deferred_never_does_not_purge);
 }

test/unit/pac_decay.c

@@ -1,6 +1,8 @@
 #include "test/jemalloc_test.h"
 #include "test/arena_util.h"
 
+#include "jemalloc/internal/deferral.h"
+#include "jemalloc/internal/pac.h"
 #include "jemalloc/internal/ticker.h"
 
 static nstime_monotonic_t *nstime_monotonic_orig;
@@ -439,8 +441,35 @@ TEST_BEGIN(test_decay_never) {
 }
 TEST_END
 
+/*
+ * pac_time_until_deferred_work is the public PAC scheduling entry the
+ * background thread uses to decide its next wakeup.  Exercised here on a real
+ * arena through the public interface (no direct decay-state manipulation).
+ */
+TEST_BEGIN(test_pac_time_until_deferred_work) {
+	test_skip_if(is_background_thread_enabled());
+	test_skip_if(opt_hpa);
+
+	unsigned arena_ind = do_arena_create(1000, -1);
+	tsdn_t  *tsdn = tsd_tsdn(tsd_fetch());
+	pac_t   *pac = &arena_get(tsdn, arena_ind, false)->pa_shard.pac;
+
+	/* Idle: nothing pending -> the scheduler reports the maximum wait. */
+	expect_u64_eq(pac_time_until_deferred_work(tsdn, pac), DEFERRED_WORK_MAX,
+	    "Idle PAC should defer for the maximum interval");
+
+	/* Dirty pages under a finite decay_ms -> a finite wait is scheduled. */
+	generate_dirty(arena_ind, PAGE);
+	expect_u64_lt(pac_time_until_deferred_work(tsdn, pac), DEFERRED_WORK_MAX,
+	    "Pending dirty decay should schedule a finite deferred-work time");
+
+	do_arena_destroy(arena_ind);
+}
+TEST_END
+
 int
 main(void) {
 	return test(test_decay_ticks, test_decay_ticker,
-	    test_decay_nonmonotonic, test_decay_now, test_decay_never);
+	    test_decay_nonmonotonic, test_decay_now, test_decay_never,
+	    test_pac_time_until_deferred_work);
 }

test/unit/pac_decay.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-export MALLOC_CONF="dirty_decay_ms:1000,muzzy_decay_ms:1000,tcache_max:1024"
+export MALLOC_CONF="dirty_decay_ms:1000,muzzy_decay_ms:1000,tcache_max:1024,background_thread:false"