From c688a0b2db1504a1da63678b35881438ad908f9a Mon Sep 17 00:00:00 2001
From: Chris Kennelly <ckennelly@google.com>
Date: Thu, 10 Sep 2026 12:30:02 -0700
Subject: [PATCH] Eliminate an unnecessary clock check for unsampled trackers.

Last allocation time is only used for sampled page trackers.  We also augment
the existing test to cover a sampled get/put sequence.

PiperOrigin-RevId: 979331505
---
 tcmalloc/huge_page_filler.h       |  4 ++--
 tcmalloc/huge_page_filler_test.cc | 39 ++++++++++++++++++++++++++++++-
 2 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/tcmalloc/huge_page_filler.h b/tcmalloc/huge_page_filler.h
index b79ab51f2..329206b0d 100644
--- a/tcmalloc/huge_page_filler.h
+++ b/tcmalloc/huge_page_filler.h
@@ -1233,11 +1233,11 @@ HugePageFiller<TrackerType>::TryGet(Length n, SpanAllocInfo span_alloc_info) {
   TC_ASSERT(type == AccessDensityPrediction::kSparse || pt->HasDenseSpans());
 
   // Log previous features before modifying the page tracker.
-  const auto now = clock_.now();
   if (pt->GetTagState().sampled_for_tagging) {
+    const auto now = clock_.now();
     pt->RecordFeatures();
+    pt->SetLastAllocationTime(now);
   }
-  pt->SetLastAllocationTime(now);
   const auto page_allocation = pt->Get(n, span_alloc_info);
   AddToFillerList(pt);
   pages_allocated_[type] += n;
diff --git a/tcmalloc/huge_page_filler_test.cc b/tcmalloc/huge_page_filler_test.cc
index 02b2bb924..986c5e9f4 100644
--- a/tcmalloc/huge_page_filler_test.cc
+++ b/tcmalloc/huge_page_filler_test.cc
@@ -714,7 +714,7 @@ TEST_F(FillerTest, ClockCalls) {
     page2 = res.page;
   }
   EXPECT_EQ(alloc_pt, pt);
-  EXPECT_EQ(FakeClock::now_calls(), 2);
+  EXPECT_EQ(FakeClock::now_calls(), 1);
   EXPECT_EQ(FakeClock::freq_calls(), 0);
 
   // 3. Put (partially freed hugepage).
@@ -741,6 +741,43 @@ TEST_F(FillerTest, ClockCalls) {
   EXPECT_EQ(FakeClock::now_calls(), 2);
   EXPECT_EQ(FakeClock::freq_calls(), 1);
 
+  // 5. Contribute and wait for pt to be sampled.
+  while (true) {
+    PageHeapSpinLockHolder l;
+    page1 = pt->Get(Length(1), info).page;
+    filler_.Contribute(pt, /*donated=*/false, info);
+    if (pt->GetTagState().sampled_for_tagging) {
+      break;
+    }
+    filler_.Put(pt, Range(page1, Length(1)), info);
+  }
+
+  FakeClock::ResetCalls();
+
+  {
+    PageHeapSpinLockHolder l;
+    auto res = filler_.TryGet(Length(1), info);
+    alloc_pt = res.pt;
+    page2 = res.page;
+  }
+
+  EXPECT_EQ(alloc_pt, pt);
+  EXPECT_EQ(FakeClock::now_calls(), 2);
+  EXPECT_EQ(FakeClock::freq_calls(), 0);
+
+  FakeClock::ResetCalls();
+
+  {
+    PageHeapSpinLockHolder l;
+    put_res1 = filler_.Put(alloc_pt, Range(page2, Length(1)), info);
+    put_res2 = filler_.Put(pt, Range(page1, Length(1)), info);
+  }
+
+  EXPECT_EQ(put_res1, nullptr);
+  EXPECT_EQ(put_res2, pt);
+  EXPECT_EQ(FakeClock::now_calls(), 3);
+  EXPECT_EQ(FakeClock::freq_calls(), 1);
+
   delete pt;
 }