netdev - Re: [PATCH net-next v3] rps: selective flow shedding during softnet overflow

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Date:	Tue, 23 Apr 2013 12:18:58 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willem de Bruijn <willemb@...gle.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	stephen@...workplumber.org
Subject: Re: [PATCH net-next v3] rps: selective flow shedding during softnet
 overflow

Hi Willem

On Tue, 2013-04-23 at 14:46 -0400, Willem de Bruijn wrote:

> +
> +static bool skb_flow_limit(struct sk_buff *skb, unsigned int qlen)
> +{
> +#ifdef CONFIG_NET_FLOW_LIMIT
> +	struct sd_flow_limit *fl;
> +	struct softnet_data *sd;
> +	unsigned int old_flow, new_flow;
> +
> +	if (qlen < (netdev_max_backlog >> 1))
> +		return false;
> +
> +	sd = &per_cpu(softnet_data, smp_processor_id());

sd = __get_cpu_var(softnet_data);

> +
> +	rcu_read_lock();
> +	fl = rcu_dereference(sd->flow_limit);
> +	if (fl) {
> +		new_flow = skb_get_rxhash(skb) & (fl->num_buckets - 1);
> +		old_flow = fl->history[fl->history_head];
> +		fl->history[fl->history_head] = new_flow;
> +
> +		fl->history_head++;
> +		fl->history_head &= FLOW_LIMIT_HISTORY - 1;
> +
> +		if (likely(fl->buckets[old_flow]))
> +			fl->buckets[old_flow]--;
> +
> +		if (++fl->buckets[new_flow] > (FLOW_LIMIT_HISTORY >> 1)) {
> +			fl->count++;
> +			rcu_read_unlock();
> +			return true;
> +		}
> +	}
> +	rcu_read_unlock();
> +#endif
> +	return false;
> +}
> +

...

 
> +#ifdef CONFIG_NET_FLOW_LIMIT
> +static DEFINE_MUTEX(flow_limit_update_mutex);
> +
> +static int flow_limit_cpu_sysctl(ctl_table *table, int write,
> +				 void __user *buffer, size_t *lenp,
> +				 loff_t *ppos)
> +{
> +	struct sd_flow_limit *cur;
> +	struct softnet_data *sd;
> +	cpumask_var_t mask;
> +	int i, len, ret = 0;
> +
> +	if (!alloc_cpumask_var(&mask, GFP_KERNEL))
> +		return -ENOMEM;
> +
> +	if (write) {
> +		ret = cpumask_parse_user(buffer, *lenp, mask);
> +		if (ret)
> +			goto done;
> +
> +		mutex_lock(&flow_limit_update_mutex);
> +		len = sizeof(*cur) + netdev_flow_limit_table_len;
> +		for_each_possible_cpu(i) {
> +			sd = &per_cpu(softnet_data, i);
> +			cur = rcu_dereference_protected(sd->flow_limit,
> +				      lockdep_is_held(flow_limit_update_mutex));
> +			if (cur && !cpumask_test_cpu(i, mask)) {
> +				RCU_INIT_POINTER(sd->flow_limit, NULL);
> +				synchronize_rcu();
> +				kfree(cur);
> +			} else if (!cur && cpumask_test_cpu(i, mask)) {
> +				cur = kzalloc(len, GFP_KERNEL);
> +				if (!cur) {
> +					/* not unwinding previous changes */
> +					ret = -ENOMEM;
> +					goto write_unlock;
> +				}
> +				cur->num_buckets = netdev_flow_limit_table_len;
> +				rcu_assign_pointer(sd->flow_limit, cur);
> +			}
> +		}
> +write_unlock:
> +		synchronize_rcu();

I believe you do not need this synchronize_rcu() call.

> +		mutex_unlock(&flow_limit_update_mutex);
> +	} else {
> +		if (*ppos || !*lenp) {
> +			*lenp = 0;
> +			goto done;
> +		}
> +
> +		cpumask_clear(mask);
> +		rcu_read_lock();
> +		for_each_possible_cpu(i) {
> +			sd = &per_cpu(softnet_data, i);
> +			if (rcu_dereference(sd->flow_limit))
> +				cpumask_set_cpu(i, mask);
> +		}
> +		rcu_read_unlock();
> +
> +		len = cpumask_scnprintf(buffer, *lenp, mask);
> +		*lenp = len + 1;
> +		*ppos += len + 1;
> +	}
> +
> +done:
> +	free_cpumask_var(mask);
> +	return ret;
> +}
> +

Thanks !


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