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

[Wanpeng Li <liwp.linux@gmail.com>]

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@linux.vnet.ibm.com>
>> 
>> 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@gmail.com>
>> ---
>>  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.

Regards,
Wanpeng Li
>
>It's very unlikely and a single call into target reclaim will drain
>all stock of the memcg, so this will self-correct quickly.
>
>And your patch won't change any of that.
>
>What you /could/ do is stick that check into refill_stock() and invoke
>res_counter_uncharge() if it gets excessive.  But I really don't see a
>practical problem here...

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