Re: [PATCH] memcg: consider per-cpu stock reserves when returning RES_USAGE for _MEM

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 21 Mar 2011 11:24:20 +0100
Michal Hocko <mhocko@suse.cz> wrote:

> [Sorry for reposting but I forgot to fully refresh the patch before
> posting...]
> 
> On Mon 21-03-11 10:34:19, Michal Hocko 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.

I think the test in LTP is bad...(it should be fuzzy.) because we cannot
avoid races...
But ok, this itself will be a problem with a large machine with many cpus.


> --- 
> From: Michal Hocko <mhocko@suse.cz>
> Subject: memcg: consider per-cpu stock reserves when returning RES_USAGE for _MEM
> 
> 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@suse.cz>

This doesn't seems correct.

> 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;

		if (memcg_stock->cached == mem)
			per_cpu_val += stock->nr_pages * PAGE_SIZE;

AND I think you doesn't handle batched uncharge.
Do you have any idea ? (Peter Zilstra's patch will make error size of
bached uncharge bigger.)

So....rather than this, just always using root memcg's code is
a good way. Could you try ?
==
        usage = mem_cgroup_recursive_stat(mem, MEM_CGROUP_STAT_CACHE);
        usage += mem_cgroup_recursive_stat(mem, MEM_CGROUP_STAT_RSS);

        if (swap)
                val += mem_cgroup_recursive_stat(mem, MEM_CGROUP_STAT_SWAPOUT);

        return val << PAGE_SHIFT;
==

Thanks,
-Kame


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

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