On Tue, 26 Jun 2012, David Rientjes wrote:
> It turns out that task->children is not an rcu-protected list so this
> doesn't work. Both (1) and (3) can be accomplished with
> rcu_read_{lock,unlock}() that can nest inside the tasklist_lock for the
> global oom killer. (We could even split the global oom killer tasklist
> locking and optimize it seperately from this patchset.)
>
> So we have a couple of options:
>
> - allow oom_kill_process() to do
>
> if (memcg)
> read_lock(&tasklist_lock);
> ...
> if (memcg)
> read_unlock(&tasklist_lock);
>
> around the iteration over the victim's children. This should solve the
> issue since any other iteration over the entire tasklist would have
> triggered the same starvation if it were that bad, or
>
> - suppress the iteration for memcg ooms and just kill the parent instead.
>
Adding Oleg for comment as well.
I did the first option but I split tasklist_lock for global oom conditions
as well. The only place we actually need it is when iterating the
victim's children since that list is not rcu-protected. If this happens
to be too painful for parallel memcg ooms then we can look to protecting
it, but it shouldn't be a problem for global ooms because of the global
oom killer's zonelist serialization.
It's a tough patch to review, but the basics are that
- oom_kill_process() is made to no longer need tasklist_lock; it's only
taken for the iteration over children and everything else, including
dump_header() is protected by rcu_read_lock() for kernels enabling
/proc/sys/vm/oom_dump_tasks,
- oom_kill_process() assumes that we have a reference to p, the victim,
when it's called. It can release this reference and grab a child's
reference if necessary and drops it before returning, and
- select_bad_process() does not require tasklist_lock, it gets
protected by rcu_read_lock() as well.
Comments?
memcontrol.c | 2 --
oom_kill.c | 33 ++++++++++++++++++++++-----------
2 files changed, 22 insertions(+), 13 deletions(-)
---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1522,10 +1522,8 @@ void __mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
if (!chosen)
return;
points = chosen_points * 1000 / totalpages;
- read_lock(&tasklist_lock);
oom_kill_process(chosen, gfp_mask, order, points, totalpages, memcg,
NULL, "Memory cgroup out of memory");
- read_unlock(&tasklist_lock);
put_task_struct(chosen);
}
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -336,7 +336,7 @@ enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
/*
* Simple selection loop. We chose the process with the highest
- * number of 'points'. We expect the caller will lock the tasklist.
+ * number of 'points'.
*
* (not docbooked, we don't want this one cluttering up the manual)
*/
@@ -348,6 +348,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
struct task_struct *chosen = NULL;
unsigned long chosen_points = 0;
+ rcu_read_lock();
do_each_thread(g, p) {
unsigned int points;
@@ -370,6 +371,9 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
chosen_points = points;
}
} while_each_thread(g, p);
+ if (chosen)
+ get_task_struct(chosen);
+ rcu_read_unlock();
*ppoints = chosen_points * 1000 / totalpages;
return chosen;
@@ -385,8 +389,6 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
* are not shown.
* State information includes task's pid, uid, tgid, vm size, rss, cpu, oom_adj
* value, oom_score_adj value, and name.
- *
- * Call with tasklist_lock read-locked.
*/
static void dump_tasks(const struct mem_cgroup *memcg, const nodemask_t *nodemask)
{
@@ -394,6 +396,7 @@ static void dump_tasks(const struct mem_cgroup *memcg, const nodemask_t *nodemas
struct task_struct *task;
pr_info("[ pid ] uid tgid total_vm rss cpu oom_adj oom_score_adj name\n");
+ rcu_read_lock();
for_each_process(p) {
if (oom_unkillable_task(p, memcg, nodemask))
continue;
@@ -415,6 +418,7 @@ static void dump_tasks(const struct mem_cgroup *memcg, const nodemask_t *nodemas
task->signal->oom_score_adj, task->comm);
task_unlock(task);
}
+ rcu_read_unlock();
}
static void dump_header(struct task_struct *p, gfp_t gfp_mask, int order,
@@ -454,6 +458,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
*/
if (p->flags & PF_EXITING) {
set_tsk_thread_flag(p, TIF_MEMDIE);
+ put_task_struct(p);
return;
}
@@ -471,6 +476,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
* parent. This attempts to lose the minimal amount of work done while
* still freeing memory.
*/
+ read_lock(&tasklist_lock);
do {
list_for_each_entry(child, &t->children, sibling) {
unsigned int child_points;
@@ -483,15 +489,23 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
child_points = oom_badness(child, memcg, nodemask,
totalpages);
if (child_points > victim_points) {
+ put_task_struct(victim);
victim = child;
victim_points = child_points;
+ get_task_struct(victim);
}
}
} while_each_thread(p, t);
+ read_unlock(&tasklist_lock);
- victim = find_lock_task_mm(victim);
- if (!victim)
+ rcu_read_lock();
+ p = find_lock_task_mm(victim);
+ if (!p) {
+ rcu_read_unlock();
+ put_task_struct(victim);
return;
+ } else
+ victim = p;
/* mm cannot safely be dereferenced after task_unlock(victim) */
mm = victim->mm;
@@ -522,9 +536,11 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
task_unlock(p);
do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
}
+ rcu_read_unlock();
set_tsk_thread_flag(victim, TIF_MEMDIE);
do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
+ put_task_struct(victim);
}
#undef K
@@ -545,9 +561,7 @@ static void check_panic_on_oom(enum oom_constraint constraint, gfp_t gfp_mask,
if (constraint != CONSTRAINT_NONE)
return;
}
- read_lock(&tasklist_lock);
dump_header(NULL, gfp_mask, order, NULL, nodemask);
- read_unlock(&tasklist_lock);
panic("Out of memory: %s panic_on_oom is enabled\n",
sysctl_panic_on_oom == 2 ? "compulsory" : "system-wide");
}
@@ -720,10 +734,10 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
mpol_mask = (constraint == CONSTRAINT_MEMORY_POLICY) ? nodemask : NULL;
check_panic_on_oom(constraint, gfp_mask, order, mpol_mask);
- read_lock(&tasklist_lock);
if (sysctl_oom_kill_allocating_task &&
!oom_unkillable_task(current, NULL, nodemask) &&
current->mm) {
+ get_task_struct(current);
oom_kill_process(current, gfp_mask, order, 0, totalpages, NULL,
nodemask,
"Out of memory (oom_kill_allocating_task)");
@@ -734,7 +748,6 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
/* Found nothing?!?! Either we hang forever, or we panic. */
if (!p) {
dump_header(NULL, gfp_mask, order, NULL, mpol_mask);
- read_unlock(&tasklist_lock);
panic("Out of memory and no killable processes...\n");
}
if (PTR_ERR(p) != -1UL) {
@@ -743,8 +756,6 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
killed = 1;
}
out:
- read_unlock(&tasklist_lock);
-
/*
* Give "p" a good chance of killing itself before we
* retry to allocate memory unless "p" is current
--
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