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Subject[PATCH 2/4] cpuset: rewrite update_tasks_nodemask()
This patch uses cgroup_scan_tasks() to rebind tasks' vmas to new cpuset's
mems_allowed.

Not only simplify the code largely, but also avoid allocating an array
to hold mm pointers of all the tasks in the cpuset. This array can be big
(size > PAGESIZE) if we have lots of tasks in that cpuset, thus has a
chance to fail the allocation when under memory stress.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
kernel/cpuset.c | 109 ++++++++++++++++++++-----------------------------------
1 files changed, 39 insertions(+), 70 deletions(-)
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 20c2db2..01a111f 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1026,6 +1026,31 @@ static void cpuset_migrate_mm(struct mm_struct *mm, const nodemask_t *from,
mutex_unlock(&callback_mutex);
}

+/*
+ * Rebind task's vmas to cpuset's new mems_allowed, and migrate pages to new
+ * nodes if memory_migrate flag is set. Called with cgroup_mutex held.
+ */
+static void cpuset_change_nodemask(struct task_struct *p,
+ struct cgroup_scanner *scan)
+{
+ struct mm_struct *mm;
+ struct cpuset *cs;
+ int migrate;
+ const nodemask_t *oldmem = scan->data;
+
+ mm = get_task_mm(p);
+ if (!mm)
+ return;
+
+ cs = cgroup_cs(scan->cg);
+ migrate = is_memory_migrate(cs);
+
+ mpol_rebind_mm(mm, &cs->mems_allowed);
+ if (migrate)
+ cpuset_migrate_mm(mm, oldmem, &cs->mems_allowed);
+ mmput(mm);
+}
+
static void *cpuset_being_rebound;

/**
@@ -1038,88 +1063,32 @@ static void *cpuset_being_rebound;
*/
static int update_tasks_nodemask(struct cpuset *cs, const nodemask_t *oldmem)
{
- struct task_struct *p;
- struct mm_struct **mmarray;
- int i, n, ntasks;
- int migrate;
- int fudge;
- struct cgroup_iter it;
int retval;
+ struct cgroup_scanner scan;

cpuset_being_rebound = cs; /* causes mpol_dup() rebind */

- fudge = 10; /* spare mmarray[] slots */
- fudge += cpumask_weight(cs->cpus_allowed);/* imagine 1 fork-bomb/cpu */
- retval = -ENOMEM;
-
- /*
- * Allocate mmarray[] to hold mm reference for each task
- * in cpuset cs. Can't kmalloc GFP_KERNEL while holding
- * tasklist_lock. We could use GFP_ATOMIC, but with a
- * few more lines of code, we can retry until we get a big
- * enough mmarray[] w/o using GFP_ATOMIC.
- */
- while (1) {
- ntasks = cgroup_task_count(cs->css.cgroup); /* guess */
- ntasks += fudge;
- mmarray = kmalloc(ntasks * sizeof(*mmarray), GFP_KERNEL);
- if (!mmarray)
- goto done;
- read_lock(&tasklist_lock); /* block fork */
- if (cgroup_task_count(cs->css.cgroup) <= ntasks)
- break; /* got enough */
- read_unlock(&tasklist_lock); /* try again */
- kfree(mmarray);
- }
-
- n = 0;
-
- /* Load up mmarray[] with mm reference for each task in cpuset. */
- cgroup_iter_start(cs->css.cgroup, &it);
- while ((p = cgroup_iter_next(cs->css.cgroup, &it))) {
- struct mm_struct *mm;
-
- if (n >= ntasks) {
- printk(KERN_WARNING
- "Cpuset mempolicy rebind incomplete.\n");
- break;
- }
- mm = get_task_mm(p);
- if (!mm)
- continue;
- mmarray[n++] = mm;
- }
- cgroup_iter_end(cs->css.cgroup, &it);
- read_unlock(&tasklist_lock);
+ scan.cg = cs->css.cgroup;
+ scan.test_task = NULL;
+ scan.process_task = cpuset_change_nodemask;
+ scan.heap = NULL;
+ scan.data = (nodemask_t *)oldmem;

/*
- * Now that we've dropped the tasklist spinlock, we can
- * rebind the vma mempolicies of each mm in mmarray[] to their
- * new cpuset, and release that mm. The mpol_rebind_mm()
- * call takes mmap_sem, which we couldn't take while holding
- * tasklist_lock. Forks can happen again now - the mpol_dup()
- * cpuset_being_rebound check will catch such forks, and rebind
- * their vma mempolicies too. Because we still hold the global
- * cgroup_mutex, we know that no other rebind effort will
- * be contending for the global variable cpuset_being_rebound.
+ * The mpol_rebind_mm() call takes mmap_sem, which we couldn't
+ * take while holding tasklist_lock. Forks can happen - the
+ * mpol_dup() cpuset_being_rebound check will catch such forks,
+ * and rebind their vma mempolicies too. Because we still hold
+ * the global cgroup_mutex, we know that no other rebind effort
+ * will be contending for the global variable cpuset_being_rebound.
* It's ok if we rebind the same mm twice; mpol_rebind_mm()
* is idempotent. Also migrate pages in each mm to new nodes.
*/
- migrate = is_memory_migrate(cs);
- for (i = 0; i < n; i++) {
- struct mm_struct *mm = mmarray[i];
-
- mpol_rebind_mm(mm, &cs->mems_allowed);
- if (migrate)
- cpuset_migrate_mm(mm, oldmem, &cs->mems_allowed);
- mmput(mm);
- }
+ retval = cgroup_scan_tasks(&scan);

/* We're done rebinding vmas to this cpuset's new mems_allowed. */
- kfree(mmarray);
cpuset_being_rebound = NULL;
- retval = 0;
-done:
+
return retval;
}

-- 1.5.4.rc3

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