Date:	Fri, 9 Oct 2009 13:17:51 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.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

On Fri, 9 Oct 2009 09:31:42 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * 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?

yes.


> > 
> > 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,

Per-cpu is bad because we can't be robust against task migration.
(I mean per-cpu cannot be clean/simple as this patch.)

Per-memcg doesn't work well. If we desgin per-memcg, we'll use a some logic
like percpu counter. batch-legnth of it cannot be enough big.
And we need fine grain synchronization more than this patch.
 

> I suspect would add some locking
> overhead, per-cpu would require synchronization across cpu's while
> uncharging, is that where per-task helps? 

Yes, it's one of reason for this design.

> I suspect per-mm, per-signal will have the issues above.
Per-mm, per-signal can't be help. mulit-thread can do file truncations (and unmap)
in parallel.

The biggest reason for per-task is that I'd like to change this behavior by
calling _context_. Per-task batch means that we can make use of caller's context.
When uncharge is called by try_to_unmap(), __remove_mapping() in vmscan.c,
we do direct uncharge, for example.


>  
> > 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?
>
TIF_MEMDIE means OOM happens. I take "reduce usage as fast as possible"
scheme here.

> > +
> > +	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. 
> 

Thanks,
-Kame

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