[PATCH] cpusets: oom_kill tweaks

This patch series extends the use of the cpuset attribute 'mem_exclusive'
to support cpuset configurations that:
 1) allow GFP_KERNEL allocations to come from a potentially larger
    set of memory nodes than GFP_USER allocations, and
 2) can constrain the oom killer to tasks running in cpusets in
    a specified subtree of the cpuset hierarchy.

Here's an example usage scenario.  For a few hours or more, a large NUMA
system at a University is to be divided in two halves, with a bunch of student
jobs running in half the system under some form of batch manager, and with a
big research project running in the other half.  Each of the student jobs is
placed in a small cpuset, but should share the classic Unix time share
facilities, such as buffered pages of files in /bin and /usr/lib.  The big
research project wants no interference whatsoever from the student jobs, and
has highly tuned, unusual memory and i/o patterns that intend to make full use
of all the main memory on the nodes available to it.

In this example, we have two big sibling cpusets, one of which is further
divided into a more dynamic set of child cpusets.

We want kernel memory allocations constrained by the two big cpusets, and user
allocations constrained by the smaller child cpusets where present.  And we
require that the oom killer not operate across the two halves of this system,
or else the first time a student job runs amuck, the big research project will
likely be first inline to get shot.

Tweaking /proc/<pid>/oom_adj is not ideal -- if the big research project
really does run amuck allocating memory, it should be shot, not some other
task outside the research projects mem_exclusive cpuset.

I propose to extend the use of the 'mem_exclusive' flag of cpusets to manage
such scenarios.  Let memory allocations for user space (GFP_USER) be
constrained by a tasks current cpuset, but memory allocations for kernel space
(GFP_KERNEL) by constrained by the nearest mem_exclusive ancestor of the
current cpuset, even though kernel space allocations will still _prefer_ to
remain within the current tasks cpuset, if memory is easily available.

Let the oom killer be constrained to consider only tasks that are in
overlapping mem_exclusive cpusets (it won't help much to kill a task that
normally cannot allocate memory on any of the same nodes as the ones on which
the current task can allocate.)

The current constraints imposed on setting mem_exclusive are unchanged.  A
cpuset may only be mem_exclusive if its parent is also mem_exclusive, and a
mem_exclusive cpuset may not overlap any of its siblings memory nodes.

This patch was presented on linux-mm in early July 2005, though did not
generate much feedback at that time.  It has been built for a variety of
arch's using cross tools, and built, booted and tested for function on SN2
(ia64).

There are 4 patches in this set:
  1) Some minor cleanup, and some improvements to the code layout
     of one routine to make subsequent patches cleaner.
  2) Add another GFP flag - __GFP_HARDWALL.  It marks memory
     requests for USER space, which are tightly confined by the
     current tasks cpuset.
  3) Now memory requests (such as KERNEL) that not marked HARDWALL can
     if short on memory, look in the potentially larger pool of memory
     defined by the nearest mem_exclusive ancestor cpuset of the current
     tasks cpuset.
  4) Finally, modify the oom killer to skip any task whose mem_exclusive
     cpuset doesn't overlap ours.

Patch (1), the one time I looked on an SN2 (ia64) build, actually saved 32
bytes of kernel text space.  Patch (2) has no affect on the size of kernel
text space (it just adds a preprocessor flag).  Patches (3) and (4) added
about 600 bytes each of kernel text space, mostly in kernel/cpuset.c, which
matters only if CONFIG_CPUSET is enabled.

This patch:

This patch applies a few comment and code cleanups to mm/oom_kill.c prior to
applying a few small patches to improve cpuset management of memory placement.

The comment changed in oom_kill.c was seriously misleading.  The code layout
change in select_bad_process() makes room for adding another condition on
which a process can be spared the oom killer (see the subsequent
cpuset_nodes_overlap patch for this addition).

Also a couple typos and spellos that bugged me, while I was here.

This patch should have no material affect.

Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/oom_kill.c

@@ -6,8 +6,8 @@
  *	for goading me into coding this file...
  *
  *  The routines in this file are used to kill a process when
- *  we're seriously out of memory. This gets called from kswapd()
- *  in linux/mm/vmscan.c when we really run out of memory.
+ *  we're seriously out of memory. This gets called from __alloc_pages()
+ *  in mm/page_alloc.c when we really run out of memory.
  *
  *  Since we won't call these routines often (on a well-configured
  *  machine) this file will double as a 'coding guide' and a signpost
@@ -26,7 +26,7 @@
 /**
  * oom_badness - calculate a numeric value for how bad this task has been
  * @p: task struct of which task we should calculate
- * @p: current uptime in seconds
+ * @uptime: current uptime in seconds
  *
  * The formula used is relatively simple and documented inline in the
  * function. The main rationale is that we want to select a good task
@@ -57,9 +57,9 @@ unsigned long badness(struct task_struct *p, unsigned long uptime)
 
 	/*
 	 * Processes which fork a lot of child processes are likely
-	 * a good choice. We add the vmsize of the childs if they
+	 * a good choice. We add the vmsize of the children if they
 	 * have an own mm. This prevents forking servers to flood the
-	 * machine with an endless amount of childs
+	 * machine with an endless amount of children
 	 */
 	list_for_each(tsk, &p->children) {
 		struct task_struct *chld;
@@ -143,28 +143,32 @@ static struct task_struct * select_bad_process(void)
 	struct timespec uptime;
 
 	do_posix_clock_monotonic_gettime(&uptime);
-	do_each_thread(g, p)
+	do_each_thread(g, p) {
+		unsigned long points;
+		int releasing;
+
 		/* skip the init task with pid == 1 */
-		if (p->pid > 1 && p->oomkilladj != OOM_DISABLE) {
-			unsigned long points;
+		if (p->pid == 1)
+			continue;
+		if (p->oomkilladj == OOM_DISABLE)
+			continue;
+		/*
+		 * This is in the process of releasing memory so for wait it
+		 * to finish before killing some other task by mistake.
+		 */
+		releasing = test_tsk_thread_flag(p, TIF_MEMDIE) ||
+						p->flags & PF_EXITING;
+		if (releasing && !(p->flags & PF_DEAD))
+			return ERR_PTR(-1UL);
+		if (p->flags & PF_SWAPOFF)
+			return p;
 
-			/*
-			 * This is in the process of releasing memory so wait it
-			 * to finish before killing some other task by mistake.
-			 */
-			if ((unlikely(test_tsk_thread_flag(p, TIF_MEMDIE)) || (p->flags & PF_EXITING)) &&
-			    !(p->flags & PF_DEAD))
-				return ERR_PTR(-1UL);
-			if (p->flags & PF_SWAPOFF)
-				return p;
-
-			points = badness(p, uptime.tv_sec);
-			if (points > maxpoints || !chosen) {
-				chosen = p;
-				maxpoints = points;
-			}
+		points = badness(p, uptime.tv_sec);
+		if (points > maxpoints || !chosen) {
+			chosen = p;
+			maxpoints = points;
 		}
-	while_each_thread(g, p);
+	} while_each_thread(g, p);
 	return chosen;
 }
 
@@ -189,7 +193,8 @@ static void __oom_kill_task(task_t *p)
 		return;
 	}
 	task_unlock(p);
-	printk(KERN_ERR "Out of Memory: Killed process %d (%s).\n", p->pid, p->comm);
+	printk(KERN_ERR "Out of Memory: Killed process %d (%s).\n",
+							p->pid, p->comm);
 
 	/*
 	 * We give our sacrificial lamb high priority and access to