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

[Johannes Weiner <hannes@cmpxchg.org>]

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

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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