Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1304319

Re: [PATCH] net: add per device sg_max_frags for skb

Show all headers | View raw

On 01/06/2016 02:59 PM, David Laight wrote:
> From: Hans Westgaard Ry
>> Sent: 06 January 2016 13:16
>> Devices may have limits on the number of fragments in an skb they
>> support. Current codebase uses a constant as maximum for number of
>> fragments (MAX_SKB_FRAGS) one skb can hold and use.
>>
>> When enabling scatter/gather and running traffic with many small
>> messages the codebase uses the maximum number of fragments and thereby
>> violates the max for certain devices.
>>
>> An example of such a violation is when running IPoIB on a HCA
>> supporting 16 SGE on an architecture with 4K pagesize. The
>> MAX_SKB_FRAGS will be 17 (64K/4K+1) and because IPoIB adds yet another
>> segment we end up with send_requests with 18 SGE resulting in
>> kernel-panic.
>>
>> The patch allows the device to limit the maximum number fragments used
>> in one skb.
> This doesn't seem to me to be the correct way to fix this.
> Anything that adds an extra fragment (in this case IPoIB) should allow
> for the skb already having the maximum number of fragments.
> Fully linearising the skb is overkill, but I think the first fragment
> can be added to the linear part of the skb.
>
> 	David
>
>
When IpoIB handles a skb-request it converts fragments to SGEs to
be handled by a HCA.
The problem arises when the HCA have a limited number of SGEs less than 
MAX_SKB_FRAGS.
(it gets a little worse since IPoIB need to yet another segment)
I have not found any easy way of fixing this with currenct codebase.

Hans

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[PATCH] net: add per device sg_max_frags for skb Hans Westgaard Ry <hans.westgaard.ry@oracle.com> - 2016-01-06 14:20 +0100
  RE: [PATCH] net: add per device sg_max_frags for skb David Laight <David.Laight@ACULAB.COM> - 2016-01-06 15:10 +0100
    Re: [PATCH] net: add per device sg_max_frags for skb Hans Westgaard Ry <hans.westgaard.ry@oracle.com> - 2016-01-08 11:00 +0100
      RE: [PATCH] net: add per device sg_max_frags for skb David Laight <David.Laight@ACULAB.COM> - 2016-01-08 11:40 +0100
      Re: [PATCH] net: add per device sg_max_frags for skb Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-01-08 12:50 +0100
        Re: [PATCH] net: add per device sg_max_frags for skb Hans Westgaard Ry <hans.westgaard.ry@oracle.com> - 2016-01-13 15:00 +0100
          Re: [PATCH] net: add per device sg_max_frags for skb Eric Dumazet <edumazet@google.com> - 2016-01-13 15:30 +0100
          Re: [PATCH] net: add per device sg_max_frags for skb Eric Dumazet <edumazet@google.com> - 2016-01-13 15:30 +0100
            Re: [PATCH] net: add per device sg_max_frags for skb Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-01-13 16:10 +0100
            Re: [PATCH] net: add per device sg_max_frags for skb David Miller <davem@davemloft.net> - 2016-01-13 16:40 +0100
              Re: [PATCH] net: add per device sg_max_frags for skb Eric Dumazet <edumazet@google.com> - 2016-01-13 16:50 +0100
          Re: [PATCH] net: add per device sg_max_frags for skb ebiederm@xmission.com (Eric W. Biederman) - 2016-01-13 22:20 +0100
  Re: [PATCH] net: add per device sg_max_frags for skb Eric Dumazet <eric.dumazet@gmail.com> - 2016-01-06 15:10 +0100
    Re: [PATCH] net: add per device sg_max_frags for skb Hans Westgaard Ry <hans.westgaard.ry@oracle.com> - 2016-01-08 11:10 +0100

csiph-web