netdev - Re: [RFC PATCH] openvswitch: use percpu flow stats

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Date:   Fri, 19 Aug 2016 18:09:53 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     cascardo@...hat.com, dev@...nvswitch.org, netdev@...r.kernel.org
Subject: Re: [RFC PATCH] openvswitch: use percpu flow stats

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 19 Aug 2016 12:56:56 -0700

> On Fri, 2016-08-19 at 16:47 -0300, Thadeu Lima de Souza Cascardo wrote:
>> Instead of using flow stats per NUMA node, use it per CPU. When using
>> megaflows, the stats lock can be a bottleneck in scalability.
> 
> ...
> 
>>  
>>  	flow_cache = kmem_cache_create("sw_flow", sizeof(struct sw_flow)
>> -				       + (nr_node_ids
>> +				       + (num_possible_cpus()
>>  					  * sizeof(struct flow_stats *)),
>>  				       0, 0, NULL);
>>  	if (flow_cache == NULL)
> 
> This is bogus.
> 
> Please use nr_cpu_ids instead of num_possible_cpus().

Then how would hot-plugged cpus be handled properly?

The only other way is you'd have to free all objects in the current
flow_cache, destroy it, then make a new kmem_cache and reallocate
all of the existing objects and hook them back up every time a cpu
up event occurs.

That doesn't make any sense at all.

Therefore, the size of the objects coming out of "sw_flow" have
to accomodate all possible cpu indexes.