Fix runtime initialization on linux with cpu hotplug enabled

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