[Devel] Re: [PATCH 03/10] memcg: Reclaim when more than one page needed.

[KAMEZAWA Hiroyuki]

On Mon, 27 Feb 2012 14:58:46 -0800
Suleiman Souhlal <ssouhlal at FreeBSD.org> wrote:

> From: Hugh Dickins <hughd at google.com>
> 
> mem_cgroup_do_charge() was written before slab accounting, and expects
> three cases: being called for 1 page, being called for a stock of 32 pages,
> or being called for a hugepage.  If we call for 2 pages (and several slabs
> used in process creation are such, at least with the debug options I had),
> it assumed it's being called for stock and just retried without reclaiming.
> 
> Fix that by passing down a minsize argument in addition to the csize;
> and pass minsize to consume_stock() also, so that it can draw on stock
> for higher order slabs, instead of accumulating an increasing surplus
> of stock, as its "nr_pages == 1" tests previously caused.
> 
> And what to do about that (csize == PAGE_SIZE && ret) retry?  If it's
> needed at all (and presumably is since it's there, perhaps to handle
> races), then it should be extended to more than PAGE_SIZE, yet how far?

IIRC, this hack was added to avoid very-easy-oom-kill in a small memcg.
If we can support dirty_ratio? and stop page reclaim to wait until
writeback end, we can remove this, I think.

> And should there be a retry count limit, of what?  For now retry up to
> COSTLY_ORDER (as page_alloc.c does), stay safe with a cond_resched(),
> and make sure not to do it if __GFP_NORETRY.
> 

Could you divide the changes to consume_stock() and mem_cgroup_do_charge() ?

In these days, I personally don't like magical retry count..
Let's see what happens if we can wait I/O for memcg. We may not have to
have any retry and be able to clean up this loop.

Thanks,
-Kame


> Signed-off-by: Hugh Dickins <hughd at google.com>
> Signed-off-by: Suleiman Souhlal <suleiman at google.com>
> ---
>  mm/memcontrol.c |   35 +++++++++++++++++++----------------
>  1 files changed, 19 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6f44fcb..c82ca1c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1928,19 +1928,19 @@ static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
>  static DEFINE_MUTEX(percpu_charge_mutex);
>  
>  /*
> - * Try to consume stocked charge on this cpu. If success, one page is consumed
> - * from local stock and true is returned. If the stock is 0 or charges from a
> - * cgroup which is not current target, returns false. This stock will be
> - * refilled.
> + * Try to consume stocked charge on this cpu. If success, nr_pages pages are
> + * consumed from local stock and true is returned. If the stock is 0 or
> + * charges from a cgroup which is not current target, returns false.
> + * This stock will be refilled.
>   */
> -static bool consume_stock(struct mem_cgroup *memcg)
> +static bool consume_stock(struct mem_cgroup *memcg, int nr_pages)
>  {
>  	struct memcg_stock_pcp *stock;
>  	bool ret = true;
>  
>  	stock = &get_cpu_var(memcg_stock);
> -	if (memcg == stock->cached && stock->nr_pages)
> -		stock->nr_pages--;
> +	if (memcg == stock->cached && stock->nr_pages >= nr_pages)
> +		stock->nr_pages -= nr_pages;
>  	else /* need to call res_counter_charge */
>  		ret = false;
>  	put_cpu_var(memcg_stock);
> @@ -2131,7 +2131,7 @@ enum {
>  };
>  
>  static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> -				unsigned int nr_pages, bool oom_check)
> +    unsigned int nr_pages, unsigned int min_pages, bool oom_check)
>  {
>  	unsigned long csize = nr_pages * PAGE_SIZE;
>  	struct mem_cgroup *mem_over_limit;
> @@ -2154,18 +2154,18 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	} else
>  		mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
>  	/*
> -	 * nr_pages can be either a huge page (HPAGE_PMD_NR), a batch
> -	 * of regular pages (CHARGE_BATCH), or a single regular page (1).
> -	 *
>  	 * Never reclaim on behalf of optional batching, retry with a
>  	 * single page instead.
>  	 */
> -	if (nr_pages == CHARGE_BATCH)
> +	if (nr_pages > min_pages)
>  		return CHARGE_RETRY;
>  
>  	if (!(gfp_mask & __GFP_WAIT))
>  		return CHARGE_WOULDBLOCK;
>  
> +	if (gfp_mask & __GFP_NORETRY)
> +		return CHARGE_NOMEM;
> +
>  	ret = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
>  	if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
>  		return CHARGE_RETRY;
> @@ -2178,8 +2178,10 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	 * unlikely to succeed so close to the limit, and we fall back
>  	 * to regular pages anyway in case of failure.
>  	 */
> -	if (nr_pages == 1 && ret)
> +	if (nr_pages <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER) && ret) {
> +		cond_resched();
>  		return CHARGE_RETRY;
> +	}
>  
>  	/*
>  	 * At task move, charge accounts can be doubly counted. So, it's
> @@ -2253,7 +2255,7 @@ again:
>  		VM_BUG_ON(css_is_removed(&memcg->css));
>  		if (mem_cgroup_is_root(memcg))
>  			goto done;
> -		if (nr_pages == 1 && consume_stock(memcg))
> +		if (consume_stock(memcg, nr_pages))
>  			goto done;
>  		css_get(&memcg->css);
>  	} else {
> @@ -2278,7 +2280,7 @@ again:
>  			rcu_read_unlock();
>  			goto done;
>  		}
> -		if (nr_pages == 1 && consume_stock(memcg)) {
> +		if (consume_stock(memcg, nr_pages)) {
>  			/*
>  			 * It seems dagerous to access memcg without css_get().
>  			 * But considering how consume_stok works, it's not
> @@ -2313,7 +2315,8 @@ again:
>  			nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
>  		}
>  
> -		ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, oom_check);
> +		ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, nr_pages,
> +		    oom_check);
>  		switch (ret) {
>  		case CHARGE_OK:
>  			break;
> -- 
> 1.7.7.3
> 
>