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

Wanpeng Li
2012-06-24T10:33:16Z

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@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.
>
>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.