Re: [PATCH 06/18] wifi: iwlwifi: keep the TSO and workaround pages mapped

["Berg, Benjamin" <benjamin.berg@intel.com>]

Hi,

have you applied all patches? It'll be slightly slower until you apply
the last patch in the series (and break completely unless you have the
bugfix obviously).

A slowdown to 80MBit/s seems a bit like we are getting packet loss and
the AMSDU is simply not send out. I suppose, that could happen if some
of the DMA mapping is failing, but I really do not have a good idea
right now.

I do think that these patches have gone through quite some verification
and were working well for us.


As for the value of the patchset, the thing to keep in mind is that we
send out large SKBs that are spanning multiple pages. Before the patch,
we would do mapping as below, where h1, … and m1, … are each mappings
that only span a small part of a page (h_n is the header for the nth
packet while m_n is the corresponding message data):

TSO page: |- h1 -|- h2 -|- h3 -|- h4 -| …
SKB page 1: |- m1     -|- m2      -|
SKB page 2: |- m2 -|-  m3  -|- m4 -|
SKB page 3: |- m4 -|- …

With the patches applied, each of the pages will be mapped in a single
chunk. This brings down the number of DMA mappings and also avoids
partial mappings which can very be particularly expensive depending on
the IOMMU configuration.

Only the last patch in the series removes the old mappings (obviously
you also need the later bugfix commit). So you might see a slight
slowdown if the patch "wifi: iwlwifi: use already mapped data when
TXing an AMSDU" is not applied.

But not a slowdown to 80MBit/s …

Benjamin

On Thu, 2024-07-18 at 12:20 -0700, Ben Greear wrote:
> On 7/11/24 09:09, Ben Greear wrote:
> > On 7/10/24 23:15, Berg, Benjamin wrote:
> > > Hi Ben,
> > > 
> > > yes, you need to apply:
> > > 
> > > commit 003eae5a28c6c9d50290a4ac9b955be912f24c9f
> > > Author: Benjamin Berg <benjamin.berg@intel.com>
> > > Date:   Tue Jul 9 14:31:49 2024 +0200
> > > 
> > >      wifi: iwlwifi: correctly reference TSO page information
> > > 
> > > 
> > > I had not fully tested the last revision and the error slipped
> > > unfortunately.
> > 
> > Hello Benjamin,
> > 
> > Sorry I did not notice that patch on the mailing list on my own.  I
> > re-applied
> > the 6/18 and 7/18 patches, and the fix you mention above, and
> > system appears stable.
> 
> Hello,
> 
> We found another regression in our patched 6.10-ish kernel.  Before I
> apply these 3
> patches:
> 
> wifi: iwlwifi: keep the TSO and workaround pages mapped
> wifi: iwlwifi: use already mapped data when TXing an AMSDU
> wifi: iwlwifi: correctly reference TSO page information
> 
> Then I see around 4Gbps TCP upload on my test rig.  After this, it
> runs very poorly,
> perhaps bouncing up to high speed for a second or two, but mostly
> averaging 80Mbps
> or so after it runs for a bit.
> 
> What are these patches trying to solve, and are you able to see good
> TCP upload performance
> with these patches applied?
> 
> Thanks,
> Ben
> 

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