netdev - Re: [PATCH net-next v2 03/10] tun: switch to net core provided statistics counters

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Message-ID: <20201106081846.27212e9e@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Fri, 6 Nov 2020 08:18:46 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     Willem de Bruijn <willemb@...gle.com>,
        David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Harald Welte <laforge@...monks.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        osmocom-net-gprs@...ts.osmocom.org, wireguard@...ts.zx2c4.com,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH net-next v2 03/10] tun: switch to net core provided
 statistics counters

On Fri, 6 Nov 2020 09:27:45 +0100 Heiner Kallweit wrote:
> On 06.11.2020 08:48, Heiner Kallweit wrote:
> > On 06.11.2020 02:14, Jakub Kicinski wrote:  
> >> On Wed, 4 Nov 2020 15:25:24 +0100 Heiner Kallweit wrote:  
> >>> @@ -1066,7 +1054,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
> >>>  	return NETDEV_TX_OK;
> >>>  
> >>>  drop:
> >>> -	this_cpu_inc(tun->pcpu_stats->tx_dropped);
> >>> +	dev->stats.tx_dropped++;
> >>>  	skb_tx_error(skb);
> >>>  	kfree_skb(skb);
> >>>  	rcu_read_unlock();  
> >>
> >> This is no longer atomic. Multiple CPUs may try to update it at the
> >> same time.
> >>
> >> Do you know what the story on dev->rx_dropped is? The kdoc says drivers
> >> are not supposed to use it but:
> >>
> >> drivers/net/ipvlan/ipvlan_core.c:               atomic_long_inc(&skb->dev->rx_dropped);
> >> drivers/net/macvlan.c:  atomic_long_inc(&skb->dev->rx_dropped);
> >> drivers/net/vxlan.c:            atomic_long_inc(&vxlan->dev->rx_dropped);
> >>
> >> Maybe tun can use it, too?
> >>  
> > Thanks, yes that should be possible. Here we speak about tx_dropped,
> > but AFAICS the same applies as for rx_dropped. Will change it accordingly
> > in a v3.
> >   
> For rx_dropped and tx_dropped it's easy, however tun also has a per-cpu
> counter for rx_frame_errors that is incremented if virtio_net_hdr_to_skb()
> fails. Not sure how to deal best with this one.

Umpf, yeah, so I'd probably add an atomic long to struct tun_struct,
but then you'll need to keep the ndo implementation instead of using
dev_get_tstats64 directly. I can't think of a better way, tho.