Andrew Morton <a...@linux-foundation.org> writes:
> On Wed, 02 May 2018 14:21:35 -0500 ebied...@xmission.com (Eric W. Biederman)
> wrote:
>
>> Recently it was reported that mm_update_next_owner could get into
>> cases where it was executing it's fallback for_each_process part of
>> the loop and thus taking up a lot of time.
>>
>> To deal with this replace mm->owner with mm->memcg. This just reduces
>> the complexity of everything. As much as possible I have maintained
>> the current semantics. There are two siginificant exceptions. During
>> fork the memcg of the process calling fork is charged rather than
>> init_css_set. During memory cgroup migration the charges are migrated
>> not if the process is the owner of the mm, but if the process being
>> migrated has the same memory cgroup as the mm.
>>
>> I believe it was a bug if init_css_set is charged for memory activity
>> during fork, and the old behavior was simply a consequence of the new
>> task not having tsk->cgroup not initialized to it's proper cgroup.
>>
>> Durhing cgroup migration only thread group leaders are allowed to
>> migrate. Which means in practice there should only be one. Linux
>> tasks created with CLONE_VM are the only exception, but the common
>> cases are already ruled out. Processes created with vfork have a
>> suspended parent and can do nothing but call exec so they should never
>> show up. Threads of the same cgroup are not the thread group leader
>> so also should not show up. That leaves the old LinuxThreads library
>> which is probably out of use by now, and someone doing something very
>> creative with cgroups, and rolling their own threads with CLONE_VM.
>> So in practice I don't think the difference charge migration will
>> affect anyone.
>>
>> To ensure that mm->memcg is updated appropriately I have implemented
>> cgroup "attach" and "fork" methods. This ensures that at those
>> points the mm pointed to the task has the appropriate memory cgroup.
>>
>> For simplicity instead of introducing a new mm lock I simply use
>> exchange on the pointer where the mm->memcg is updated to get
>> atomic updates.
>>
>> Looking at the history effectively this change is a revert. The
>> reason given for adding mm->owner is so that multiple cgroups can be
>> attached to the same mm. In the last 8 years a second user of
>> mm->owner has not appeared. A feature that has never used, makes the
>> code more complicated and has horrible worst case performance should
>> go.
>
> Cleanliness nit: I'm not sure that the removal and open-coding of
> mem_cgroup_from_task() actually improved things. Should we restore
> it?
While writing the patch itself removing mem_cgroup_from_task forced
thinking about which places should use mm->memcg and which places
should use an alternative.
If we want to add it back afterwards with a second patch I don't mind.
I just don't want to have that in the same patch as opportunities get
lost to look at how the memory cgroup should be derived.
Eric
> --- a/mm/memcontrol.c~memcg-replace-mm-owner-with-mm-memcg-fix
> +++ a/mm/memcontrol.c
> @@ -664,6 +664,11 @@ static void memcg_check_events(struct me
> }
> }
>
> +static inline struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
> +{
> + return mem_cgroup_from_css(task_css(p, memory_cgrp_id));
> +}
> +
> struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
> {
> struct mem_cgroup *memcg = NULL;
> @@ -1011,7 +1016,7 @@ bool task_in_mem_cgroup(struct task_stru
> * killed to prevent needlessly killing additional tasks.
> */
> rcu_read_lock();
> - task_memcg = mem_cgroup_from_css(task_css(task,
> memory_cgrp_id));
> + task_memcg = mem_cgroup_from_task(task);
> css_get(&task_memcg->css);
> rcu_read_unlock();
> }
> @@ -4829,7 +4834,7 @@ static int mem_cgroup_can_attach(struct
> if (!move_flags)
> return 0;
>
> - from = mem_cgroup_from_css(task_css(p, memory_cgrp_id));
> + from = mem_cgroup_from_task(p);
>
> VM_BUG_ON(from == memcg);
>
> @@ -5887,7 +5892,7 @@ void mem_cgroup_sk_alloc(struct sock *sk
> }
>
> rcu_read_lock();
> - memcg = mem_cgroup_from_css(task_css(current, memory_cgrp_id));
> + memcg = mem_cgroup_from_task(current);
> if (memcg == root_mem_cgroup)
> goto out;
> if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !memcg->tcpmem_active)
> _
