Re: [PATCH V4 5/6] memcg: patch mem_cgroup_{begin,end}_update_page_stat() out if only root memcg exists

[Sha Zhengju <handai.szj@gmail.com>]

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Add cc to Glauber

On Thu, Jul 11, 2013 at 10:56 PM, Michal Hocko <mhocko@suse.cz> wrote:
> On Sat 06-07-13 01:33:43, Sha Zhengju wrote:
>> From: Sha Zhengju <handai.szj@taobao.com>
>>
>> If memcg is enabled and no non-root memcg exists, all allocated
>> pages belongs to root_mem_cgroup and wil go through root memcg
>> statistics routines.  So in order to reduce overheads after adding
>> memcg dirty/writeback accounting in hot paths, we use jump label to
>> patch mem_cgroup_{begin,end}_update_page_stat() in or out when not
>> used.
>
> I do not think this is enough. How much do you save? One atomic read.
> This doesn't seem like a killer.
>
> I hoped we could simply not account at all and move counters to the root
> cgroup once the label gets enabled.

I have thought of this approach before, but it would probably run into
another issue, e.g, each zone has a percpu stock named ->pageset to
optimize the increment and decrement operations, and I haven't figure out a
simpler and cheaper approach to handle that stock numbers if moving global
counters to root cgroup, maybe we can just leave them and can afford the
approximation?

Glauber have already done lots of works here, in his previous patchset he
also tried to move some global stats to root (
http://comments.gmane.org/gmane.linux.kernel.cgroups/6291). May I steal
some of your ideas here, Glauber? :P


>
> Besides that, the current patch is racy. Consider what happens when:
>
> mem_cgroup_begin_update_page_stat
>                                         arm_inuse_keys
>
mem_cgroup_move_account
> mem_cgroup_move_account_page_stat
> mem_cgroup_end_update_page_stat
>
> The race window is small of course but it is there. I guess we need
> rcu_read_lock at least.

Yes, you're right. I'm afraid we need to take care of the racy in the next
updates as well. But mem_cgroup_begin/end_update_page_stat() already have
rcu lock, so here we maybe only need a synchronize_rcu() after changing
memcg_inuse_key?


>
>> If no non-root memcg comes to life, we do not need to accquire moving
>> locks, so patch them out.
>>
>> cc: Michal Hocko <mhocko@suse.cz>
>> cc: Greg Thelen <gthelen@google.com>
>> cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> cc: Andrew Morton <akpm@linux-foundation.org>
>> cc: Fengguang Wu <fengguang.wu@intel.com>
>> cc: Mel Gorman <mgorman@suse.de>
>> ---
>>  include/linux/memcontrol.h |   15 +++++++++++++++
>>  mm/memcontrol.c            |   23 ++++++++++++++++++++++-
>>  2 files changed, 37 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index ccd35d8..0483e1a 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -55,6 +55,13 @@ struct mem_cgroup_reclaim_cookie {
>>  };
>>
>>  #ifdef CONFIG_MEMCG
>> +
>> +extern struct static_key memcg_inuse_key;
>> +static inline bool mem_cgroup_in_use(void)
>> +{
>> +     return static_key_false(&memcg_inuse_key);
>> +}
>> +
>>  /*
>>   * All "charge" functions with gfp_mask should use GFP_KERNEL or
>>   * (gfp_mask & GFP_RECLAIM_MASK). In current implementatin, memcg
doesn't
>> @@ -159,6 +166,8 @@ static inline void
mem_cgroup_begin_update_page_stat(struct page *page,
>>  {
>>       if (mem_cgroup_disabled())
>>               return;
>> +     if (!mem_cgroup_in_use())
>> +             return;
>>       rcu_read_lock();
>>       *locked = false;
>>       if (atomic_read(&memcg_moving))
>> @@ -172,6 +181,8 @@ static inline void
mem_cgroup_end_update_page_stat(struct page *page,
>>  {
>>       if (mem_cgroup_disabled())
>>               return;
>> +     if (!mem_cgroup_in_use())
>> +             return;
>>       if (*locked)
>>               __mem_cgroup_end_update_page_stat(page, flags);
>>       rcu_read_unlock();
>> @@ -215,6 +226,10 @@ void mem_cgroup_print_bad_page(struct page *page);
>>  #endif
>>  #else /* CONFIG_MEMCG */
>>  struct mem_cgroup;
>> +static inline bool mem_cgroup_in_use(void)
>> +{
>> +     return false;
>> +}
>>
>>  static inline int mem_cgroup_newpage_charge(struct page *page,
>>                                       struct mm_struct *mm, gfp_t
gfp_mask)
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 9126abc..a85f7c5 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -463,6 +463,13 @@ enum res_type {
>>  #define MEM_CGROUP_RECLAIM_SHRINK_BIT        0x1
>>  #define MEM_CGROUP_RECLAIM_SHRINK    (1 <<
MEM_CGROUP_RECLAIM_SHRINK_BIT)
>>
>> +/* static_key used for marking memcg in use or not. We use this jump
label to
>> + * patch some memcg page stat accounting code in or out.
>> + * The key will be increased when non-root memcg is created, and be
decreased
>> + * when memcg is destroyed.
>> + */
>> +struct static_key memcg_inuse_key;
>> +
>>  /*
>>   * The memcg_create_mutex will be held whenever a new cgroup is created.
>>   * As a consequence, any change that needs to protect against new child
cgroups
>> @@ -630,10 +637,22 @@ static void disarm_kmem_keys(struct mem_cgroup
*memcg)
>>  }
>>  #endif /* CONFIG_MEMCG_KMEM */
>>
>> +static void disarm_inuse_keys(struct mem_cgroup *memcg)
>> +{
>> +     if (!mem_cgroup_is_root(memcg))
>> +             static_key_slow_dec(&memcg_inuse_key);
>> +}
>> +
>> +static void arm_inuse_keys(void)
>> +{
>> +     static_key_slow_inc(&memcg_inuse_key);
>> +}
>> +
>>  static void disarm_static_keys(struct mem_cgroup *memcg)
>>  {
>>       disarm_sock_keys(memcg);
>>       disarm_kmem_keys(memcg);
>> +     disarm_inuse_keys(memcg);
>>  }
>>
>>  static void drain_all_stock_async(struct mem_cgroup *memcg);
>> @@ -2298,7 +2317,6 @@ void mem_cgroup_update_page_stat(struct page *page,
>>  {
>>       struct mem_cgroup *memcg;
>>       struct page_cgroup *pc = lookup_page_cgroup(page);
>> -     unsigned long uninitialized_var(flags);
>>
>>       if (mem_cgroup_disabled())
>>               return;
>> @@ -6293,6 +6311,9 @@ mem_cgroup_css_online(struct cgroup *cont)
>>       }
>>
>>       error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
>> +     if (!error)
>> +             arm_inuse_keys();
>> +
>>       mutex_unlock(&memcg_create_mutex);
>>       return error;
>>  }
>> --
>> 1.7.9.5
>>
>> --
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>
> --
> Michal Hocko
> SUSE Labs



--
Thanks,
Sha

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