Subject: Re: [PATCH 3/3] iptables: lock free counters (alternate version)

From: Eric Dumazet <[email-masked]>
Date: Tue, 03 Feb 2009 21:20:04 +0100

Paul E. McKenney a écrit :
> On Tue, Feb 03, 2009 at 08:00:16PM +0100, Eric Dumazet wrote:
>> Stephen Hemminger a écrit :
>>> This is an alternative to earlier RCU/seqcount_t version of counters.
>>> The counters operate as usual without locking, but when counters are rotated
>>> around the CPU's entries.  RCU is used in two ways, first to handle the
>>> counter rotation, second for replace.
>> Is it a working patch or just a prototype ?
>>
>>> Signed-off-by: Stephen Hemminger <[email-masked]>
>>>
>>> ---
>>>  include/linux/netfilter/x_tables.h |   10 +++-
>>>  net/ipv4/netfilter/arp_tables.c    |   73 +++++++++++++++++++++++++++---------
>>>  net/ipv4/netfilter/ip_tables.c     |   68 ++++++++++++++++++++++++---------
>>>  net/ipv6/netfilter/ip6_tables.c    |   75 ++++++++++++++++++++++++++-----------
>>>  net/netfilter/x_tables.c           |   43 +++++++++++++++------
>>>  5 files changed, 197 insertions(+), 72 deletions(-)
>>>
>>> --- a/include/linux/netfilter/x_tables.h	2009-02-02 15:06:39.893751845 -0800
>>> +++ b/include/linux/netfilter/x_tables.h	2009-02-02 15:28:10.022574005 -0800
>>> @@ -353,7 +353,7 @@ struct xt_table
>>>  	unsigned int valid_hooks;
>>>  
>>>  	/* Lock for the curtain */
>>> -	rwlock_t lock;
>>> +	struct mutex lock;
>>>  
>>>  	/* Man behind the curtain... */
>>>  	struct xt_table_info *private;
>>> @@ -383,9 +383,15 @@ struct xt_table_info
>>>  	unsigned int hook_entry[NF_INET_NUMHOOKS];
>>>  	unsigned int underflow[NF_INET_NUMHOOKS];
>>>  
>>> +	/* For the dustman... */
>>> +	union {
>>> +		struct rcu_head rcu;
>>> +		struct work_struct work;
>>> +	};
>>> +
>>>  	/* ipt_entry tables: one per CPU */
>>>  	/* Note : this field MUST be the last one, see XT_TABLE_INFO_SZ */
>>> -	char *entries[1];
>>> +	void *entries[1];
>>>  };
>>>  
>>>  #define XT_TABLE_INFO_SZ (offsetof(struct xt_table_info, entries) \
>>> --- a/net/ipv4/netfilter/ip_tables.c	2009-02-02 15:06:29.684249364 -0800
>>> +++ b/net/ipv4/netfilter/ip_tables.c	2009-02-02 15:14:13.256499021 -0800
>>> @@ -347,10 +347,12 @@ ipt_do_table(struct sk_buff *skb,
>>>  	mtpar.family  = tgpar.family = NFPROTO_IPV4;
>>>  	tgpar.hooknum = hook;
>>>  
>>> -	read_lock_bh(&table->lock);
>>>  	IP_NF_ASSERT(table->valid_hooks & (1 << hook));
>>> -	private = table->private;
>>> -	table_base = (void *)private->entries[smp_processor_id()];
>>> +
>>> +	rcu_read_lock_bh();
>>> +	private = rcu_dereference(table->private);
>>> +	table_base = rcu_dereference(private->entries[smp_processor_id()]);
>>> +
>>>  	e = get_entry(table_base, private->hook_entry[hook]);
>>>  
>>>  	/* For return from builtin chain */
>>> @@ -445,7 +447,7 @@ ipt_do_table(struct sk_buff *skb,
>>>  		}
>>>  	} while (!hotdrop);
>>>  
>>> -	read_unlock_bh(&table->lock);
>>> +	rcu_read_unlock_bh();
>>>  
>>>  #ifdef DEBUG_ALLOW_ALL
>>>  	return NF_ACCEPT;
>>> @@ -892,45 +894,73 @@ set_entry_to_counter(const struct ipt_en
>>>  	return 0;
>>>  }
>>>  
>>> +static inline int
>>> +set_counter_to_entry(struct ipt_entry *e,
>>> +		     const struct ipt_counters total[],
>>> +		     unsigned int *i)
>>> +{
>>> +	SET_COUNTER(e->counters, total[*i].bcnt, total[*i].pcnt);
>>> +
>>> +	(*i)++;
>>> +	return 0;
>>> +}
>>> +
>>> +
>>>  static void
>>> -get_counters(const struct xt_table_info *t,
>>> +get_counters(struct xt_table_info *t,
>>>  	     struct xt_counters counters[])
>>>  {
>>>  	unsigned int cpu;
>>>  	unsigned int i;
>>>  	unsigned int curcpu;
>>> +	struct ipt_entry *e;
>>>  
>>> -	/* Instead of clearing (by a previous call to memset())
>>> -	 * the counters and using adds, we set the counters
>>> -	 * with data used by 'current' CPU
>>> -	 * We dont care about preemption here.
>>> -	 */
>>> +	preempt_disable();
>>>  	curcpu = raw_smp_processor_id();
>>> -
>>> +	e = t->entries[curcpu];
>>>  	i = 0;
>>> -	IPT_ENTRY_ITERATE(t->entries[curcpu],
>>> +	IPT_ENTRY_ITERATE(e,
>>>  			  t->size,
>>>  			  set_entry_to_counter,
>> Hum, current cpu might be interrupted by NIC, since you only disabled preemption.
>> set_entry_to_counter() might get garbage.
>> I suppose I already mentioned it :)
>>
>>>  			  counters,
>>>  			  &i);
>>>  
>>>  	for_each_possible_cpu(cpu) {
>>> +		void *p;
>>> +
>>>  		if (cpu == curcpu)
>>>  			continue;
>>> +
>>> +		/* Swizzle the values and wait */
>>> +		e->counters = ((struct xt_counters) { 0, 0 });
>> I dont see what you want to do here...
>>
>> e->counters is the counter associated with rule #0
>>
>>> +		p = t->entries[cpu];
>>> +		rcu_assign_pointer(t->entries[cpu], e);
>>> +		synchronize_net();
>>
>> Oh well, not this synchronize_net() :)
>>
>> This wont provide atomic sampling of counters for whole CPUS, and introduce large delays
>> on big machines (NR_CPUS >= 64)
> 
> Why would this not provide the moral equivalent of atomic sampling?
> The code above switches to another counter set, and waits for a grace
> period.  Shouldn't this mean that all CPUs that were incrementing the
> old set of counters have finished doing so, so that the aggregate count
> covers all CPUs that started their increments before the pointer switch?
> Same as acquiring a write lock, which would wait for all CPUs that
> started their increments before starting the write-lock acquisition.
> CPUs that started their increments after starting the write acquisition
> would not be accounted for in the total, same as the RCU approach.
> 
> Steve's approach does delay reading out the counters, but it avoids
> delaying any CPU trying to increment the counters.

I see your point, but this is not what Stephen implemented.

So.. CPU will increments which counters, if not delayed ?

How counters will be synced again after our 'iptables -L' finished ?

"iptable -L" is not supposed to miss some counters updates (only some packets
 might be droped at NIC level because we spend time in the collection).
If packets matches some rules, we really want up2date counters.

Maybe we need for this collection an extra "cpu", to collect 
all increments that were done when CPUs where directed to a 
"secondary table/counters"

> 
> So what am I missing here?
> 

Well, I saw one synchronize_net() inside the for_each_possible_cpu(cpu) loop.
Say we have NR_CPUS=4096, how long will it takes to perform "iptables -L" ?

General/intuitive idea would be :

switch pointers to a newly allocated table (and zeroed counters)
wait one RCU grace period
collect/sum all counters of "old" table + (all cpus) into user provided table
restore previous table
wait one RCU grace period
disable_bh()
collect/sum all counters of "new and temporary" table (all cpus) and
reinject them into local cpu table (this cpu should not be interrupted)
enable_bh()

This way, "iptables -L" is not too expensive and doesnt block packet processing at all.

Thanks Paul