netdev - Re: Performance regression on kernels 3.10 and newer

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Message-ID: <CA+mtBx-c0HCAES9PCyGn7cChZsX0KaLgdziFL_e1iRmkdG0dsQ@mail.gmail.com>
Date:	Thu, 14 Aug 2014 16:25:53 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: Performance regression on kernels 3.10 and newer

On Thu, Aug 14, 2014 at 4:20 PM, David Miller <davem@...emloft.net> wrote:
> From: Alexander Duyck <alexander.h.duyck@...el.com>
> Date: Thu, 14 Aug 2014 16:16:36 -0700
>
>> Are you sure about each socket having it's own DST?  Everything I see
>> seems to indicate it is somehow associated with IP.
>
> Right it should be, unless you have exception entries created by path
> MTU or redirects.
>
> WRT prequeue, it does the right thing for dumb apps that block in
> receive.  But because it causes the packet to cross domains as it
> does, we can't do a lot of tricks which we normally can do, and that's
> why the refcounting on the dst is there now.
>
> Perhaps we can find a clever way to elide that refcount, who knows.

I don't know if it's the same problem, but I did post a patch back in
January that would resolve false sharing of dst->__refcnt and rt_genid
in the same cacheline. We could revisit that if it helps.

Tom

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