netdev - Re: Performance regression on kernels 3.10 and newer

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Date:	Sat, 06 Sep 2014 08:46:24 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	therbert@...gle.com, alexander.h.duyck@...el.com,
	netdev@...r.kernel.org
Subject: Re: Performance regression on kernels 3.10 and newer

On Sat, 2014-09-06 at 08:27 -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> Alexander Duyck reported high false sharing on dst refcount in tcp stack
> when prequeue is used. prequeue is the mechanism used when a thread is
> blocked in recvmsg()/read() on the TCP socket.
> 
> We already try to use RCU in input path as much as possible, but we were
> forced to take a refcount on the dst when skb escaped RCU protected
> region. When/if the user thread runs on different cpu, dst_release()
> will then touch dst refcount again.
> 
> Commit 093162553c33 (tcp: force a dst refcount when prequeue packet)
> was an example of a race fix.
> 
> It turns out the only remaining usage of skb->dst for a packet stored
> in a TCP socket prequeue is IP early demux.
> 
> We can add a logic to detect when IP early demux is probably going
> to use skb->dst.
> 
> In IPv4 stack, we also can drop dst when skb has to be queued into
> socket backlog. IPv6 needs extra care before doing the same.
> 
> Many thanks to Alexander for providing a nice bug report, git bisection,
> and reproducer.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Alexander Duyck <alexander.h.duyck@...el.com>

Works fine on IPv4, not on IPv6, I will submit a v2 (with a proper title
btw)



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