Subject: Re: [PATCH] tun: use netif_receive_skb instead of netif_rx_ni

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 11 Feb 2014 17:47:02 +0200

On Tue, Feb 11, 2014 at 07:24:04AM -0800, Eric Dumazet wrote:
> On Tue, 2014-02-11 at 22:25 +0800, Qin Chuanyu wrote:
> > we could xmit directly instead of going through softirq to gain 
> > throughput and lantency improved.
> > test model: VM-Host-Host just do transmit. with vhost thread and nic
> > interrupt bind cpu1. netperf do throuhput test and qperf do lantency test.
> > Host OS: suse11sp3, Guest OS: suse11sp3
> > 
> > latency result(us):
> > packet_len 64 256 512 1460
> > old(UDP)   44  47  48   66
> > new(UDP)   38  41  42   66
> > 
> > old(TCP)   52  55  70  117
> > new(TCP)   45  48  61  114
> > 
> > throughput result(Gbit/s):
> > packet_len   64   512   1024   1460
> > old(UDP)   0.42  2.02   3.75   4.68
> > new(UDP)   0.45  2.14   3.77   5.06
> > 
> > TCP due to the latency, client couldn't send packet big enough
> > to get benefit from TSO of nic, so the result show it will send
> > more packet per sencond but get lower throughput.
> > 
> > Eric mentioned that it would has problem with cgroup, but the patch
> > had been sent by Herbert Xu.
> > patch_id f845172531fb7410c7fb7780b1a6e51ee6df7d52
> > 
> > Signed-off-by: Chuanyu Qin <[email-masked]>
> > ---
> >   drivers/net/tun.c |    4 +++-
> >   1 files changed, 3 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index 44c4db8..90b4e58 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1184,7 +1184,9 @@ static ssize_t tun_get_user(struct tun_struct 
> > *tun, struct tun_file *tfile,
> >   	skb_probe_transport_header(skb, 0);
> > 
> >   	rxhash = skb_get_hash(skb);
> > -	netif_rx_ni(skb);
> > +	rcu_read_lock_bh();
> > +	netif_receive_skb(skb);
> > +	rcu_read_unlock_bh();
> > 
> >   	tun->dev->stats.rx_packets++;
> >   	tun->dev->stats.rx_bytes += len;
> 
> I already said this patch is not good :
> 
> rcu_read_lock_bh() makes no sense here.
> 
> What is really needed is local_bh_disable();
> 
> Herbert patch ( http://patchwork.ozlabs.org/patch/52963/ ) had a much
> cleaner form.
> 
> Just use it, CC him, credit him, please ?
> 

But not before making sure (testing) that patch does not break the
cgroups classifier please.

-- 
MST