Update Environment.ProcessorCount on Windows to take into account the processor affinity mask (#45943)

- Similarly to cases on Unixes where sched_getaffinity is available
- If `GCCpuGroup` and `Thread_UseAllCpuGroups` are both enabled, I'm not sure if the `CPUGroupInfo` count of active processors takes affinity into account as the docs are not clear, for now I'm not modifying that path until I can verify it
- Otherwise, a process that is started with a specific processor affinity mask still shows full CPU count
- This is one of the differences in the portable managed thread pool implementation, which relies on Environment.ProcessorCount, as opposed to the native thread pool, which uses the affinity mask
- After this change, in affinitized cases on Windows the behavior is consistent perf-wise with Linux in similar situations:
  - The portable thread pool uses the same worker thread count as the native thread pool
  - `Environment.ProcessorCount` returns the number of processors the the process is affinitized to, which may be less than it would have returned before

1 files changed, 6 insertions, 16 deletions

diff --git a/src/coreclr/classlibnative/bcltype/system.cpp b/src/coreclr/classlibnative/bcltype/system.cpp
index 91a39ef9d08..2bcf2d56990 100644
--- a/src/coreclr/classlibnative/bcltype/system.cpp
+++ b/src/coreclr/classlibnative/bcltype/system.cpp
@@ -185,29 +185,19 @@ INT32 QCALLTYPE SystemNative::GetProcessorCount()
 #ifndef TARGET_UNIX
     CPUGroupInfo::EnsureInitialized();
 
-    if(CPUGroupInfo::CanEnableThreadUseAllCpuGroups())
+    if (CPUGroupInfo::CanEnableThreadUseAllCpuGroups())
     {
         processorCount = CPUGroupInfo::GetNumActiveProcessors();
     }
+    else
 #endif // !TARGET_UNIX
-    // Processor count will be 0 if CPU groups are disabled/not supported
-    if(processorCount == 0)
     {
-        SYSTEM_INFO systemInfo;
-        ZeroMemory(&systemInfo, sizeof(systemInfo));
-
-        GetSystemInfo(&systemInfo);
-
-        processorCount = systemInfo.dwNumberOfProcessors;
+        // This similar to GetSystemInfo() + dwNumberOfProcessors except:
+        // - GetCurrentProcessCpuCount() tries to take into account the processor affinity mask where applicable
+        // - GetCurrentProcessCpuCount() on Unixes tries to take into account cgroups CPU quota limits where applicable
+        processorCount = GetCurrentProcessCpuCount();
     }
 
-#ifdef TARGET_UNIX
-    uint32_t cpuLimit;
-
-    if (PAL_GetCpuLimit(&cpuLimit) && cpuLimit < (uint32_t)processorCount)
-        processorCount = cpuLimit;
-#endif
-
     END_QCALL;
 
     return processorCount;