Subject: Re: [PATCH] /proc/net/tcp, overhead removed

From: Eric Dumazet <[email-masked]>
Date: Tue, 29 Sep 2009 09:56:29 +0200

Yakov Lerner a écrit :
> Take 2. 
> 
> "Sharp improvement in performance of /proc/net/tcp when number of 
> sockets is large and hashsize is large. 
> O(numsock * hashsize) time becomes O(numsock + hashsize). On slow
> processors, speed difference can be x100 and more."
> 
> I must say that I'm not fully satisfied with my choice of "st->sbucket" 
> for the new preserved index. The better name would be "st->snum". 
> Re-using "st->sbucket" saves 4 bytes, and keeps the patch to one sourcefile.
> But "st->sbucket" has different meaning in OPENREQ and LISTEN states;
> this can be confusing. 
> Maybe better add "snum" member to struct tcp_iter_state ?
> 
> Shall I change subject when sending "take N+1", or keep the old subject ?
> 
> Signed-off-by: Yakov Lerner <[email-masked]>
> ---
>  net/ipv4/tcp_ipv4.c |   35 +++++++++++++++++++++++++++++++++--
>  1 files changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index 7cda24b..e4c4f19 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -1994,13 +1994,14 @@ static inline int empty_bucket(struct tcp_iter_state *st)
>  		hlist_nulls_empty(&tcp_hashinfo.ehash[st->bucket].twchain);
>  }
>  
> -static void *established_get_first(struct seq_file *seq)
> +static void *established_get_first_after(struct seq_file *seq, int bucket)
>  {
>  	struct tcp_iter_state *st = seq->private;
>  	struct net *net = seq_file_net(seq);
>  	void *rc = NULL;
>  
> -	for (st->bucket = 0; st->bucket < tcp_hashinfo.ehash_size; ++st->bucket) {
> +	for (st->bucket = bucket; st->bucket < tcp_hashinfo.ehash_size;
> +	     ++st->bucket) {
>  		struct sock *sk;
>  		struct hlist_nulls_node *node;
>  		struct inet_timewait_sock *tw;
> @@ -2010,6 +2011,8 @@ static void *established_get_first(struct seq_file *seq)
>  		if (empty_bucket(st))
>  			continue;
>  
> +		st->sbucket = st->num;
> +
>  		spin_lock_bh(lock);
>  		sk_nulls_for_each(sk, node, &tcp_hashinfo.ehash[st->bucket].chain) {
>  			if (sk->sk_family != st->family ||
> @@ -2036,6 +2039,11 @@ out:
>  	return rc;
>  }
>  
> +static void *established_get_first(struct seq_file *seq)
> +{
> +	return established_get_first_after(seq, 0);
> +}
> +
>  static void *established_get_next(struct seq_file *seq, void *cur)
>  {
>  	struct sock *sk = cur;
> @@ -2064,6 +2072,9 @@ get_tw:
>  		while (++st->bucket < tcp_hashinfo.ehash_size &&
>  				empty_bucket(st))
>  			;
> +
> +		st->sbucket = st->num;
> +
>  		if (st->bucket >= tcp_hashinfo.ehash_size)
>  			return NULL;
>  
> @@ -2107,6 +2118,7 @@ static void *tcp_get_idx(struct seq_file *seq, loff_t pos)
>  
>  	if (!rc) {
>  		st->state = TCP_SEQ_STATE_ESTABLISHED;
> +		st->sbucket = 0;
>  		rc	  = established_get_idx(seq, pos);
>  	}
>  
> @@ -2116,6 +2128,25 @@ static void *tcp_get_idx(struct seq_file *seq, loff_t pos)
>  static void *tcp_seq_start(struct seq_file *seq, loff_t *pos)
>  {
>  	struct tcp_iter_state *st = seq->private;
> +
> +	if (*pos && *pos >= st->sbucket &&
> +	    (st->state == TCP_SEQ_STATE_ESTABLISHED ||
> +	     st->state == TCP_SEQ_STATE_TIME_WAIT)) {
> +		void *cur;
> +		int nskip;
> +
> +		/* for states estab and tw, st->sbucket is index (*pos) */
> +		/* corresponding to the beginning of bucket st->bucket */
> +
> +		st->num = st->sbucket;
> +		/* jump to st->bucket, then skip (*pos - st->sbucket) items */
> +		st->state = TCP_SEQ_STATE_ESTABLISHED;
> +		cur = established_get_first_after(seq, st->bucket);
> +		for (nskip = *pos - st->num; cur && nskip > 0; --nskip)
> +			cur = established_get_next(seq, cur);
> +		return cur;
> +	}
> +
>  	st->state = TCP_SEQ_STATE_LISTENING;
>  	st->num = 0;
>  	return *pos ? tcp_get_idx(seq, *pos - 1) : SEQ_START_TOKEN;

Just in case you are working on "take 3" of the patch, there is a fondamental problem.

All the scalability problems come from the fact that tcp_seq_start()
*has* to rescan all the tables from the begining, because of lseek() capability
on /proc/net/tcp file 

We probably could disable llseek() (on other positions than start of the file),
and rely only on internal state (listening/established hashtable, hash bucket, position in chain)

I cannot imagine how an application could rely on lseek() on >0 position in this file.