[scudo] Update secondary cache time-based release logic

[JoshuaMBa]

Secondary cache entries are now released to the OS from least recent to most recent entries. This helps to avoid unnecessary scans of the cache since entries ready to be released (specifically, entries that are considered old relative to the configurable release interval) will always be at the tail of the list of committed entries by the LRU ordering. For this same reason, the OldestTime variable is no longer needed to indicate when releases are necessary so it has been removed.

[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Joshua Baehring (JoshuaMBa)

Changes

Secondary cache entries are now released to the OS from least recent to most recent entries. This helps to avoid unnecessary scans of the cache since entries ready to be released (specifically, entries that are considered old relative to the configurable release interval) will always be at the tail of the list of committed entries by the LRU ordering. For this same reason, the OldestTime variable is no longer needed to indicate when releases are necessary so it has been removed.

---

Full diff: https://github.com/llvm/llvm-project/pull/107507.diff

1 Files Affected:

• (modified) compiler-rt/lib/scudo/standalone/secondary.h (+31-21)

diff --git a/compiler-rt/lib/scudo/standalone/secondary.h b/compiler-rt/lib/scudo/standalone/secondary.h
index 1a232b9b9fb2d7..de3038e66d1b52 100644
--- a/compiler-rt/lib/scudo/standalone/secondary.h
+++ b/compiler-rt/lib/scudo/standalone/secondary.h
@@ -318,8 +318,6 @@ class MapAllocatorCache {
         }
         CachedBlock PrevEntry = Quarantine[QuarantinePos];
         Quarantine[QuarantinePos] = Entry;
-        if (OldestTime == 0)
-          OldestTime = Entry.Time;
         Entry = PrevEntry;
       }
 
@@ -331,9 +329,6 @@ class MapAllocatorCache {
       }
 
       insert(Entry);
-
-      if (OldestTime == 0)
-        OldestTime = Entry.Time;
     } while (0);
 
     for (MemMapT &EvictMemMap : EvictionMemMaps)
@@ -514,6 +509,11 @@ class MapAllocatorCache {
     return (EntriesCount >= atomic_load_relaxed(&MaxEntriesCount));
   }
 
+  inline void advanceDecommitBase() REQUIRES(Mutex) {
+    if (DecommitBase != CachedBlock::InvalidEntry)
+      DecommitBase = Entries[DecommitBase].Prev;
+  }
+
   void insert(const CachedBlock &Entry) REQUIRES(Mutex) {
     DCHECK_LT(EntriesCount, atomic_load_relaxed(&MaxEntriesCount));
 
@@ -532,6 +532,9 @@ class MapAllocatorCache {
       Entries[LRUHead].Prev = static_cast<u16>(FreeIndex);
     }
 
+    if (DecommitBase == CachedBlock::InvalidEntry)
+      DecommitBase = static_cast<u16>(FreeIndex);
+
     Entries[FreeIndex] = Entry;
     Entries[FreeIndex].Next = LRUHead;
     Entries[FreeIndex].Prev = CachedBlock::InvalidEntry;
@@ -549,6 +552,9 @@ class MapAllocatorCache {
 
     Entries[I].invalidate();
 
+    if (I == DecommitBase)
+      advanceDecommitBase();
+
     if (I == LRUHead)
       LRUHead = Entries[I].Next;
     else
@@ -590,35 +596,36 @@ class MapAllocatorCache {
     }
   }
 
-  void releaseIfOlderThan(CachedBlock &Entry, u64 Time) REQUIRES(Mutex) {
-    if (!Entry.isValid() || !Entry.Time)
-      return;
-    if (Entry.Time > Time) {
-      if (OldestTime == 0 || Entry.Time < OldestTime)
-        OldestTime = Entry.Time;
-      return;
-    }
+  inline void release(CachedBlock &Entry) {
+    DCHECK(Entry.Time != 0);
     Entry.MemMap.releaseAndZeroPagesToOS(Entry.CommitBase, Entry.CommitSize);
     Entry.Time = 0;
   }
 
   void releaseOlderThan(u64 Time) EXCLUDES(Mutex) {
     ScopedLock L(Mutex);
-    if (!EntriesCount || OldestTime == 0 || OldestTime > Time)
+    if (!EntriesCount)
       return;
-    OldestTime = 0;
-    for (uptr I = 0; I < Config::getQuarantineSize(); I++)
-      releaseIfOlderThan(Quarantine[I], Time);
-    for (uptr I = 0; I < Config::getEntriesArraySize(); I++)
-      releaseIfOlderThan(Entries[I], Time);
-  }
 
+    for (uptr I = 0; I < Config::getQuarantineSize(); I++) {
+      CachedBlock &ReleaseEntry = Quarantine[I];
+      if (!ReleaseEntry.isValid() || ReleaseEntry.Time > Time)
+        continue;
+      release(ReleaseEntry);
+    }
+
+    // Release oldest entries first by releasing from decommit base
+    while (DecommitBase != CachedBlock::InvalidEntry &&
+           Entries[DecommitBase].Time <= Time) {
+      release(Entries[DecommitBase]);
+      advanceDecommitBase();
+    }
+  }
   HybridMutex Mutex;
   u32 EntriesCount GUARDED_BY(Mutex) = 0;
   u32 QuarantinePos GUARDED_BY(Mutex) = 0;
   atomic_u32 MaxEntriesCount = {};
   atomic_uptr MaxEntrySize = {};
-  u64 OldestTime GUARDED_BY(Mutex) = 0;
   atomic_s32 ReleaseToOsIntervalMs = {};
   u32 CallsToRetrieve GUARDED_BY(Mutex) = 0;
   u32 SuccessfulRetrieves GUARDED_BY(Mutex) = 0;
@@ -633,6 +640,9 @@ class MapAllocatorCache {
   u16 LRUTail GUARDED_BY(Mutex) = 0;
   // The AvailableHead is the top of the stack of available entries
   u16 AvailableHead GUARDED_BY(Mutex) = 0;
+  // The DecommitBase is the least recently used entry that has not
+  // been released
+  u16 DecommitBase GUARDED_BY(Mutex) = 0;
 };
 
 template <typename Config> class MapAllocator {

[ChiaHungDuan]

I haven't got time to reproduce the problem detected by the buildbot. I'll try it later this week and update accordingly.

[JoshuaMBa]

I haven't got time to reproduce the problem detected by the buildbot. I'll try it later this week and update accordingly.

Ok, sounds good. It looks like a scudo unit test failed, which is weird because I'm not getting it on my end. Could be dependent on the configuration, but I also had to do some weird workarounds to get the tests running on my local machine.