Use uv_thread_getaffinity when --threads=auto

src/sys.c

@@ -597,6 +597,30 @@ typedef DWORD (WINAPI *GAPC)(WORD);
 #endif
 #endif
 
+static int num_threads_bound(void) JL_NOTSAFEPOINT
+{
+#ifdef _OS_DARWIN_
+    return INT_MAX;
+#else
+    int masksize = uv_cpumask_size();
+    uv_thread_t tid = uv_thread_self();
+    char *cpumask = (char *)calloc(masksize, sizeof(char));
+    int err = uv_thread_getaffinity(&tid, cpumask, masksize);
+    if (err) {
+        free(cpumask);
+        jl_safe_printf("WARNING: failed to get thread affinity (%s %d)\n", uv_err_name(err),
+                       err);
+        return INT_MAX;
+    }
+    int n = 0;
+    for (size_t i = 0; i < masksize; i++) {
+        n += cpumask[i];
+    }
+    free(cpumask);
+    return n;
+#endif
+}
+
 // Apple's M1 processor is a big.LITTLE style processor, with 4x "performance"
 // cores, and 4x "efficiency" cores.  Because Julia expects to be able to run
 // things like heavy linear algebra workloads on all cores, it's best for us
@@ -608,6 +632,7 @@ typedef DWORD (WINAPI *GAPC)(WORD);
 
 JL_DLLEXPORT int jl_cpu_threads(void) JL_NOTSAFEPOINT
 {
+    int upper_bound = num_threads_bound();
 #if defined(HW_AVAILCPU) && defined(HW_NCPU)
     size_t len = 4;
     int32_t count;
@@ -631,22 +656,24 @@ JL_DLLEXPORT int jl_cpu_threads(void) JL_NOTSAFEPOINT
         }
     }
 #endif
-    return count;
+    return upper_bound < count ? upper_bound : count;
 #elif defined(_SC_NPROCESSORS_ONLN)
     long count = sysconf(_SC_NPROCESSORS_ONLN);
     if (count < 1)
         return 1;
-    return count;
+    return upper_bound < count ? upper_bound : count;
 #elif defined(_OS_WINDOWS_)
     //Try to get WIN7 API method
     GAPC gapc;
     if (jl_dlsym(jl_kernel32_handle, "GetActiveProcessorCount", (void **)&gapc, 0)) {
-        return gapc(ALL_PROCESSOR_GROUPS);
+        DWORD count = gapc(ALL_PROCESSOR_GROUPS);
+        return upper_bound < count ? upper_bound : count;
     }
     else { //fall back on GetSystemInfo
         SYSTEM_INFO info;
         GetSystemInfo(&info);
-        return info.dwNumberOfProcessors;
+        DWORD count = info.dwNumberOfProcessors;
+        return upper_bound < count ? upper_bound : count;
     }
 #else
 #warning "cpu core detection not defined for this platform"