Re: For review: pthread_setaffinity_np.3

From: "Michael Kerrisk" <mtk.manpages-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>
To: Loic Domaigne <tech-Z4JMKDdsf89Wk0Htik3J/w@public.gmane.org>,
	Paul Jackson <pj-sJ/iWh9BUns@public.gmane.org>
Cc: linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	josv-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org,
	"brian m. carlson"
	<sandals-spVehEqlxw627WubY2PhZQivdfXVPZ6z@public.gmane.org>,
	Bert Wesarg <bert.wesarg-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>,
	Stefan Puiu
	<stefanpuiuro-/E1597aS9LQAvxtiuMwx3w@public.gmane.org>,
	Karsten Weiss
	<K.Weiss-Pt+Xe7GJXK+P2YhJcF5u+nqWYbMAw+HU@public.gmane.org>
Subject: Re: For review: pthread_setaffinity_np.3
Date: Fri, 14 Nov 2008 10:38:56 -0500	[thread overview]
Message-ID: <cfd18e0f0811140738i33f2c671kc99bbb7750f2fa44@mail.gmail.com> (raw)
In-Reply-To: <cfd18e0f0811140704l18e5741fgdbb5a6c52fb8cbe7-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

[CC += "Paul Jackson" <pj-sJ/iWh9BUns@public.gmane.org>, who might want to Ack the patch below]

>> This might be obvious to all of you, but it wasn't to me... The CPU set used
>> for the affinity is the intersection of the set passed to
>> pthread_setaffinity_np() and the set of processors supported by the
>> kernel...
>>
>> That's why your example shall work, even if the system where it runs has
>> less than 8 processors.
>
> It's true.  The page should say something to make this clearer.  I
> added this text to NOTES
>
>       After a call to  pthread_setaffinity_np(3),  the  set  of
>       CPUs  on which the thread will actually run is the inter-
>       section of the set specified in the cpuset  argument  and
>       the set of CPUs actually present on the system.  The sys-
>       tem may further restrict the set of  CPUs  on  which  the
>       thread  runs  if  the  "cpuset"  mechanism  described  in
>       cpuset(7) is  being  used.   These  restrictions  on  the
>       actual  set  of  CPUs  on  which  the thread will run are
>       silently imposed by the kernel.

Of course, the sched_setaffinity.2 page also deserves similar text.  I
made the change below.

Look ok?

Cheers,

Michael

--- a/man2/sched_setaffinity.2
+++ b/man2/sched_setaffinity.2

@@ -122,7 +122,10 @@ A supplied memory address was invalid.
 .B EINVAL
 The affinity bit mask
 .I mask
-contains no processors that are physically on the system.
+contains no processors that are currently physically on the system
+and permitted to the process according to any restrictions that
+may be imposed by the "cpuset" mechanism described in
+.BR cpuset (7).
 .TP
 .B EINVAL
 .RB ( sched_getaffinity ()
@@ -157,6 +160,19 @@ argument was removed, but was then restored in glibc 2.3.4,
 with type
 .SH "CONFORMING TO"
 These system calls are Linux-specific.
 .SH "NOTES"
+After a call to
+.BR sched_setaffinity (),
+the set of CPUs on which the process will actually run is
+the intersection of the set specified in the
+.I mask
+argument and the set of CPUs actually present on the system.
+The system may further restrict the set of CPUs on which the process
+runs if the "cpuset" mechanism described in
+.BR cpuset (7)
+is being used.
+These restrictions on the actual set of CPUs on which the process
+will run are silently imposed by the kernel.
+
 .BR sched_setscheduler (2)
 has a description of the Linux scheduling scheme.
 .PP
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