Re: [PATCH v5 net-next 1/5] net: add napi_id and hash

[Eric Dumazet <erdnetdev@gmail.com>]

On Mon, 2013-05-27 at 10:44 +0300, Eliezer Tamir wrote:
> Adds a napi_id and a hashing mechanism to lookup a napi by id.
> This will be used by subsequent patches to implement low latency
> Ethernet device polling.
> Based on a code sample by Eric Dumazet.
> 
> Signed-off-by: Eliezer Tamir <eliezer.tamir@linux.intel.com>
> ---
> 
>  include/linux/netdevice.h |   29 +++++++++++++++++++++++++++++
>  net/core/dev.c            |   44 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 73 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index ea7b6bc..d1ec8b1 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -324,12 +324,15 @@ struct napi_struct {
>  	struct sk_buff		*gro_list;
>  	struct sk_buff		*skb;
>  	struct list_head	dev_list;
> +	struct hlist_node	napi_hash_node;
> +	int			napi_id;
>  };
>  
>  enum {
>  	NAPI_STATE_SCHED,	/* Poll is scheduled */
>  	NAPI_STATE_DISABLE,	/* Disable pending */
>  	NAPI_STATE_NPSVC,	/* Netpoll - don't dequeue from poll_list */
> +	NAPI_STATE_HASHED,	/* In NAPI hash */
>  };
>  
>  enum gro_result {
> @@ -446,6 +449,32 @@ extern void __napi_complete(struct napi_struct *n);
>  extern void napi_complete(struct napi_struct *n);
>  
>  /**
> + *	napi_hash_add - add a NAPI to global hashtable
> + *	@napi: napi context
> + *
> + * generate a new napi_id and store a @napi under it in napi_hash
> + */
> +extern void napi_hash_add(struct napi_struct *napi);
> +
> +/**
> + *	napi_hash_del - remove a NAPI from blobal table

global

> + *	@napi: napi context
> + *
> + * Warning: caller must observe rcu grace period
> + * before freeing memory containing @NAPI

 @napi

> + */
> +extern void napi_hash_del(struct napi_struct *napi);
> +
> +/**
> + *	napi_by_is - lookup a NAPI by napi_id

napi_by_id

> + *	@napi_id: hashed napi_id
> + *
> + * lookup napi_id in napi_hash table

 @napi_id

> + * must be called under rcu_read_lock()
> + */
> +extern struct napi_struct *napi_by_id(int napi_id);
> +
> +/**
>   *	napi_disable - prevent NAPI from scheduling
>   *	@n: napi context
>   *
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 50c02de..283ab14 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -129,6 +129,7 @@
>  #include <linux/inetdevice.h>
>  #include <linux/cpu_rmap.h>
>  #include <linux/static_key.h>
> +#include <linux/hashtable.h>
>  
>  #include "net-sysfs.h"
>  
> @@ -166,6 +167,10 @@ static struct list_head offload_base __read_mostly;
>  DEFINE_RWLOCK(dev_base_lock);
>  EXPORT_SYMBOL(dev_base_lock);
>  
> +atomic_t napi_gen_id;

Not sure we need an atomic, we are protected by RTNL anyway.

> +
> +DEFINE_HASHTABLE(napi_hash, 8);
> +
>  seqcount_t devnet_rename_seq;
>  
>  static inline void dev_base_seq_inc(struct net *net)
> @@ -4113,6 +4118,45 @@ void napi_complete(struct napi_struct *n)
>  }
>  EXPORT_SYMBOL(napi_complete);
>  
> +void napi_hash_add(struct napi_struct *napi)
> +{
> +	if (!test_and_set_bit(NAPI_STATE_HASHED, &napi->state)) {
> +
> +		/* 0 is not a valid id */
> +		napi->napi_id = 0;
> +		while (!napi->napi_id)
> +			napi->napi_id = atomic_inc_return(&napi_gen_id);
> +
> +		hlist_add_head_rcu(&napi->napi_hash_node,
> +			&napi_hash[napi->napi_id % HASH_SIZE(napi_hash)]);
> +	}
> +}
> +EXPORT_SYMBOL_GPL(napi_hash_add);
> +
> +/* Warning : caller is responsible to make sure rcu grace period
> + * is respected before freeing memory containing @napi
> + */
> +void napi_hash_del(struct napi_struct *napi)
> +{
> +	if (test_and_clear_bit(NAPI_STATE_HASHED, &napi->state))
> +		hlist_del_rcu(&napi->napi_hash_node);
> +}
> +EXPORT_SYMBOL_GPL(napi_hash_del);
> +
> +/* must be called under rcu_read_lock(), as we dont take a reference */
> +struct napi_struct *napi_by_id(int napi_id)
> +{
> +	unsigned int hash = napi_id % HASH_SIZE(napi_hash);
> +	struct napi_struct *napi;
> +
> +	hlist_for_each_entry_rcu(napi, &napi_hash[hash], napi_hash_node)
> +		if (napi->napi_id == napi_id)
> +			return napi;
> +
> +	return NULL;
> +}
> +EXPORT_SYMBOL_GPL(napi_by_id);
> +
>  void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
>  		    int (*poll)(struct napi_struct *, int), int weight)
>  {
>