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Date: Wed, 14 Sep 2022 22:10:43 +0000 From: Vladimir Oltean <vladimir.oltean@....com> To: Vinicius Costa Gomes <vinicius.gomes@...el.com> CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Xiaoliang Yang <xiaoliang.yang_1@....com>, Rui Sousa <rui.sousa@....com>, Claudiu Manoil <claudiu.manoil@....com>, Alexandre Belloni <alexandre.belloni@...tlin.com>, "UNGLinuxDriver@...rochip.com" <UNGLinuxDriver@...rochip.com>, Andrew Lunn <andrew@...n.ch>, Vivien Didelot <vivien.didelot@...il.com>, Florian Fainelli <f.fainelli@...il.com>, Michael Walle <michael@...le.cc>, Maxim Kochetkov <fido_max@...ox.ru>, Colin Foster <colin.foster@...advantage.com>, Richie Pearn <richard.pearn@....com>, Kurt Kanzenbach <kurt@...utronix.de>, Vladimir Oltean <olteanv@...il.com>, Jesse Brandeburg <jesse.brandeburg@...el.com>, Tony Nguyen <anthony.l.nguyen@...el.com>, Giuseppe Cavallaro <peppe.cavallaro@...com>, Alexandre Torgue <alexandre.torgue@...s.st.com>, Jose Abreu <joabreu@...opsys.com>, Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, Gerhard Engleder <gerhard@...leder-embedded.com>, Grygorii Strashko <grygorii.strashko@...com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH net-next 04/13] net/sched: taprio: allow user input of per-tc max SDU On Wed, Sep 14, 2022 at 02:43:02PM -0700, Vinicius Costa Gomes wrote: > > @@ -416,6 +417,9 @@ static int taprio_enqueue_one(struct sk_buff *skb, struct Qdisc *sch, > > struct Qdisc *child, struct sk_buff **to_free) > > { > > struct taprio_sched *q = qdisc_priv(sch); > > + struct net_device *dev = qdisc_dev(sch); > > + int prio = skb->priority; > > + u8 tc; > > > > /* sk_flags are only safe to use on full sockets. */ > > if (skb->sk && sk_fullsock(skb->sk) && sock_flag(skb->sk, SOCK_TXTIME)) { > > @@ -427,6 +431,12 @@ static int taprio_enqueue_one(struct sk_buff *skb, struct Qdisc *sch, > > return qdisc_drop(skb, sch, to_free); > > } > > > > + /* Devices with full offload are expected to honor this in hardware */ > > + tc = netdev_get_prio_tc_map(dev, prio); > > + if (q->max_sdu[tc] && > > + q->max_sdu[tc] < max_t(int, 0, skb->len - skb_mac_header_len(skb))) > > + return qdisc_drop(skb, sch, to_free); > > + > > One minor idea, perhaps if you initialize q->max_sdu[] with a value that > you could use to compare here (2^32 - 1), this comparison could be > simplified. The issue is that that value would become invalid for a > maximum SDU, not a problem for ethernet. Could do (and the fact that U32_MAX becomes a reserved value shouldn't be a problem for any linklayer), but if I optimize the code for this one place, I need, in turn, to increase the complexity in the netlink dump and in the offload procedures, to hide what I've done. If I look at the difference in generated code, maybe it's worth it (I get rid of a "cbz" instruction). Maybe it's worth simply creating a shadow array of q->max_sdu[], but which is also adjusted for something like dev->hard_header_len (also a fast path invariant)? This way, we could only check for q->max_frm_len[tc] > skb->len and save even more checks in the fast path.
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