Date:	Fri, 9 Oct 2009 09:31:42 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH 1/2] memcg: coalescing uncharge at unmap and truncation

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-10-02 14:01:26]:

> 
> In massive parallel enviroment, res_counter can be a performance bottleneck.
> One strong techinque to reduce lock contention is reducing calls by
> coalescing some amount of calls into 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 have a chance in uncharge at unmap/truncation.

A chance to improve scalability?

> 
> This patch is a for coalescing uncharge. For avoiding scattering memcg's
> structure to functions under /mm, this patch adds memcg batch uncharge
> information to the task. 
>

Is there a reason for associating batch with the task rather than
per-cpu or per-memcg? per-memcg, I suspect would add some locking
overhead, per-cpu would require synchronization across cpu's while
uncharging, is that where per-task helps?  I suspect per-mm,
per-signal will have the issues above.
 
> The degree of coalescing depends on callers
>   - at invalidate/trucate... pagevec size
>   - at unmap ....ZAP_BLOCK_SIZE
> (memory itself will be freed in this degree.)
> Then, we'll not coalescing too much.
> 
> Changelog(now):
>  - rebased onto the latest mmotm + softlimit fix patches.
> 
> Changelog(old):
>  - unified patch for callers
>  - added commetns.
>  - make ->do_batch as bool.
>  - removed css_get() at el. We don't need it.
> 
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  include/linux/memcontrol.h |   13 ++++++
>  include/linux/sched.h      |    7 +++
>  mm/memcontrol.c            |   91 ++++++++++++++++++++++++++++++++++++++++++---
>  mm/memory.c                |    2 
>  mm/truncate.c              |    6 ++
>  5 files changed, 113 insertions(+), 6 deletions(-)
> 
> Index: mmotm-2.6.31-Sep28/include/linux/memcontrol.h
> ===================================================================
> --- mmotm-2.6.31-Sep28.orig/include/linux/memcontrol.h
> +++ mmotm-2.6.31-Sep28/include/linux/memcontrol.h
> @@ -54,6 +54,11 @@ 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);
> +
> +/* For coalescing uncharge for reducing memcg' overhead*/
> +extern void mem_cgroup_uncharge_start(void);
> +extern void mem_cgroup_uncharge_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 +156,14 @@ 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-Sep28/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.31-Sep28.orig/mm/memcontrol.c
> +++ mmotm-2.6.31-Sep28/mm/memcontrol.c
> @@ -1826,6 +1826,49 @@ void mem_cgroup_cancel_charge_swapin(str
>  	css_put(&mem->css);
>  }
> 
> +static void
> +__do_uncharge(struct mem_cgroup *mem, const enum charge_type ctype)
> +{
> +	struct memcg_batch_info *batch = NULL;
> +	bool uncharge_memsw = true;
> +	/* If swapout, usage of swap doesn't decrease */
> +	if (!do_swap_account || ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
> +		uncharge_memsw = false;
> +	/*
> +	 * do_batch > 0 when unmapping pages or inode invalidate/truncate.
> +	 * In those cases, all pages freed continously can be expected to be in
> +	 * the same cgroup and we have chance to coalesce uncharges.
> +	 * And, we do uncharge one by one if this is killed by OOM.
> +	 */
> +	if (!current->memcg_batch.do_batch || test_thread_flag(TIF_MEMDIE))
> +		goto direct_uncharge;

Should we also not uncharge the current batch when the task is dying?

> +
> +	batch = &current->memcg_batch;
> +	/*
> +	 * In usual, we do css_get() when we remember memcg pointer.
> +	 * But in this case, we keep res->usage until end of a series of
> +	 * uncharges. Then, it's ok to ignore memcg's refcnt.
> +	 */
> +	if (!batch->memcg)
> +		batch->memcg = mem;
> +	/*
> +	 * In typical case, batch->memcg == mem. This means we can
> +	 * merge a series of uncharges to an uncharge of res_counter.
> +	 * If not, we uncharge res_counter ony by one.
> +	 */
> +	if (batch->memcg != mem)
> +		goto direct_uncharge;
> +	/* remember freed charge and uncharge it later */
> +	batch->pages += PAGE_SIZE;
> +	if (uncharge_memsw)
> +		batch->memsw += PAGE_SIZE;
> +	return;
> +direct_uncharge:
> +	res_counter_uncharge(&mem->res, PAGE_SIZE);
> +	if (uncharge_memsw)
> +		res_counter_uncharge(&mem->memsw, PAGE_SIZE);
> +	return;
> +}
> 
>  /*
>   * uncharge if !page_mapped(page)
> @@ -1874,12 +1917,8 @@ __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_uncharge(mem, ctype);
>  	if (ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
>  		mem_cgroup_swap_statistics(mem, true);
>  	mem_cgroup_charge_statistics(mem, pc, false);
> @@ -1925,6 +1964,46 @@ void mem_cgroup_uncharge_cache_page(stru
>  	__mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_CACHE);
>  }
> 
> +/*
> + * batch_start/batch_end is called in unmap_page_range/invlidate/trucate.
> + * In that cases, pages are freed continuously and we can expect pages
> + * are in the same memcg. All these calls itself limits the number of
> + * pages freed at once, then uncharge_start/end() is called properly.
> + */
> +
> +void mem_cgroup_uncharge_start(void)
> +{
> +	if (!current->memcg_batch.do_batch) {
> +		current->memcg_batch.memcg = NULL;
> +		current->memcg_batch.pages = 0;
> +		current->memcg_batch.memsw = 0;
> +	}
> +	current->memcg_batch.do_batch++;
> +}
> +
> +void mem_cgroup_uncharge_end(void)
> +{
> +	struct mem_cgroup *mem;
> +
> +	if (!current->memcg_batch.do_batch)
> +		return;
> +
> +	current->memcg_batch.do_batch--;
> +	if (current->memcg_batch.do_batch) /* Nested ? */
> +		return;
> +
> +	mem = current->memcg_batch.memcg;
> +	if (!mem)
> +		return;
> +	/* This "mem" is valid bacause we hide charges behind us. */
> +	if (current->memcg_batch.pages)
> +		res_counter_uncharge(&mem->res, current->memcg_batch.pages);
> +	if (current->memcg_batch.memsw)
> +		res_counter_uncharge(&mem->memsw, current->memcg_batch.memsw);
> +	/* Not necessary. but forget this pointer */
> +	current->memcg_batch.memcg = NULL;
> +}
> +
>  #ifdef CONFIG_SWAP
>  /*
>   * called after __delete_from_swap_cache() and drop "page" account.
> Index: mmotm-2.6.31-Sep28/include/linux/sched.h
> ===================================================================
> --- mmotm-2.6.31-Sep28.orig/include/linux/sched.h
> +++ mmotm-2.6.31-Sep28/include/linux/sched.h
> @@ -1549,6 +1549,13 @@ 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 {
> +		int do_batch;
> +		struct mem_cgroup *memcg;
> +		long pages, memsw;
> +	} memcg_batch;
> +#endif
>  };
> 
>  /* Future-safe accessor for struct task_struct's cpus_allowed. */
> Index: mmotm-2.6.31-Sep28/mm/memory.c
> ===================================================================
> --- mmotm-2.6.31-Sep28.orig/mm/memory.c
> +++ mmotm-2.6.31-Sep28/mm/memory.c
> @@ -940,6 +940,7 @@ static unsigned long unmap_page_range(st
>  		details = NULL;
> 
>  	BUG_ON(addr >= end);
> +	mem_cgroup_uncharge_start();
>  	tlb_start_vma(tlb, vma);
>  	pgd = pgd_offset(vma->vm_mm, addr);
>  	do {
> @@ -952,6 +953,7 @@ static unsigned long unmap_page_range(st
>  						zap_work, details);
>  	} while (pgd++, addr = next, (addr != end && *zap_work > 0));
>  	tlb_end_vma(tlb, vma);
> +	mem_cgroup_uncharge_end();
> 
>  	return addr;
>  }
> Index: mmotm-2.6.31-Sep28/mm/truncate.c
> ===================================================================
> --- mmotm-2.6.31-Sep28.orig/mm/truncate.c
> +++ mmotm-2.6.31-Sep28/mm/truncate.c
> @@ -272,6 +272,7 @@ void truncate_inode_pages_range(struct a
>  			pagevec_release(&pvec);
>  			break;
>  		}
> +		mem_cgroup_uncharge_start();
>  		for (i = 0; i < pagevec_count(&pvec); i++) {
>  			struct page *page = pvec.pages[i];
> 
> @@ -286,6 +287,7 @@ void truncate_inode_pages_range(struct a
>  			unlock_page(page);
>  		}
>  		pagevec_release(&pvec);
> +		mem_cgroup_uncharge_end();
>  	}
>  }
>  EXPORT_SYMBOL(truncate_inode_pages_range);
> @@ -327,6 +329,7 @@ unsigned long invalidate_mapping_pages(s
>  	pagevec_init(&pvec, 0);
>  	while (next <= end &&
>  			pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) {
> +		mem_cgroup_uncharge_start();
>  		for (i = 0; i < pagevec_count(&pvec); i++) {
>  			struct page *page = pvec.pages[i];
>  			pgoff_t index;
> @@ -354,6 +357,7 @@ unsigned long invalidate_mapping_pages(s
>  				break;
>  		}
>  		pagevec_release(&pvec);
> +		mem_cgroup_uncharge_end();
>  		cond_resched();
>  	}
>  	return ret;
> @@ -428,6 +432,7 @@ int invalidate_inode_pages2_range(struct
>  	while (next <= end && !wrapped &&
>  		pagevec_lookup(&pvec, mapping, next,
>  			min(end - next, (pgoff_t)PAGEVEC_SIZE - 1) + 1)) {
> +		mem_cgroup_uncharge_start();
>  		for (i = 0; i < pagevec_count(&pvec); i++) {
>  			struct page *page = pvec.pages[i];
>  			pgoff_t page_index;
> @@ -477,6 +482,7 @@ int invalidate_inode_pages2_range(struct
>  			unlock_page(page);
>  		}
>  		pagevec_release(&pvec);
> +		mem_cgroup_uncharge_end();
>  		cond_resched();
>  	}
>  	return ret;
>

The patch overall looks good, just some questions about it. 

-- 
	Balbir
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