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

Wanpeng Li
2012-06-24T10:08:36Z

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...