netdev - Re: [PATCH v2 2/2] openvswitch: use percpu flow stats

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Date:   Thu, 15 Sep 2016 20:36:43 -0300
From:   Thadeu Lima de Souza Cascardo <cascardo@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, dev@...nvswitch.org, pshelar@....org,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH v2 2/2] openvswitch: use percpu flow stats

On Thu, Sep 15, 2016 at 04:09:26PM -0700, Eric Dumazet wrote:
> On Thu, 2016-09-15 at 19:11 -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.
> > 
> > On a E5-2690 12-core system, usual throughput went from ~4Mpps to
> > ~15Mpps when forwarding between two 40GbE ports with a single flow
> > configured on the datapath.
> > 
> > This has been tested on a system with possible CPUs 0-7,16-23. After
> > module removal, there were no corruption on the slab cache.
> > 
> > Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...hat.com>
> > Cc: pravin shelar <pshelar@....org>
> > ---
> 
> > +	/* We open code this to make sure cpu 0 is always considered */
> > +	for (cpu = 0; cpu < nr_cpu_ids; cpu = cpumask_next(cpu, cpu_possible_mask))
> > +		if (flow->stats[cpu])
> >  			kmem_cache_free(flow_stats_cache,
> > -					(struct flow_stats __force *)flow->stats[node]);
> > +					(struct flow_stats __force *)flow->stats[cpu]);
> >  	kmem_cache_free(flow_cache, flow);
> >  }
> >  
> > @@ -757,7 +749,7 @@ int ovs_flow_init(void)
> >  	BUILD_BUG_ON(sizeof(struct sw_flow_key) % sizeof(long));
> >  
> >  	flow_cache = kmem_cache_create("sw_flow", sizeof(struct sw_flow)
> > -				       + (nr_node_ids
> > +				       + (nr_cpu_ids
> >  					  * sizeof(struct flow_stats *)),
> >  				       0, 0, NULL);
> >  	if (flow_cache == NULL)
> 
> Well, if you switch to percpu stats, better use normal
> alloc_percpu(struct flow_stats)
> 
> The code was dealing with per node allocation so could not use existing
> helper.
> 
> No need to keep this forever.

The problem is that the alloc_percpu uses a global spinlock and that affects
some workloads on OVS that creates lots of flows, as described in commit
9ac56358dec1a5aa7f4275a42971f55fad1f7f35 ("datapath: Per NUMA node flow
stats.").

This problem would not happen on this version as the flow allocation does not
suffer from the same scalability problem as when using alloc_percpu.

Cascardo.