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Date: Mon, 26 Apr 2010 10:24:15 +0800 From: Herbert Xu <herbert@...dor.apana.org.au> To: "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org Subject: RPS and forwarding Hi: I'm sorry I didn't have time to jump into the RPS discussions earlier, so in a way I'm just getting what I deserved :) Anyway, I am specifically concerned about the possibility of reordering of forwarded traffic. As RPS is doing fuzzy matching, it is possible (and quite likely if rps_sock_flow_table is small) for a forwarded flow to be hashed to the same index as a local flow. In that case we may end up redirecting a forwarded flow. That in itself is undesirable because for forwarded flows the best solution is to stay on the ingress CPU. What's worse is that if the local flow bounces around different CPUs, the forwarded flow will follow it. For a local flow RPS can guarantee original ordering (assuming we're not doing anything weird like netfilter queueing), but this doesn't work for forwarded flows. Even if netif_receive_skb has completed, the forwarded packet may still be sitting in a hardware TX queue, selected based on the processing CPU. If you then bounce the forwarded flow then packets may be placed in a different hardware TX queue, causing reordering. BTW, selecting hardware TX queues for forwarded flows based on the rxhash is not a good solution, as that causes cache-line bouncing between CPUs. Apart from not using RPS on routers, I suppose people doing forwarding will simply have to maintain a constant RPS table, and forgo its local redirection capabilities. Cheers, -- Visit Openswan at http://www.openswan.org/ Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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