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

Hans Westgaard Ry [off-list ref] writes:

On 01/08/2016 12:47 PM, Hannes Frederic Sowa wrote:

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On 08.01.2016 10:55, Hans Westgaard Ry wrote:

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On 01/06/2016 02:59 PM, David Laight wrote:

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From: Hans Westgaard Ry

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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.

I think because of the complex forwarding nature, a global counter which
driver's can reduce during initialization time is the only solution I see
right now without changing the layout of the skb later on.

Unfortunately this doesn't resolve the cases were virtual machines inject gso
skbs, for those there still needs to be a slow path to do the reformatting of
the skb. :/

Bye,
Hannes

The use-case for this patch is an application which sends many small messages,
by write(2) on a TCP socket which has Nagle enabled. A scatter-gather capable
NIC (potentially also supporting tso) will then be asked to send an skb
containing up to MAX_SKB_FRAGS worth of fragments (17 considering a 4kb page
size, hypothetically 65 considering an arch supporting 1kb page size).

Now, if the NIC hardware supports less _gather-fragments_, said hardware must
run with scatter-gather disabled - or - the NIC driver has to implement a
partial linearization of the skb to reduce #frags to what the hardware
supports. The latter is far from elegant, and must be implemented in all NIC
drivers which have this restriction.

This patch provides the flexibility to choose the maximum number of fragments
that can be passed down to the NIC in order to
utilize the NIC SG hardware features.


In our view we are discussing two different issues:

   1. Is it reasonable that a NIC can restrict #frags in an skb when
transmitting?
   2. If yes to the above, how is this implemented the best possible way.

Thanks a lot for feedback on the implementation from David Laight, Eric Dumazet
and Hannes Fredreric Sowa.

What do you think?

*Scratches my head*  Why doesn't someone fix the infiniband firmware so
that it supports more scatter gather entries?

Last I looked everything like this in infiniband was all implemented in
firmware and there is only one vendor to pick on, so it should be
comparatively easy to just fix the hardware so it does not have this
limitation.

Eric