[PATCH] sys_sched_getaffinity() & hotplug

Change sched_getaffinity() so that it returns a bitmap that indicates the
legally schedulable cpus that a task is allowed to run on.

Without this patch, if CONFIG_HOTPLUG_CPU is enabled, sched_getaffinity()
unconditionally returns (at least on IA64) a mask with NR_CPUS bits set.
This conveys no useful infornmation except for a kernel compile option.

This fixes a breakage we obseved running recent kernels. We have MPI jobs
that use sched_getaffinity() to determine where to place their threads.
Placing them on non-existant cpus is problematic :-)
Signed-off-by: Jack Steiner <steiner@sgi.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Nathan Lynch <ntl@pobox.com>
Cc: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -4031,7 +4031,7 @@ long sched_getaffinity(pid_t pid, cpumask_t *mask)
 		goto out_unlock;
 
 	retval = 0;
-	cpus_and(*mask, p->cpus_allowed, cpu_possible_map);
+	cpus_and(*mask, p->cpus_allowed, cpu_online_map);
 
 out_unlock:
 	read_unlock(&tasklist_lock);