Fix JULIA_EXCLUSIVE setting affinity on non-worker threads

With JULIA_EXCLUSIVE=1, we would try to give both worker and interactive threads
an exclusive CPU, causing --threads=auto to produce a "Too many threads
requested" error.

Fixes JuliaLang#50702.

Author: xal-0

src/threading.c

@@ -794,6 +794,9 @@ void jl_start_threads(void)
 {
     int nthreads = jl_atomic_load_relaxed(&jl_n_threads);
     int ngcthreads = jl_n_gcthreads;
+    int nthreadsi = jl_n_threads_per_pool[0];
+    int nmutator_threads = nthreads - ngcthreads;
+
     int cpumasksize = uv_cpumask_size();
     char *cp;
     int i, exclusive;
@@ -812,32 +815,33 @@ void jl_start_threads(void)
     // according to a 'compact' policy
     // non-exclusive: no affinity settings; let the kernel move threads about
     if (exclusive) {
-        if (nthreads > jl_cpu_threads()) {
+        if (nmutator_threads - nthreadsi > jl_cpu_threads()) {
             jl_printf(JL_STDERR, "ERROR: Too many threads requested for %s option.\n", MACHINE_EXCLUSIVE_NAME);
             exit(1);
         }
         memset(mask, 0, cpumasksize);
-        mask[0] = 1;
-        uvtid = uv_thread_self();
-        uv_thread_setaffinity(&uvtid, mask, NULL, cpumasksize);
-        mask[0] = 0;
+
+        if (nthreadsi == 0) {
+            mask[0] = 1;
+            uvtid = uv_thread_self();
+            uv_thread_setaffinity(&uvtid, mask, NULL, cpumasksize);
+            mask[0] = 0;
+        }
     }
 
     // create threads
     uv_barrier_init(&thread_init_done, nthreads);
 
     // GC/System threads need to be after the worker threads.
-    int nmutator_threads = nthreads - ngcthreads;
-
     for (i = 1; i < nmutator_threads; ++i) {
         jl_threadarg_t *t = (jl_threadarg_t *)malloc_s(sizeof(jl_threadarg_t)); // ownership will be passed to the thread
         t->tid = i;
         t->barrier = &thread_init_done;
         uv_thread_create(&uvtid, jl_threadfun, t);
-        if (exclusive) {
-            mask[i] = 1;
+        if (exclusive && i > nthreadsi) {
+            mask[i - nthreadsi] = 1;
             uv_thread_setaffinity(&uvtid, mask, NULL, cpumasksize);
-            mask[i] = 0;
+            mask[i - nthreadsi] = 0;
         }
         uv_thread_detach(&uvtid);
     }

test/threads.jl

@@ -123,10 +123,11 @@ if AFFINITY_SUPPORTED
     end
 end
 
-function get_nthreads(options = ``; cpus = nothing)
+function get_nthreads(options = ``; cpus = nothing, exclusive = false)
     cmd = `$(Base.julia_cmd()) --startup-file=no $(options)`
     cmd = `$cmd -e "print(Threads.threadpoolsize())"`
-    cmd = addenv(cmd, "JULIA_EXCLUSIVE" => "0", "JULIA_NUM_THREADS" => "auto")
+    cmd = addenv(cmd, "JULIA_EXCLUSIVE" => exclusive ? "1" : "0",
+        "JULIA_NUM_THREADS" => "auto")
     if cpus !== nothing
         cmd = setcpuaffinity(cmd, cpus)
     end
@@ -138,6 +139,7 @@ end
         allowed_cpus = findall(uv_thread_getaffinity())
         if length(allowed_cpus) ≥ 2
             @test get_nthreads() ≥ 2
+            @test get_nthreads(exclusive = true) ≥ 2
             @test get_nthreads(cpus = allowed_cpus[1:1]) == 1
             @test get_nthreads(cpus = allowed_cpus[2:2]) == 1
             @test get_nthreads(cpus = allowed_cpus[1:2]) == 2