[libc-commits] [libc] [libc] Add a config option to disable slab reclaiming (PR #151599)
via libc-commits
libc-commits at lists.llvm.org
Thu Jul 31 14:50:50 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libc
Author: Joseph Huber (jhuber6)
<details>
<summary>Changes</summary>
Summary:
Without slab reclaiming this interface is much simpler and it can speed
up cases with a lot of churn. Basically, wastes memory for performance.
---
Full diff: https://github.com/llvm/llvm-project/pull/151599.diff
6 Files Affected:
- (modified) libc/config/config.json (+6)
- (modified) libc/config/gpu/amdgpu/config.json (+5)
- (modified) libc/config/gpu/nvptx/config.json (+5)
- (modified) libc/docs/configure.rst (+2)
- (modified) libc/src/__support/GPU/CMakeLists.txt (+5)
- (modified) libc/src/__support/GPU/allocator.cpp (+12-4)
``````````diff
diff --git a/libc/config/config.json b/libc/config/config.json
index d53b2936edb07..fe077d16342de 100644
--- a/libc/config/config.json
+++ b/libc/config/config.json
@@ -119,6 +119,12 @@
"doc": "Force the size of time_t to 64 bits, even on platforms where compatibility considerations would otherwise make it 32-bit."
}
},
+ "malloc": {
+ "LIBC_CONF_GPU_MALLOC_DISABLE_SLAB_RECLAIMING": {
+ "value": false,
+ "doc": "The malloc implementation will return unused slabs to system memory."
+ }
+ },
"general": {
"LIBC_ADD_NULL_CHECKS": {
"value": true,
diff --git a/libc/config/gpu/amdgpu/config.json b/libc/config/gpu/amdgpu/config.json
index 30ae10e2cfd61..d3918eff8a59d 100644
--- a/libc/config/gpu/amdgpu/config.json
+++ b/libc/config/gpu/amdgpu/config.json
@@ -36,5 +36,10 @@
"LIBC_CONF_MATH_OPTIMIZATIONS": {
"value": "(LIBC_MATH_SKIP_ACCURATE_PASS | LIBC_MATH_SMALL_TABLES | LIBC_MATH_NO_ERRNO | LIBC_MATH_NO_EXCEPT)"
}
+ },
+ "malloc": {
+ "LIBC_CONF_GPU_MALLOC_DISABLE_SLAB_RECLAIMING": {
+ "value": false
+ }
}
}
diff --git a/libc/config/gpu/nvptx/config.json b/libc/config/gpu/nvptx/config.json
index 30ae10e2cfd61..d3918eff8a59d 100644
--- a/libc/config/gpu/nvptx/config.json
+++ b/libc/config/gpu/nvptx/config.json
@@ -36,5 +36,10 @@
"LIBC_CONF_MATH_OPTIMIZATIONS": {
"value": "(LIBC_MATH_SKIP_ACCURATE_PASS | LIBC_MATH_SMALL_TABLES | LIBC_MATH_NO_ERRNO | LIBC_MATH_NO_EXCEPT)"
}
+ },
+ "malloc": {
+ "LIBC_CONF_GPU_MALLOC_DISABLE_SLAB_RECLAIMING": {
+ "value": false
+ }
}
}
diff --git a/libc/docs/configure.rst b/libc/docs/configure.rst
index 109412225634f..4b005bb177da1 100644
--- a/libc/docs/configure.rst
+++ b/libc/docs/configure.rst
@@ -32,6 +32,8 @@ to learn about the defaults for your platform and target.
- ``LIBC_CONF_ERRNO_MODE``: The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_DEFAULT, LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, LIBC_ERRNO_MODE_SYSTEM, and LIBC_ERRNO_MODE_SYSTEM_INLINE.
* **"general" options**
- ``LIBC_ADD_NULL_CHECKS``: Add nullptr checks in the library's implementations to some functions for which passing nullptr is undefined behavior.
+* **"malloc" options**
+ - ``LIBC_CONF_GPU_MALLOC_DISABLE_SLAB_RECLAIMING``: The malloc implementation will return unused slabs to system memory.
* **"math" options**
- ``LIBC_CONF_FREXP_INF_NAN_EXPONENT``: The value written back to the second parameter when calling frexp/frexpf/frexpl` with `+/-Inf`/`NaN` is unspecified. Configure an explicit exp value for Inf/NaN inputs.
- ``LIBC_CONF_MATH_OPTIMIZATIONS``: Configures optimizations for math functions. Values accepted are LIBC_MATH_SKIP_ACCURATE_PASS, LIBC_MATH_SMALL_TABLES, LIBC_MATH_NO_ERRNO, LIBC_MATH_NO_EXCEPT, and LIBC_MATH_FAST.
diff --git a/libc/src/__support/GPU/CMakeLists.txt b/libc/src/__support/GPU/CMakeLists.txt
index f8fdfeb9da9df..e6884a31b6a5b 100644
--- a/libc/src/__support/GPU/CMakeLists.txt
+++ b/libc/src/__support/GPU/CMakeLists.txt
@@ -3,6 +3,10 @@ if(NOT LIBC_TARGET_OS_IS_GPU)
return()
endif()
+if(LIBC_CONF_MALLOC_DISABLE_SLAB_RECLAIMING)
+ list(APPEND malloc_config_copts "-DLIBC_CONF_MALLOC_DISABLE_SLAB_RECLAIMING")
+endif()
+
add_header_library(
utils
HDRS
@@ -23,4 +27,5 @@ add_object_library(
libc.src.__support.CPP.bit
libc.src.__support.CPP.new
.utils
+ ${malloc_config_copts}
)
diff --git a/libc/src/__support/GPU/allocator.cpp b/libc/src/__support/GPU/allocator.cpp
index bd0a55cb938fb..a55ff6b967546 100644
--- a/libc/src/__support/GPU/allocator.cpp
+++ b/libc/src/__support/GPU/allocator.cpp
@@ -39,6 +39,13 @@ constexpr static uint32_t MIN_ALIGNMENT = MIN_SIZE - 1;
// The number of times to attempt claiming an in-progress slab allocation.
constexpr static uint32_t MAX_TRIES = 1024;
+// Configuration for whether or not we will return unused slabs to memory.
+#ifdef LIBC_CONF_MALLOC_DISABLE_SLAB_RECLAIMING
+constexpr static bool RECLAIM = false;
+#else
+constexpr static bool RECLAIM = true;
+#endif
+
static_assert(!(ARRAY_SIZE & (ARRAY_SIZE - 1)), "Must be a power of two");
namespace impl {
@@ -368,7 +375,7 @@ struct GuardPtr {
// and obtain exclusive rights to deconstruct it. If the CAS failed either
// another thread resurrected the counter and we quit, or a parallel read
// helped us invalidating it. For the latter, claim that flag and return.
- if (counter.fetch_sub(n, cpp::MemoryOrder::RELAXED) == n) {
+ if (counter.fetch_sub(n, cpp::MemoryOrder::RELAXED) == n && RECLAIM) {
uint32_t expected = 0;
if (counter.compare_exchange_strong(expected, INVALID,
cpp::MemoryOrder::RELAXED,
@@ -386,8 +393,9 @@ struct GuardPtr {
// thread.
uint64_t read() {
auto val = counter.load(cpp::MemoryOrder::RELAXED);
- if (val == 0 && counter.compare_exchange_strong(
- val, INVALID | HELPED, cpp::MemoryOrder::RELAXED))
+ if (val == 0 && RECLAIM &&
+ counter.compare_exchange_strong(val, INVALID | HELPED,
+ cpp::MemoryOrder::RELAXED))
return 0;
return (val & INVALID) ? 0 : val;
}
@@ -421,7 +429,7 @@ struct GuardPtr {
return nullptr;
cpp::atomic_thread_fence(cpp::MemoryOrder::ACQUIRE);
- return ptr.load(cpp::MemoryOrder::RELAXED);
+ return RECLAIM ? ptr.load(cpp::MemoryOrder::RELAXED) : expected;
}
// Finalize the associated memory and signal that it is ready to use by
``````````
</details>
https://github.com/llvm/llvm-project/pull/151599
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