diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index e109b1ebe1cee4..415d31ccd3633b 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -948,7 +948,7 @@ Miscellaneous
    Return the number of CPUs in the system.
 
    This number is not equivalent to the number of CPUs the current process can
-   use.  The number of usable CPUs can be obtained with
+   use.  On some Unix platforms, the number of usable CPUs can be obtained with
    ``len(os.sched_getaffinity(0))``
 
    May raise :exc:`NotImplementedError`.
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 371d59e9c31a45..1e5325166056ec 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -4691,7 +4691,7 @@ Miscellaneous System Information
    Return the number of CPUs in the system. Returns ``None`` if undetermined.
 
    This number is not equivalent to the number of CPUs the current process can
-   use.  The number of usable CPUs can be obtained with
+   use.  On some Unix platforms, the number of usable CPUs can be obtained with
    ``len(os.sched_getaffinity(0))``
 
 
diff --git a/Misc/NEWS.d/next/Documentation/2021-04-17-18-04-34.bpo-43881.99JWsY.rst b/Misc/NEWS.d/next/Documentation/2021-04-17-18-04-34.bpo-43881.99JWsY.rst
new file mode 100644
index 00000000000000..9ce59da56cda97
--- /dev/null
+++ b/Misc/NEWS.d/next/Documentation/2021-04-17-18-04-34.bpo-43881.99JWsY.rst
@@ -0,0 +1 @@
+Add platform availabity information for :func:`os.sched_getaffinity` where its usage is recommended.
\ No newline at end of file
diff --git a/Modules/clinic/posixmodule.c.h b/Modules/clinic/posixmodule.c.h
index 36bb7c3ee2e300..b0bbd42c68e86b 100644
--- a/Modules/clinic/posixmodule.c.h
+++ b/Modules/clinic/posixmodule.c.h
@@ -7947,7 +7947,7 @@ PyDoc_STRVAR(os_cpu_count__doc__,
 "Return the number of CPUs in the system; return None if indeterminable.\n"
 "\n"
 "This number is not equivalent to the number of CPUs the current process can\n"
-"use.  The number of usable CPUs can be obtained with\n"
+"use.  On some Unix platforms, the number of usable CPUs can be obtained with\n"
 "``len(os.sched_getaffinity(0))``");
 
 #define OS_CPU_COUNT_METHODDEF    \
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 65e8d5e7bd984d..8fd7a493f8615e 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -13175,7 +13175,7 @@ os.cpu_count
 Return the number of CPUs in the system; return None if indeterminable.
 
 This number is not equivalent to the number of CPUs the current process can
-use.  The number of usable CPUs can be obtained with
+use.  On some Unix platforms, the number of usable CPUs can be obtained with
 ``len(os.sched_getaffinity(0))``
 [clinic start generated code]*/