Re: [RFC][PATCH 2/5] memcg: uncharge in batched manner

[Balbir Singh]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2009-08-28 13:24:38]:

> 
> In massive parallel enviroment, res_counter can be a performance bottleneck.
> This patch is a trial for reducing lock contention.
> One strong techinque to reduce lock contention is reducing calls by
> batching some amount of calls int one.
> 
> Considering charge/uncharge chatacteristic,
> 	- charge is done one by one via demand-paging.
> 	- uncharge is done by
> 		- in chunk at munmap, truncate, exit, execve...
> 		- one by one via vmscan/paging.
> 
> It seems we hace a chance to batched-uncharge.
> This patch is a base patch for batched uncharge. For avoiding
> scattering memcg's structure, this patch adds memcg batch uncharge
> information to the task. please see start/end usage in next patch.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
>  include/linux/memcontrol.h |   12 +++++++
>  include/linux/sched.h      |    7 ++++
>  mm/memcontrol.c            |   70 +++++++++++++++++++++++++++++++++++++++++----
>  3 files changed, 83 insertions(+), 6 deletions(-)
> 
> Index: mmotm-2.6.31-Aug27/include/linux/memcontrol.h
> ===================================================================
> --- mmotm-2.6.31-Aug27.orig/include/linux/memcontrol.h
> +++ mmotm-2.6.31-Aug27/include/linux/memcontrol.h
> @@ -54,6 +54,10 @@ extern void mem_cgroup_rotate_lru_list(s
>  extern void mem_cgroup_del_lru(struct page *page);
>  extern void mem_cgroup_move_lists(struct page *page,
>  				  enum lru_list from, enum lru_list to);
> +
> +extern void mem_cgroup_uncharge_batch_start(void);
> +extern void mem_cgroup_uncharge_batch_end(void);
> +
>  extern void mem_cgroup_uncharge_page(struct page *page);
>  extern void mem_cgroup_uncharge_cache_page(struct page *page);
>  extern int mem_cgroup_shmem_charge_fallback(struct page *page,
> @@ -151,6 +155,14 @@ static inline void mem_cgroup_cancel_cha
>  {
>  }
> 
> +static inline void mem_cgroup_uncharge_batch_start(void)
> +{
> +}
> +
> +static inline void mem_cgroup_uncharge_batch_start(void)
> +{
> +}
> +
>  static inline void mem_cgroup_uncharge_page(struct page *page)
>  {
>  }
> Index: mmotm-2.6.31-Aug27/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.31-Aug27.orig/mm/memcontrol.c
> +++ mmotm-2.6.31-Aug27/mm/memcontrol.c
> @@ -1837,7 +1837,35 @@ void mem_cgroup_cancel_charge_swapin(str
>  	css_put(&mem->css);
>  }
> 
> +static bool
> +__do_batch_uncharge(struct mem_cgroup *mem, const enum charge_type ctype)
> +{
> +	struct memcg_batch_info *batch = NULL;
> +	bool uncharge_memsw;
> +	/* If swapout, usage of swap doesn't decrease */
> +	if (do_swap_account && (ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT))
> +		uncharge_memsw = false;
> +	else
> +		uncharge_memsw = true;
>
> +	if (current->memcg_batch.do_batch) {
> +		batch = &current->memcg_batch;
> +		if (batch->memcg == NULL) {
> +			batch->memcg = mem;
> +			css_get(&mem->css);
> +		}
> +	}
> +	if (!batch || batch->memcg != mem) {
> +		res_counter_uncharge(&mem->res, PAGE_SIZE);
> +		if (uncharge_memsw)
> +			res_counter_uncharge(&mem->memsw, PAGE_SIZE);

Could you please add a comment stating that if memcg is different that
we do a direct uncharge else we batch.

> +	} else {
> +		batch->pages += PAGE_SIZE;
> +		if (uncharge_memsw)
> +			batch->memsw += PAGE_SIZE;
> +	}
> +	return soft_limit_excess;
> +}
>  /*
>   * uncharge if !page_mapped(page)
>   */
> @@ -1886,12 +1914,8 @@ __mem_cgroup_uncharge_common(struct page
>  		break;
>  	}
> 
> -	if (!mem_cgroup_is_root(mem)) {
> -		res_counter_uncharge(&mem->res, PAGE_SIZE);
> -		if (do_swap_account &&
> -				(ctype != MEM_CGROUP_CHARGE_TYPE_SWAPOUT))
> -			res_counter_uncharge(&mem->memsw, PAGE_SIZE);
> -	}
> +	if (!mem_cgroup_is_root(mem))
> +		__do_batch_uncharge(mem, ctype);

Now I am beginning to think we need a cond_mem_cgroup_is_not_root()
function.

>  	if (ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
>  		mem_cgroup_swap_statistics(mem, true);
>  	mem_cgroup_charge_statistics(mem, pc, false);
> @@ -1938,6 +1962,40 @@ void mem_cgroup_uncharge_cache_page(stru
>  	__mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_CACHE);
>  }
> 
> +void mem_cgroup_uncharge_batch_start(void)
> +{
> +	VM_BUG_ON(current->memcg_batch.do_batch);
> +	/* avoid batch if killed by OOM */
> +	if (test_thread_flag(TIF_MEMDIE))
> +		return;
> +	current->memcg_batch.do_batch = 1;
> +	current->memcg_batch.memcg = NULL;
> +	current->memcg_batch.pages = 0;
> +	current->memcg_batch.memsw = 0;
> +}
> +
> +void mem_cgroup_uncharge_batch_end(void)
> +{
> +	struct mem_cgroup *mem;
> +
> +	if (!current->memcg_batch.do_batch)
> +		return;
> +
> +	current->memcg_batch.do_batch = 0;
> +
> +	mem = current->memcg_batch.memcg;
> +	if (!mem)
> +		return;
> +	if (current->memcg_batch.pages)
> +		res_counter_uncharge(&mem->res,
> +				     current->memcg_batch.pages, NULL);
> +	if (current->memcg_batch.memsw)
> +		res_counter_uncharge(&mem->memsw,
> +				     current->memcg_batch.memsw, NULL);
> +	/* we got css's refcnt */
> +	cgroup_release_and_wakeup_rmdir(&mem->css);


Does this effect deleting of a group and delay it by a large amount?

> +}
> +
>  #ifdef CONFIG_SWAP
>  /*
>   * called after __delete_from_swap_cache() and drop "page" account.
> Index: mmotm-2.6.31-Aug27/include/linux/sched.h
> ===================================================================
> --- mmotm-2.6.31-Aug27.orig/include/linux/sched.h
> +++ mmotm-2.6.31-Aug27/include/linux/sched.h
> @@ -1540,6 +1540,13 @@ struct task_struct {
>  	unsigned long trace_recursion;
>  #endif /* CONFIG_TRACING */
>  	unsigned long stack_start;
> +#ifdef CONFIG_CGROUP_MEM_RES_CTLR /* memcg uses this to do batch job */
> +	struct memcg_batch_info {
> +		bool do_batch;
> +		struct mem_cgroup *memcg;
> +		long pages, memsw;
> +	} memcg_batch;
> +#endif
>  };
> 
>  /* Future-safe accessor for struct task_struct's cpus_allowed. */
> 
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-- 
	Balbir
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