Re: cgroup: real meaning of memory.usage_in_bytes

[Ying Han]

On Mon, Mar 21, 2011 at 2:34 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
> On Fri 18-03-11 16:25:32, Michal Hocko wrote:
> [...]
>> According to our documention this is a reasonable test case:
>> Documentation/cgroups/memory.txt:
>> memory.usage_in_bytes           # show current memory(RSS+Cache) usage.
>>
>> This however doesn't work after your commit:
>> cdec2e4265d (memcg: coalesce charging via percpu storage)
>>
>> because since then we are charging in bulks so we can end up with
>> rss+cache <= usage_in_bytes.
> [...]
>> I think we have several options here
>>       1) document that the value is actually >= rss+cache and it shows
>>          the guaranteed charges for the group
>>       2) use rss+cache rather then res->count
>>       3) remove the file
>>       4) call drain_all_stock_sync before asking for the value in
>>          mem_cgroup_read
>>       5) collect the current amount of stock charges and subtract it
>>          from the current res->count value
>>
>> 1) and 2) would suggest that the file is actually not very much useful.
>> 3) is basically the interface change as well
>> 4) sounds little bit invasive as we basically lose the advantage of the
>> pool whenever somebody reads the file. Btw. for who is this file
>> intended?
>> 5) sounds like a compromise
>
> I guess that 4) is really too invasive - for no good reason so here we
> go with the 5) solution.
> ---
> From: Michal Hocko <mhocko@xxxxxxx>
> Subject: Drain memcg_stock before returning res->count value
>
> Since cdec2e4265d (memcg: coalesce charging via percpu storage) commit we
> are charging resource counter in batches. This means that the current
> res->count value doesn't show the real consumed value (rss+cache as we
> describe in the documentation) but rather a promissed charges for future.
> We are pre-charging CHARGE_SIZE bulk at once and subsequent charges are
> satisfied from the per-cpu cgroup_stock pool.
>
> We have seen a report that one of the LTP testcases checks exactly this
> condition so the test fails.
>
> As this exported value is a part of kernel->userspace interface we should
> try to preserve the original (and documented) semantic.
>
> This patch fixes the issue by collecting the current usage of each per-cpu
> stock and subtracting it from the current res counter value.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> Index: linus_tree/mm/memcontrol.c
> ===================================================================
> --- linus_tree.orig/mm/memcontrol.c     2011-03-18 16:09:11.000000000 +0100
> +++ linus_tree/mm/memcontrol.c  2011-03-21 10:21:55.000000000 +0100
> @@ -3579,13 +3579,30 @@ static unsigned long mem_cgroup_recursiv
>        return val;
>  }
>
> +static u64 mem_cgroup_current_usage(struct mem_cgroup *mem)
> +{
> +       u64 val = res_counter_read_u64(&mem->res, RES_USAGE);
> +       u64 per_cpu_val = 0;
> +       int cpu;
> +
> +       get_online_cpus();
> +       for_each_online_cpu(cpu) {
> +               struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> +
> +               per_cpu_val += stock->nr_pages * PAGE_SIZE;
> +       }
> +       put_online_cpus();
> +
> +       return (val > per_cpu_val)? val - per_cpu_val: 0;
> +}
> +
>  static inline u64 mem_cgroup_usage(struct mem_cgroup *mem, bool swap)
>  {
>        u64 val;
>
>        if (!mem_cgroup_is_root(mem)) {
>                if (!swap)
> -                       return res_counter_read_u64(&mem->res, RES_USAGE);
> +                       return mem_cgroup_current_usage(mem);
>                else
>                        return res_counter_read_u64(&mem->memsw, RES_USAGE);
>        }

Michal,

Can you help to post the test result after applying the patch?

--Ying

> --
> Michal Hocko
> SUSE Labs
> SUSE LINUX s.r.o.
> Lihovarska 1060/12
> 190 00 Praha 9
> Czech Republic
>
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