netdev - Re: [PATCH net-next v2 2/2] net: sched: Lockless Token Bucket (LTB) qdisc

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Date:   Thu, 09 Jul 2020 06:01:49 +0800
From:   "YU, Xiangning" <xiangning.yu@...baba-inc.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 2/2] net: sched: Lockless Token Bucket (LTB)
 qdisc

On 7/8/20 2:37 PM, Eric Dumazet wrote:
> 
> 
> On 7/8/20 9:38 AM, YU, Xiangning wrote:
>> Lockless Token Bucket (LTB) is a qdisc implementation that controls the
>> use of outbound bandwidth on a shared link. With the help of lockless
>> qdisc, and by decoupling rate limiting and bandwidth sharing, LTB is
>> designed to scale in the cloud data centers.
>>
> 
> ...
> 
> This ltb_class struct has a size of 1579584 bytes :/
> 
>> +struct ltb_class {
>> +	struct Qdisc_class_common common;
>> +	struct psched_ratecfg ratecfg;
>> +	struct psched_ratecfg ceilcfg;
>> +	u32 prio;
>> +	struct ltb_class *parent;
>> +	struct Qdisc *qdisc;
>> +	struct Qdisc *root_qdisc;
>> +	u32 classid;
>> +	struct list_head pnode;
>> +	unsigned long state; ____cacheline_aligned_in_smp
>> +
>> +	/* Aggr/drain context only */
>> +	s64 next_timestamp; ____cacheline_aligned_in_smp
>> +	int num_cpus;
>> +	int last_cpu;
>> +	s64 bw_used;
>> +	s64 last_bytes;
>> +	s64 last_timestamp;
>> +	s64 stat_bytes;
>> +	s64 stat_packets;
>> +	atomic64_t stat_drops;
>> +
>> +	/* Balance delayed work only */
>> +	s64 rate; ____cacheline_aligned_in_smp
>> +	s64 ceil;
>> +	s64 high_water;
>> +	int drop_delay;
>> +	s64 bw_allocated;
>> +	bool want_more;
>> +
>> +	/* Shared b/w aggr/drain thread and balancer */
>> +	unsigned long curr_interval; ____cacheline_aligned_in_smp
>> +	s64 bw_measured;	/* Measured actual bandwidth */
>> +	s64 maxbw;	/* Calculated bandwidth */
>> +
>> +	STRUCT_KFIFO(struct sk_buff *, SKB_QLEN) aggr_queues[MAX_CPU_COUNT];
>> +	____cacheline_aligned_in_smp
>> +	STRUCT_KFIFO(struct sk_buff *, SKB_QLEN * MAX_CPU_COUNT) drain_queue;
>> +	____cacheline_aligned_in_smp
>> +	STRUCT_KFIFO(struct sk_buff *, SKB_QLEN) fanout_queues[MAX_CPU_COUNT];
>> +	____cacheline_aligned_in_smp
>> +
>> +	struct tasklet_struct aggr_tasklet;
>> +	struct hrtimer aggr_timer;
>> +};
>> +
>>
> 
>> +
>> +static struct ltb_class *ltb_alloc_class(struct Qdisc *sch,
>> +					 struct ltb_class *parent, u32 classid,
>> +					 struct psched_ratecfg *ratecfg,
>> +					 struct psched_ratecfg *ceilcfg,
>> +					 u32 prio)
>> +{
>> +	struct ltb_sched *ltb  = qdisc_priv(sch);
>> +	struct ltb_class *cl;
>> +	int i;
>> +
>> +	if (ratecfg->rate_bytes_ps > ceilcfg->rate_bytes_ps ||
>> +	    prio < 0 || prio >= TC_LTB_NUMPRIO)
>> +		return NULL;
>> +
>> +	cl = kzalloc(sizeof(*cl), GFP_KERNEL);
> 
> This is going to fail, 2MB chunks of physically contiguous memory is unreasonable.
> 
> 2MB per class makes this qdisc very particular, especially with 1000 classes ?
> 
> In comparison, HTB class consumes less than 1 KB
> 

The main memory consumption comes from the kfifo queues. We use far more less classes than 1000 so we didn't really care that.

If supporting 1000 classes is a goal, we should be able to aggressively reduce the queue length. Currently it is set to 512 per-CPU which is a waste. Also we can dynamically allocate the kfifo queues according to CPU numbers.

Thanks,
- Xiangning