netdev - RE: [net 2/2] ixgbe: Limit use of 2K buffers on architectures with 256B or larger cache lines

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Date:   Fri, 3 Mar 2017 15:55:20 +0000
From:   "Duyck, Alexander H" <alexander.h.duyck@...el.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        "jogreene@...hat.com" <jogreene@...hat.com>
Subject: RE: [net 2/2] ixgbe: Limit use of 2K buffers on architectures with
 256B or larger cache lines

> -----Original Message-----
> From: David Laight [mailto:David.Laight@...LAB.COM]
> Sent: Friday, March 3, 2017 4:25 AM
> To: Kirsher, Jeffrey T <jeffrey.t.kirsher@...el.com>; davem@...emloft.net
> Cc: Duyck, Alexander H <alexander.h.duyck@...el.com>;
> netdev@...r.kernel.org; nhorman@...hat.com; sassmann@...hat.com;
> jogreene@...hat.com
> Subject: RE: [net 2/2] ixgbe: Limit use of 2K buffers on architectures with 256B
> or larger cache lines
> 
> From: Jeff Kirsher
> > Sent: 03 March 2017 02:25
> > From: Alexander Duyck <alexander.h.duyck@...el.com>
> >
> > On architectures that have a cache line size larger than 64 Bytes we
> > start running into issues where the amount of headroom for the frame
> > starts shrinking.
> >
> > The size of skb_shared_info on a system with a 64B L1 cache line size
> > is 320.  This increases to 384 with a 128B cache line, and 512 with a
> > 256B cache line.
> 
> Perhaps some of the CACHE_LINE_ALIGNED markers don't actually need to
> force alignment with large line sizes?
> 
> I realise some things have hard requirements for cache alignment (eg non-
> coherent dma), but others are just there to limit the number of cache lines read
> and/or dirtied.
> 
> 	David

For our purposes I think this works well enough.  Basically we wanted to guarantee we have enough headroom for XDP.  In the case of the Mellanox drivers they are guaranteeing 256 if I recall correctly.

I have some follow-up patches for net-next that will make it so that we can just do a build-time test that will determine the padding size and allow us to always guaranteed at least NET_SKB_PAD + NET_IP_ALIGN.

- Alex