Subject: Re: [PATCH] INET : removes per bucket rwlock in tcp/dccp ehash table

From: Eric Dumazet <[email-masked]>
Date: Thu, 01 Nov 2007 19:00:13 +0100

Jarek Poplawski a écrit :
> Hi,
> 
> A few doubts below: 
> 
>>  
>> +#if defined(CONFIG_SMP) || defined(CONFIG_PROVE_LOCKING)
> 
> Probably "|| defined(CONFIG_DEBUG_SPINLOCK)" is needed here.

Not sure, because DEBUG_SPINLOCK only applies to spinlocks.
Here we deal with rwlocks.

> 
>> +/*
>> + * Instead of using one rwlock for each inet_ehash_bucket, we use a table of locks
>> + * The size of this table is a power of two and depends on the number of CPUS.
>> + */
>> +# if defined(CONFIG_DEBUG_LOCK_ALLOC)
>> +#  define EHASH_LOCK_SZ 256
>> +# elif NR_CPUS >= 32
>> +#  define EHASH_LOCK_SZ	4096
>> +# elif NR_CPUS >= 16
>> +#  define EHASH_LOCK_SZ	2048
>> +# elif NR_CPUS >= 8
>> +#  define EHASH_LOCK_SZ	1024
>> +# elif NR_CPUS >= 4
>> +#  define EHASH_LOCK_SZ	512
>> +# else
>> +#  define EHASH_LOCK_SZ	256
>> +# endif
>> +#else
>> +# define EHASH_LOCK_SZ 0
>> +#endif
>> +
> 
> Looks hackish: usually DEBUG code checks "real" environment, and here it's
> a special case. But omitting locks if no SMP or DEBUG is strange. IMHO,
> there should be 1 instead of 0.

It is 0 so that no alloc is done. (see your next questions)

> 
>>  struct inet_hashinfo {
>>  	/* This is for sockets with full identity only.  Sockets here will
>>  	 * always be without wildcards and will have the following invariant:
>> @@ -100,6 +121,7 @@ struct inet_hashinfo {
>>  	 * TIME_WAIT sockets use a separate chain (twchain).
>>  	 */
>>  	struct inet_ehash_bucket	*ehash;
>> +	rwlock_t			*ehash_locks;
>>  
>>  	/* Ok, let's try this, I give up, we do need a local binding
>>  	 * TCP hash as well as the others for fast bind/connect.
>> @@ -134,6 +156,13 @@ static inline struct inet_ehash_bucket *inet_ehash_bucket(
>>  	return &hashinfo->ehash[hash & (hashinfo->ehash_size - 1)];
>>  }
>>  
>> +static inline rwlock_t *inet_ehash_lockp(
>> +	struct inet_hashinfo *hashinfo,
>> +	unsigned int hash)
>> +{
>> +	return &hashinfo->ehash_locks[hash & (EHASH_LOCK_SZ - 1)];
>> +}
>> +
> 
> Is it OK for EHASH_LOCK_SZ == 0?

At least, compiled tested and booted on UP ;)

> 
> ...
>> diff --git a/net/dccp/proto.c b/net/dccp/proto.c
>> index d849739..3b5f97a 100644
>> --- a/net/dccp/proto.c
>> +++ b/net/dccp/proto.c
>> @@ -1072,11 +1072,18 @@ static int __init dccp_init(void)
>>  	}
>>  
>>  	for (i = 0; i < dccp_hashinfo.ehash_size; i++) {
>> -		rwlock_init(&dccp_hashinfo.ehash[i].lock);
>>  		INIT_HLIST_HEAD(&dccp_hashinfo.ehash[i].chain);
>>  		INIT_HLIST_HEAD(&dccp_hashinfo.ehash[i].twchain);
>>  	}
>> -
>> +	if (EHASH_LOCK_SZ) {
> 
> Why not #ifdef then? But, IMHO, rwlock_init() should be done at least
> once here. (Similarly later for tcp.)

well, #ifdef are not so nice :)

> 
>> +		dccp_hashinfo.ehash_locks =
>> +			kmalloc(EHASH_LOCK_SZ * sizeof(rwlock_t),
>> +				GFP_KERNEL);
>> +		if (!dccp_hashinfo.ehash_locks)
>> +			goto out_free_dccp_ehash;
>> +		for (i = 0; i < EHASH_LOCK_SZ; i++)
>> +			rwlock_init(&dccp_hashinfo.ehash_locks[i]);
>> +	}
>>  	bhash_order = ehash_order;
>>  
>>  	do {
>> @@ -1091,7 +1098,7 @@ static int __init dccp_init(void)
>>  
>>  	if (!dccp_hashinfo.bhash) {
>>  		DCCP_CRIT("Failed to allocate DCCP bind hash table");
>> -		goto out_free_dccp_ehash;
>> +		goto out_free_dccp_locks;
>>  	}
>>  
>>  	for (i = 0; i < dccp_hashinfo.bhash_size; i++) {
>> @@ -1121,6 +1128,9 @@ out_free_dccp_mib:
>>  out_free_dccp_bhash:
>>  	free_pages((unsigned long)dccp_hashinfo.bhash, bhash_order);
>>  	dccp_hashinfo.bhash = NULL;
>> +out_free_dccp_locks:
>> +	kfree(dccp_hashinfo.ehash_locks);
>> +	dccp_hashinfo.ehash_locks = NULL;
>>  out_free_dccp_ehash:
>>  	free_pages((unsigned long)dccp_hashinfo.ehash, ehash_order);
>>  	dccp_hashinfo.ehash = NULL;
> 
> Isn't such kfree(dccp_hashinfo.ehash_locks) needed in dccp_fini()?
> 

Probably ! Thank you for reviewing !

Eric