Separate foreign threads into a :foreign threadpool

base/partr.jl

@@ -88,6 +88,7 @@ end
 
 function multiq_insert(task::Task, priority::UInt16)
     tpid = ccall(:jl_get_task_threadpoolid, Int8, (Any,), task)
+    @assert tpid > -1
     heap_p = multiq_size(tpid)
     tp = tpid + 1
 
@@ -124,6 +125,9 @@ function multiq_deletemin()
 
     tid = Threads.threadid()
     tp = ccall(:jl_threadpoolid, Int8, (Int16,), tid-1) + 1
+    if tp == 0 # Foreign thread
+        return nothing
+    end
     tpheaps = heaps[tp]
 
     @label retry
@@ -175,6 +179,9 @@ end
 function multiq_check_empty()
     tid = Threads.threadid()
     tp = ccall(:jl_threadpoolid, Int8, (Int16,), tid-1) + 1
+    if tp == 0 # Foreign thread
+        return true
+    end
     for i = UInt32(1):length(heaps[tp])
         if heaps[tp][i].ntasks != 0
             return false

base/task.jl

@@ -798,7 +798,7 @@ function enq_work(t::Task)
     else
         @label not_sticky
         tp = Threads.threadpool(t)
-        if Threads.threadpoolsize(tp) == 1
+        if tp === :foreign || Threads.threadpoolsize(tp) == 1
             # There's only one thread in the task's assigned thread pool;
             # use its work queue.
             tid = (tp === :interactive) ? 1 : Threads.threadpoolsize(:interactive)+1

base/threadingconstructs.jl

@@ -63,6 +63,8 @@ function _tpid_to_sym(tpid::Int8)
         return :interactive
     elseif tpid == 1
         return :default
+    elseif tpid == -1
+        return :foreign
     else
         throw(ArgumentError("Unrecognized threadpool id $tpid"))
     end
@@ -73,6 +75,8 @@ function _sym_to_tpid(tp::Symbol)
         return Int8(0)
     elseif tp === :default
         return Int8(1)
+    elseif tp == :foreign
+        return Int8(-1)
     else
         throw(ArgumentError("Unrecognized threadpool name `$(repr(tp))`"))
     end
@@ -81,7 +85,7 @@ end
 """
     Threads.threadpool(tid = threadid()) -> Symbol
 
-Returns the specified thread's threadpool; either `:default` or `:interactive`.
+Returns the specified thread's threadpool; either `:default`, `:interactive`, or `:foreign`.
 """
 function threadpool(tid = threadid())
     tpid = ccall(:jl_threadpoolid, Int8, (Int16,), tid-1)
@@ -108,6 +112,8 @@ See also: `BLAS.get_num_threads` and `BLAS.set_num_threads` in the
 function threadpoolsize(pool::Symbol = :default)
     if pool === :default || pool === :interactive
         tpid = _sym_to_tpid(pool)
+    elseif pool == :foreign
+        error("Threadpool size of `:foreign` is indeterminant")
     else
         error("invalid threadpool specified")
     end
@@ -151,7 +157,7 @@ function threading_run(fun, static)
         else
             # TODO: this should be the current pool (except interactive) if there
             # are ever more than two pools.
-            ccall(:jl_set_task_threadpoolid, Cint, (Any, Int8), t, _sym_to_tpid(:default))
+            @assert ccall(:jl_set_task_threadpoolid, Cint, (Any, Int8), t, _sym_to_tpid(:default)) == 1
         end
         tasks[i] = t
         schedule(t)
@@ -357,10 +363,10 @@ end
 
 function _spawn_set_thrpool(t::Task, tp::Symbol)
     tpid = _sym_to_tpid(tp)
-    if _nthreads_in_pool(tpid) == 0
+    if tpid == -1 || _nthreads_in_pool(tpid) == 0
         tpid = _sym_to_tpid(:default)
     end
-    ccall(:jl_set_task_threadpoolid, Cint, (Any, Int8), t, tpid)
+    @assert ccall(:jl_set_task_threadpoolid, Cint, (Any, Int8), t, tpid) == 1
     nothing
 end
 

src/partr.c

@@ -70,7 +70,7 @@ JL_DLLEXPORT int jl_set_task_tid(jl_task_t *task, int16_t tid) JL_NOTSAFEPOINT
 
 JL_DLLEXPORT int jl_set_task_threadpoolid(jl_task_t *task, int8_t tpid) JL_NOTSAFEPOINT
 {
-    if (tpid < 0 || tpid >= jl_n_threadpools)
+    if (tpid < -1 || tpid >= jl_n_threadpools)
         return 0;
     task->threadpoolid = tpid;
     return 1;

src/threading.c

@@ -332,7 +332,7 @@ JL_DLLEXPORT int8_t jl_threadpoolid(int16_t tid) JL_NOTSAFEPOINT
         if (tid < n)
             return (int8_t)i;
     }
-    return 0; // everything else uses threadpool 0 (though does not become part of any threadpool)
+    return -1; // everything else uses threadpool -1 (does not belong to any threadpool)
 }
 
 jl_ptls_t jl_init_threadtls(int16_t tid)