From 89e8a244b97e48f1f30e898b6f32acca477f2a13 Mon Sep 17 00:00:00 2001 From: David Rientjes Date: Wed, 2 Nov 2011 13:38:39 -0700 Subject: [PATCH] cpusets: avoid looping when storing to mems_allowed if one node remains set {get,put}_mems_allowed() exist so that general kernel code may locklessly access a task's set of allowable nodes without having the chance that a concurrent write will cause the nodemask to be empty on configurations where MAX_NUMNODES > BITS_PER_LONG. This could incur a significant delay, however, especially in low memory conditions because the page allocator is blocking and reclaim requires get_mems_allowed() itself. It is not atypical to see writes to cpuset.mems take over 2 seconds to complete, for example. In low memory conditions, this is problematic because it's one of the most imporant times to change cpuset.mems in the first place! The only way a task's set of allowable nodes may change is through cpusets by writing to cpuset.mems and when attaching a task to a generic code is not reading the nodemask with get_mems_allowed() at the same time, and then clearing all the old nodes. This prevents the possibility that a reader will see an empty nodemask at the same time the writer is storing a new nodemask. If at least one node remains unchanged, though, it's possible to simply set all new nodes and then clear all the old nodes. Changing a task's nodemask is protected by cgroup_mutex so it's guaranteed that two threads are not changing the same task's nodemask at the same time, so the nodemask is guaranteed to be stored before another thread changes it and determines whether a node remains set or not. Signed-off-by: David Rientjes Cc: Miao Xie Cc: KOSAKI Motohiro Cc: Nick Piggin Cc: Paul Menage Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/cpuset.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/cpuset.c b/kernel/cpuset.c index 10131fdaff70..ed0ff443f036 100644 --- a/kernel/cpuset.c +++ b/kernel/cpuset.c @@ -949,6 +949,8 @@ static void cpuset_migrate_mm(struct mm_struct *mm, const nodemask_t *from, static void cpuset_change_task_nodemask(struct task_struct *tsk, nodemask_t *newmems) { + bool masks_disjoint = !nodes_intersects(*newmems, tsk->mems_allowed); + repeat: /* * Allow tasks that have access to memory reserves because they have @@ -963,7 +965,6 @@ repeat: nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems); mpol_rebind_task(tsk, newmems, MPOL_REBIND_STEP1); - /* * ensure checking ->mems_allowed_change_disable after setting all new * allowed nodes. @@ -980,9 +981,11 @@ repeat: /* * Allocation of memory is very fast, we needn't sleep when waiting - * for the read-side. + * for the read-side. No wait is necessary, however, if at least one + * node remains unchanged. */ - while (ACCESS_ONCE(tsk->mems_allowed_change_disable)) { + while (masks_disjoint && + ACCESS_ONCE(tsk->mems_allowed_change_disable)) { task_unlock(tsk); if (!task_curr(tsk)) yield(); -- 2.20.1