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

Johannes Weiner
2012-06-24T18:33:38Z

On Sun, Jun 24, 2012 at 06:32:58PM +0800, Wanpeng Li wrote:
> 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.

In which case, nr_pages will be bigger than CHARGE_BATCH, in which
case batch equals nr_pages, in which case stock won't be refilled:

unsigned int batch = max(CHARGE_BATCH, nr_pages);
...
if (batch > nr_pages)
refill_stock(memcg, batch - nr_pages);

We could maybe make this

if (nr_pages == 1 && batch > nr_pages)
...

for clarity, but it won't make a behavioural difference.