Re: [PATCH] mm/memcg: add MAX_CHARGE_BATCH to limit unnecessary charge overhead

[Wanpeng Li <liwp.linux-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Sun, Jun 24, 2012 at 12:19:48PM +0200, Johannes Weiner wrote:
>On Sun, Jun 24, 2012 at 06:08:26PM +0800, Wanpeng Li wrote:
>> On Sun, Jun 24, 2012 at 11:46:14AM +0200, Johannes Weiner wrote:
>> >On Sun, Jun 24, 2012 at 10:16:09AM +0800, Wanpeng Li wrote:
>> >> From: Wanpeng Li <liwp-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
>> >> 
>> >> Since exceeded unused cached charges would add pressure to
>> >> mem_cgroup_do_charge, more overhead would burn cpu cycles when
>> >> mem_cgroup_do_charge cause page reclaim or even OOM be triggered
>> >> just for such exceeded unused cached charges. Add MAX_CHARGE_BATCH
>> >> to limit max cached charges.
>> >> 
>> >> Signed-off-by: Wanpeng Li <liwp.linux-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>> >> ---
>> >>  mm/memcontrol.c |   16 ++++++++++++++++
>> >>  1 file changed, 16 insertions(+)
>> >> 
>> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> >> index 0e092eb..1ff317a 100644
>> >> --- a/mm/memcontrol.c
>> >> +++ b/mm/memcontrol.c
>> >> @@ -1954,6 +1954,14 @@ void mem_cgroup_update_page_stat(struct page *page,
>> >>   * TODO: maybe necessary to use big numbers in big irons.
>> >>   */
>> >>  #define CHARGE_BATCH	32U
>> >> +
>> >> +/*
>> >> + * Max size of charge stock. Since exceeded unused cached charges would
>> >> + * add pressure to mem_cgroup_do_charge which will cause page reclaim or
>> >> + * even oom be triggered.
>> >> + */
>> >> +#define MAX_CHARGE_BATCH 1024U
>> >> +
>> >>  struct memcg_stock_pcp {
>> >>  	struct mem_cgroup *cached; /* this never be root cgroup */
>> >>  	unsigned int nr_pages;
>> >> @@ -2250,6 +2258,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
>> >>  	unsigned int batch = max(CHARGE_BATCH, nr_pages);
>> >>  	int nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
>> >>  	struct mem_cgroup *memcg = NULL;
>> >> +	struct memcg_stock_pcp *stock;
>> >>  	int ret;
>> >>  
>> >>  	/*
>> >> @@ -2320,6 +2329,13 @@ again:
>> >>  		rcu_read_unlock();
>> >>  	}
>> >>  
>> >> +	stock = &get_cpu_var(memcg_stock);
>> >> +	if (memcg == stock->cached && stock->nr_pages) {
>> >> +		if (stock->nr_pages > MAX_CHARGE_BATCH)
>> >> +			batch = nr_pages;
>> >> +	}
>> >> +	put_cpu_var(memcg_stock);
>> >
>> >The only way excessive stock can build up is if the charging task gets
>> >rescheduled, after trying to consume stock a few lines above, to a cpu
>> >it was running on when it built up stock in the past.
>> >
>> >    consume_stock()
>> >      memcg != stock->cached:
>> >        return false
>> >    do_charge()
>> >    <reschedule>
>> >    refill_stock()
>> >      memcg == stock->cached:
>> >        stock->nr_pages += nr_pages
>> 
>> __mem_cgroup_try_charge() {
>> 	unsigned int batch = max(CHARGE_BATCH, nr_pages);
>> 	[...]
>> 	mem_cgroup_do_charge(memcg, gfp_mask, batch, oom_check);
>> 	[...]
>> 	if(batch > nr_pages)
>> 		refill_stock(memcg, batch - nr_pages);
>> }
>> 
>> Consider this scenario, If one task wants to charge nr_pages = 1,
>> then batch = max(32,1) = 32, this time 31 excess charges 
>> will be charged in mem_cgroup_do_charge and then add to stock by
>> refill_stock. Generally there are many tasks in one memory cgroup and 
>> maybe charges frequency. In this situation, limit will reach soon, 
>> and cause mem_cgroup_reclaim to call try_to_free_mem_cgroup_pages.
>
>But the stock is not a black hole that gets built up for giggles!  The
>next time the processes want to charge a page on this cpu, they will
>consume it from the stock.  Not add more pages to it.  Look at where
>consume_stock() is called.

if(nr_pages == 1 && consume_stock(memcg))
	goto done;

Only when charge one page will call consume_stock. You can see the codes
in mem_cgroup_charge_common() which also call __mem_cgroup_try_charge, 
when both transparent huge and hugetlbfs pages, nr_pages will larger than 1.