Fix runtime initialization on linux with cpu hotplug enabled (#128069)

On unix, during initialization, the runtime obtains the total number of
CPUs via `sysconf(_SC_NPROCESSORS_CONF)`. This should return the current
number of cpus that are currently present on the system. It turns out
linux has cpu hotplug support, so this number can increase. When hotplug
is enabled, the kernel reserves storage for the max possible number of
CPUs. This max number is exported in `/sys/devices/system/cpu/possible`.
The problem is that, when allocating the `cpu_set_t*` for use with
`sched_getaffinity`, this api failed because the OS expected for the cpu
set to have reserved space for the maximum amount of cpu's, not just for
the ones that are currently present.

Author: BrzVlad

src/coreclr/gc/unix/gcenv.unix.cpp

@@ -26,6 +26,7 @@
 #include <minipal/memorybarrierprocesswide.h>
 #include <minipal/thread.h>
 #include <minipal/time.h>
+#include <minipal/cpucount.h>
 
 #if HAVE_SWAPCTL
 #include <sys/swap.h>
@@ -146,7 +147,7 @@ bool GCToOSInterface::Initialize()
         return false;
     }
 
-    int configuredCpuCount = sysconf(_SC_NPROCESSORS_CONF);
+    int configuredCpuCount = minipal_get_cpu_max_possible_count();
     if (configuredCpuCount == -1)
     {
         return false;

src/coreclr/nativeaot/Runtime/unix/PalUnix.cpp

@@ -28,6 +28,7 @@
 #include "RhConfig.h"
 
 #include <unistd.h>
+#include <minipal/cpucount.h>
 #include <sched.h>
 #include <sys/mman.h>
 #include <sys/types.h>
@@ -456,10 +457,10 @@ void InitializeCurrentProcessCpuCount()
     {
 #if HAVE_SCHED_GETAFFINITY
 
-        int configuredCpuCount = sysconf(_SC_NPROCESSORS_CONF);
+        int configuredCpuCount = minipal_get_cpu_max_possible_count();
         if (configuredCpuCount == -1)
         {
-            // In the unlikely event that sysconf(_SC_NPROCESSORS_CONF) fails, just assume a reasonable default maximum number of CPUs to avoid failing.
+            // In the unlikely event that minipal_get_cpu_max_possible_count() fails, just assume a reasonable default maximum number of CPUs to avoid failing.
             configuredCpuCount = CPU_SETSIZE;
         }
 

src/coreclr/pal/src/misc/sysinfo.cpp

@@ -26,6 +26,7 @@ Revision History:
 #include <unistd.h>
 #include <minipal/utils.h>
 #include <minipal/ospagesize.h>
+#include <minipal/cpucount.h>
 #define __STDC_FORMAT_MACROS
 #include <inttypes.h>
 #include <sys/types.h>
@@ -157,10 +158,10 @@ PAL_GetLogicalCpuCountFromOS()
     {
 #if HAVE_SCHED_GETAFFINITY
 
-        int configuredCpuCount = sysconf(_SC_NPROCESSORS_CONF);
+        int configuredCpuCount = minipal_get_cpu_max_possible_count();
         if (configuredCpuCount == -1)
         {
-            // In the unlikely event that sysconf(_SC_NPROCESSORS_CONF) fails, just assume a reasonable default maximum number of CPUs to avoid failing.
+            // In the unlikely event that minipal_get_cpu_max_possible_count() fails, just assume a reasonable default maximum number of CPUs to avoid failing.
             configuredCpuCount = CPU_SETSIZE;
         }
 

src/coreclr/pal/src/thread/thread.cpp

@@ -28,6 +28,7 @@ SET_DEFAULT_DEBUG_CHANNEL(THREAD); // some headers have code with asserts, so do
 #include "pal/virtual.h"
 
 #include <minipal/thread.h>
+#include <minipal/cpucount.h>
 
 #if defined(__NetBSD__) && !HAVE_PTHREAD_GETCPUCLOCKID
 #include <sys/cdefs.h>
@@ -1392,10 +1393,10 @@ CPalThread::ThreadEntry(
     // - https://forum.snapcraft.io/t/requesting-autoconnect-for-interfaces-in-pigmeat-process-control-home/17987/13
 
     {
-        int configuredCpuCount = sysconf(_SC_NPROCESSORS_CONF);
+        int configuredCpuCount = minipal_get_cpu_max_possible_count();
         if (configuredCpuCount == -1)
         {
-            // In the unlikely event that sysconf(_SC_NPROCESSORS_CONF) fails, just assume a reasonable default maximum number of CPUs to avoid failing thread creation.
+            // In the unlikely event that minipal_get_cpu_max_possible_count() fails, just assume a reasonable default maximum number of CPUs to avoid failing thread creation.
             configuredCpuCount = CPU_SETSIZE;
         }
 

src/native/minipal/CMakeLists.txt

@@ -24,6 +24,10 @@ if(NOT WIN32 AND NOT CLR_CMAKE_TARGET_ARCH_WASM AND NOT HOST_WASM)
     list(APPEND SOURCES ospagesize.c)
 endif()
 
+if(CLR_CMAKE_HOST_UNIX)
+    list(APPEND SOURCES cpucount.c)
+endif()
+
 # Provide an object library for scenarios where we ship static libraries
 include_directories(${CLR_SRC_NATIVE_DIR} ${CMAKE_CURRENT_BINARY_DIR})
 

src/native/minipal/cpucount.c

@@ -0,0 +1,49 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#include "cpucount.h"
+
+#include <stdio.h>
+#include <unistd.h>
+
+int minipal_get_cpu_max_possible_count(void)
+{
+#if defined(__linux__)
+    FILE* f = fopen("/sys/devices/system/cpu/possible", "r");
+    if (f != NULL)
+    {
+        int maxCpu = -1;
+        for (;;)
+        {
+            int lo, hi;
+            int matched = fscanf(f, "%d-%d", &lo, &hi);
+            if (matched == 1)
+            {
+                hi = lo;
+            }
+            else if (matched != 2)
+            {
+                break;
+            }
+
+            if (maxCpu < hi)
+            {
+                maxCpu = hi;
+            }
+
+            int ch = fgetc(f);
+            if (ch == EOF || ch != ',')
+            {
+                break;
+            }
+        }
+        fclose(f);
+        if (maxCpu != -1)
+        {
+            return maxCpu + 1;
+        }
+    }
+#endif
+
+    return (int)sysconf(_SC_NPROCESSORS_CONF);
+}

src/native/minipal/cpucount.h

@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#ifndef HAVE_MINIPAL_CPUCOUNT_H
+#define HAVE_MINIPAL_CPUCOUNT_H
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif // __cplusplus
+
+// Returns the maximum number of CPUs that could ever be available on this system,
+// suitable for sizing cpu_set_t allocations via CPU_ALLOC.
+//
+// On Linux, this reads /sys/devices/system/cpu/possible to account for CPU hotplug.
+// This may be larger than the number of online or present CPUs.
+// Falls back to sysconf(_SC_NPROCESSORS_CONF) if the sysfs file is unavailable.
+int minipal_get_cpu_max_possible_count(void);
+
+#ifdef __cplusplus
+}
+#endif // __cplusplus
+
+#endif // HAVE_MINIPAL_CPUCOUNT_H