netdev - Re: [RFC PATCH] reuseport: compute the ehash only if needed

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Date:   Wed, 13 Dec 2017 15:08:55 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     pabeni@...hat.com
Cc:     netdev@...r.kernel.org, kraig@...gle.com, edumazet@...gle.com
Subject: Re: [RFC PATCH] reuseport: compute the ehash only if needed

From: Paolo Abeni <pabeni@...hat.com>
Date: Tue, 12 Dec 2017 14:09:28 +0100

> When a reuseport socket group is using a BPF filter to distribute
> the packets among the sockets, we don't need to compute any hash
> value, but the current reuseport_select_sock() requires the
> caller to compute such hash in advance.
> 
> This patch reworks reuseport_select_sock() to compute the hash value
> only if needed - missing or failing BPF filter. Since different
> hash functions have different argument types - ipv4 addresses vs ipv6
> ones - to avoid over-complicate the interface, reuseport_select_sock()
> is now a macro.
> 
> Additionally, the sk_reuseport test is move inside reuseport_select_sock,
> to avoid some code duplication.
> 
> Overall this gives small but measurable performance improvement
> under UDP flood while using SO_REUSEPORT + BPF.
> 
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>

I don't doubt that this improves the case where the hash is elided, but
I suspect it makes things slower othewise.

You're doing two function calls for an operation that used to require
just one in the bottom of the call chain.

You're also putting something onto the stack that the compiler can't
possibly optimize into purely using cpu registers to hold.