netdev - Re: [PATCH v2] Receive Packet Steering

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Date:	Mon, 04 May 2009 09:08:00 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Tom Herbert <therbert@...gle.com>
CC:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [PATCH v2] Receive Packet Steering

Tom Herbert a écrit :
> This is an update of the receive packet steering patch (RPS) based on received
> comments (thanks for all the comments).  Improvements are:
> 
> 1) Removed config option for the feature.
> 2) Made scheduling of backlog NAPI devices between CPUs lockless and much
> simpler.
> 3) Added new softirq to do defer sending IPIs for coalescing.
> 4) Imported hash from simple_rx_hash.  Eliminates modulo operation to convert
> hash to index.
> 5) If no cpu is found for packet steering, then netif_receive_skb processes
> packet inline as before without queueing.  In paritcular if RPS is not
> configured on a device the receive path is unchanged from current for
> NAPI devices (one additional conditional).
> 

Thinking again about this Tom, I also think something might/should be done
for the TX completion path. As spotted by earlier patches last weeks,
TX completion on TCP/UDP can do a lot of stuff, including wakeups (that
can be really expensive), and touching many socket cache lines.

Each cpu can call device transmit on its own, but TX completion path is taken
by one cpu only, and this can use all this cpu cycles.

(It would make sense that skb freeing is done by the same cpu that did the alloc,
especially on NUMA setups)

As this TX completion path should use same hash than one selected for RX (as
done in your patch), maybe this is something you could add, if we know
current cpu is saturated by softirq handling.

One comment about simple_hashrnd check/initialization in get_rps_cpu()

+	if (unlikely(!simple_hashrnd_initialized)) {
+		get_random_bytes(&simple_hashrnd, 4);
+		simple_hashrnd_initialized = 1;
+	}
+

You could move it to rps_map_len/rps_map initialization in store_rps_cpus()
to save a few cycles per packet.

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