On 12/03/2012 09:32 PM, Michal Hocko wrote:
> On Fri 30-11-12 17:31:25, Glauber Costa wrote:
>> Although there is arguably some value in doing this per se, the main
>> goal of this patch is to make room for the locking changes to come.
>>
>> With all the value assignment from parent happening in a context where
>> our iterators can already be used, we can safely lock against value
>> change in some key values like use_hierarchy, without resorting to the
>> cgroup core at all.
>
> I am sorry but I really do not get why online_css callback is more
> appropriate. Quite contrary. With this change iterators can see a group
> which is not fully initialized which calls for a problem (even though it
> is not one yet).
But it should be extremely easy to protect against this. It is just a
matter of not returning online css in the iterator: then we'll never see
them until they are online. This also sounds a lot more correct than
returning allocated css.
> Could you be more specific why we cannot keep the initialization in
> mem_cgroup_css_alloc? We can lock there as well, no?
>
Because we need to parent value of things like use_hierarchy and
oom_control not to change after it was copied to a child.
If we do it in css_alloc, the iterators won't be working yet - nor will
cgrp->children list, for that matter - and we will risk a situation
where another thread thinks no children exist, and flips use_hierarchy
to 1 (or oom_control, etc), right after the children already got the
value of 0.
The two other ways to solve this problem that I see, are:
1) lock in css_alloc and unlock in css_online, that tejun already ruled
out as too damn ugly (and I can't possibly disagree)
2) have an alternate indication of emptiness that is working since
css_alloc (like counting number of children).
Since I don't share your concerns about the iterator showing incomplete
memcgs - trivial to fix, if not fixed already - I deemed my approach
preferable here.
>> Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
>> ---
>> mm/memcontrol.c | 52 +++++++++++++++++++++++++++++++++++-----------------
>> 1 file changed, 35 insertions(+), 17 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index d80b6b5..b6d352f 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -5023,12 +5023,40 @@ mem_cgroup_css_alloc(struct cgroup *cont)
>> INIT_WORK(&stock->work, drain_local_stock);
>> }
>> hotcpu_notifier(memcg_cpu_hotplug_callback, 0);
>> - } else {
>> - parent = mem_cgroup_from_cont(cont->parent);
>> - memcg->use_hierarchy = parent->use_hierarchy;
>> - memcg->oom_kill_disable = parent->oom_kill_disable;
>> +
>> + res_counter_init(&memcg->res, NULL);
>> + res_counter_init(&memcg->memsw, NULL);
>> }
>>
>> + memcg->last_scanned_node = MAX_NUMNODES;
>> + INIT_LIST_HEAD(&memcg->oom_notify);
>> + atomic_set(&memcg->refcnt, 1);
>> + memcg->move_charge_at_immigrate = 0;
>> + mutex_init(&memcg->thresholds_lock);
>> + spin_lock_init(&memcg->move_lock);
>> +
>> + return &memcg->css;
>> +
>> +free_out:
>> + __mem_cgroup_free(memcg);
>> + return ERR_PTR(error);
>> +}
>> +
>> +static int
>> +mem_cgroup_css_online(struct cgroup *cont)
>> +{
>> + struct mem_cgroup *memcg, *parent;
>> + int error = 0;
>> +
>> + if (!cont->parent)
>> + return 0;
>> +
>> + memcg = mem_cgroup_from_cont(cont);
>> + parent = mem_cgroup_from_cont(cont->parent);
>> +
>> + memcg->use_hierarchy = parent->use_hierarchy;
>> + memcg->oom_kill_disable = parent->oom_kill_disable;
>> +
>> if (parent && parent->use_hierarchy) {
>> res_counter_init(&memcg->res, &parent->res);
>> res_counter_init(&memcg->memsw, &parent->memsw);
>> @@ -5050,15 +5078,8 @@ mem_cgroup_css_alloc(struct cgroup *cont)
>> if (parent && parent != root_mem_cgroup)
>> mem_cgroup_subsys.broken_hierarchy = true;
>> }
>> - memcg->last_scanned_node = MAX_NUMNODES;
>> - INIT_LIST_HEAD(&memcg->oom_notify);
>>
>> - if (parent)
>> - memcg->swappiness = mem_cgroup_swappiness(parent);
>> - atomic_set(&memcg->refcnt, 1);
>> - memcg->move_charge_at_immigrate = 0;
>> - mutex_init(&memcg->thresholds_lock);
>> - spin_lock_init(&memcg->move_lock);
>> + memcg->swappiness = mem_cgroup_swappiness(parent);
>>
>> error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
>> if (error) {
>> @@ -5068,12 +5089,8 @@ mem_cgroup_css_alloc(struct cgroup *cont)
>> * call __mem_cgroup_free, so return directly
>> */
>> mem_cgroup_put(memcg);
>> - return ERR_PTR(error);
>> }
>> - return &memcg->css;
>> -free_out:
>> - __mem_cgroup_free(memcg);
>> - return ERR_PTR(error);
>> + return error;
>> }
>>
>> static void mem_cgroup_css_offline(struct cgroup *cont)
>> @@ -5702,6 +5719,7 @@ struct cgroup_subsys mem_cgroup_subsys = {
>> .name = "memory",
>> .subsys_id = mem_cgroup_subsys_id,
>> .css_alloc = mem_cgroup_css_alloc,
>> + .css_online = mem_cgroup_css_online,
>> .css_offline = mem_cgroup_css_offline,
>> .css_free = mem_cgroup_css_free,
>> .can_attach = mem_cgroup_can_attach,
>> --
>> 1.7.11.7
>>
>
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