Re: [PATCH] memcg: reduce lock time at move charge (Was Re: [PATCH04/10] memcg: disable local interrupts in lock_page_cgroup()

[Daisuke Nishimura]

On Thu, 7 Oct 2010 15:21:11 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Thu, 7 Oct 2010 11:17:43 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
>  
> > > hmm, if we'll do that, I think we need to do that under pte_lock in
> > > mem_cgroup_move_charge_pte_range(). But, we can't do wait_on_page_writeback()
> > > under pte_lock, right? Or, we need re-organize current move-charge implementation.
> > > 
> > Nice catch. I think releaseing pte_lock() is okay. (and it should be released)
> > 
> > IIUC, task's css_set() points to new cgroup when "move" is called. Then,
> > it's not necessary to take pte_lock, I guess.
> > (And taking pte_lock too long is not appreciated..)
> > 
> > I'll write a sample patch today.
> > 
> Here.
Great!

> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> 
> Now, at task migration among cgroup, memory cgroup scans page table and moving
> account if flags are properly set.
> 
> The core code, mem_cgroup_move_charge_pte_range() does
> 
>  	pte_offset_map_lock();
> 	for all ptes in a page table:
> 		1. look into page table, find_and_get a page
> 		2. remove it from LRU.
> 		3. move charge.
> 		4. putback to LRU. put_page()
> 	pte_offset_map_unlock();
> 
> for pte entries on a 3rd(2nd) level page table.
> 
> This pte_offset_map_lock seems a bit long. This patch modifies a rountine as
> 
> 	pte_offset_map_lock()
> 	for 32 pages:
> 		      find_and_get a page
> 		      record it
> 	pte_offset_map_unlock()
> 	for all recorded pages
> 		      isolate it from LRU.
> 		      move charge
> 		      putback to LRU
> 	for all recorded pages
> 		      put_page()
> 
> Note: newly-charged pages while we move account are charged to the new group.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
>  mm/memcontrol.c |   92 ++++++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 60 insertions(+), 32 deletions(-)
> 
> Index: mmotm-0928/mm/memcontrol.c
> ===================================================================
> --- mmotm-0928.orig/mm/memcontrol.c
> +++ mmotm-0928/mm/memcontrol.c
> @@ -4475,17 +4475,22 @@ one_by_one:
>   *
>   * Called with pte lock held.
>   */
> -union mc_target {
> -	struct page	*page;
> -	swp_entry_t	ent;
> -};
>  
>  enum mc_target_type {
> -	MC_TARGET_NONE,	/* not used */
> +	MC_TARGET_NONE, /* used as failure code(0) */
>  	MC_TARGET_PAGE,
>  	MC_TARGET_SWAP,
>  };
>  
> +struct mc_target {
> +	enum mc_target_type type;
> +	union {
> +		struct page	*page;
> +		swp_entry_t	ent;
> +	} val;
> +};
> +
> +
>  static struct page *mc_handle_present_pte(struct vm_area_struct *vma,
>  						unsigned long addr, pte_t ptent)
>  {
> @@ -4561,7 +4566,7 @@ static struct page *mc_handle_file_pte(s
>  }
>  
>  static int is_target_pte_for_mc(struct vm_area_struct *vma,
> -		unsigned long addr, pte_t ptent, union mc_target *target)
> +		unsigned long addr, pte_t ptent, struct mc_target *target)
>  {
>  	struct page *page = NULL;
>  	struct page_cgroup *pc;
> @@ -4587,7 +4592,7 @@ static int is_target_pte_for_mc(struct v
>  		if (PageCgroupUsed(pc) && pc->mem_cgroup == mc.from) {
>  			ret = MC_TARGET_PAGE;
>  			if (target)
> -				target->page = page;
> +				target->val.page = page;
>  		}
>  		if (!ret || !target)
>  			put_page(page);
> @@ -4597,8 +4602,10 @@ static int is_target_pte_for_mc(struct v
>  			css_id(&mc.from->css) == lookup_swap_cgroup(ent)) {
>  		ret = MC_TARGET_SWAP;
>  		if (target)
> -			target->ent = ent;
> +			target->val.ent = ent;
>  	}
> +	if (target)
> +		target->type = ret;
>  	return ret;
>  }
>  
> @@ -4751,6 +4758,9 @@ static void mem_cgroup_cancel_attach(str
>  	mem_cgroup_clear_mc();
>  }
>  
> +
> +#define MC_MOVE_ONCE		(32)
> +
>  static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
>  				unsigned long addr, unsigned long end,
>  				struct mm_walk *walk)
> @@ -4759,26 +4769,47 @@ static int mem_cgroup_move_charge_pte_ra
>  	struct vm_area_struct *vma = walk->private;
>  	pte_t *pte;
>  	spinlock_t *ptl;
> +	struct mc_target *target;
> +	int index, num;
> +
> +	target = kzalloc(sizeof(struct mc_target) *MC_MOVE_ONCE, GFP_KERNEL);
hmm? I can't see it freed anywhere.

Considering you reset target[]->type to MC_TARGET_NONE, do you intended to
reuse targe[] while walking the page table ?
If so, how about adding a new member(struct mc_target *targe) to move_charge_struct,
and allocate/free it at mem_cgroup_move_charge() ?

Thanks,
Daisuke Nishimura.

> +	if (!target)
> +		return -ENOMEM;
>  
>  retry:
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> -	for (; addr != end; addr += PAGE_SIZE) {
> +	for (num = 0; num < MC_MOVE_ONCE && addr != end; addr += PAGE_SIZE) {
>  		pte_t ptent = *(pte++);
> -		union mc_target target;
> -		int type;
> +		ret = is_target_pte_for_mc(vma, addr, ptent, &target[num]);
> +		if (!ret)
> +			continue;
> +		target[num++].type = ret;
> +	}
> +	pte_unmap_unlock(pte - 1, ptl);
> +	cond_resched();
> +
> +	ret = 0;
> +	index = 0;
> +	do {
> +		struct mc_target *mt;
>  		struct page *page;
>  		struct page_cgroup *pc;
>  		swp_entry_t ent;
>  
> -		if (!mc.precharge)
> -			break;
> +		if (!mc.precharge) {
> +			ret = mem_cgroup_do_precharge(1);
> +			if (ret)
> +				goto out;
> +			continue;
> +		}
> +
> +		mt = &target[index++];
>  
> -		type = is_target_pte_for_mc(vma, addr, ptent, &target);
> -		switch (type) {
> +		switch (mt->type) {
>  		case MC_TARGET_PAGE:
> -			page = target.page;
> +			page = mt->val.page;
>  			if (isolate_lru_page(page))
> -				goto put;
> +				break;
>  			pc = lookup_page_cgroup(page);
>  			if (!mem_cgroup_move_account(pc,
>  						mc.from, mc.to, false)) {
> @@ -4787,11 +4818,9 @@ retry:
>  				mc.moved_charge++;
>  			}
>  			putback_lru_page(page);
> -put:			/* is_target_pte_for_mc() gets the page */
> -			put_page(page);
>  			break;
>  		case MC_TARGET_SWAP:
> -			ent = target.ent;
> +			ent = mt->val.ent;
>  			if (!mem_cgroup_move_swap_account(ent,
>  						mc.from, mc.to, false)) {
>  				mc.precharge--;
> @@ -4802,21 +4831,20 @@ put:			/* is_target_pte_for_mc() gets th
>  		default:
>  			break;
>  		}
> +	} while (index < num);
> +out:
> +	for (index = 0; index < num; index++) {
> +		if (target[index].type == MC_TARGET_PAGE)
> +			put_page(target[index].val.page);
> +		target[index].type = MC_TARGET_NONE;
>  	}
> -	pte_unmap_unlock(pte - 1, ptl);
> +
> +	if (ret)
> +		return ret;
>  	cond_resched();
>  
> -	if (addr != end) {
> -		/*
> -		 * We have consumed all precharges we got in can_attach().
> -		 * We try charge one by one, but don't do any additional
> -		 * charges to mc.to if we have failed in charge once in attach()
> -		 * phase.
> -		 */
> -		ret = mem_cgroup_do_precharge(1);
> -		if (!ret)
> -			goto retry;
> -	}
> +	if (addr != end)
> +		goto retry;
>  
>  	return ret;
>  }
> 
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