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

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Message-ID: <f7b8e306-5169-ad5c-0a5e-7ec6333e24bf@gmail.com>
Date:   Wed, 8 Jul 2020 14:37:01 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "YU, Xiangning" <xiangning.yu@...baba-inc.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 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