Re: [PATCH RFC 2/2] net: Optimize snmp stat aggregation by walking all the percpu data at once

[Eric Dumazet]

On Tue, 2015-08-25 at 13:24 +0530, Raghavendra K T wrote:
> Docker container creation linearly increased from around 1.6 sec to 7.5 sec
> (at 1000 containers) and perf data showed 50% ovehead in snmp_fold_field.
> 
> reason: currently __snmp6_fill_stats64 calls snmp_fold_field that walks
> through per cpu data of an item (iteratively for around 90 items).
> 
> idea: This patch tries to aggregate the statistics by going through
> all the items of each cpu sequentially which is reducing cache
> misses.
> 
> Docker creation got faster by more than 2x after the patch.
> 
> Result:
>                        Before           After
> Docker creation time   6.836s           3.357s
> cache miss             2.7%             1.38%
> 
> perf before:
>     50.73%  docker           [kernel.kallsyms]       [k] snmp_fold_field
>      9.07%  swapper          [kernel.kallsyms]       [k] snooze_loop
>      3.49%  docker           [kernel.kallsyms]       [k] veth_stats_one
>      2.85%  swapper          [kernel.kallsyms]       [k] _raw_spin_lock
> 
> perf after:
>     10.56%  swapper          [kernel.kallsyms]     [k] snooze_loop
>      8.72%  docker           [kernel.kallsyms]     [k] snmp_get_cpu_field
>      7.59%  docker           [kernel.kallsyms]     [k] veth_stats_one
>      3.65%  swapper          [kernel.kallsyms]     [k] _raw_spin_lock
> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
> ---
>  net/ipv6/addrconf.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> index 21c2c81..2ec905f 100644
> --- a/net/ipv6/addrconf.c
> +++ b/net/ipv6/addrconf.c
> @@ -4624,16 +4624,24 @@ static inline void __snmp6_fill_statsdev(u64 *stats, atomic_long_t *mib,
>  }
>  
>  static inline void __snmp6_fill_stats64(u64 *stats, void __percpu *mib,
> -				      int items, int bytes, size_t syncpoff)
> +					int items, int bytes, size_t syncpoff)
>  {
> -	int i;
> +	int i, c;
> +	u64 *tmp;
>  	int pad = bytes - sizeof(u64) * items;
>  	BUG_ON(pad < 0);
>  
> +	tmp = kcalloc(items, sizeof(u64), GFP_KERNEL);
> +


This is a great idea, but kcalloc()/kmalloc() can fail and you'll crash
the whole kernel at this point.



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