Subject: Re: [PATCH iproute2-next v2] ss: Shorter display format for TLS zerocopy sendfile

From: Jakub Kicinski <[email-masked]>
Date: Thu, 2 Jun 2022 09:44:28 -0700

On Thu, 2 Jun 2022 12:13:53 +0300 Maxim Mikityanskiy wrote:
> I would expect to get a comment on my patch, instead of submitting your 
> own v2

I replied to v1 that the name is not okay. And you didn't go with my
suggestion. This is a trivial matter, faster for me to send my own
patch.

> and dropping my author and signed-off-by.

Sorry, you can add it now tho.

> On 2022-06-02 02:42, Jakub Kicinski wrote:
> > Commit 21c07b45688f ("ss: Show zerocopy sendfile status of TLS
> > sockets") used "key: value" format for the sendfile read-only
> > optimization. Move to a more appropriate "flag" display format.
> > Rename the flag to something based on the assumption it allows
> > the kernel to make.  
> 
> The kernel feature is exposed to the userspace as "zerocopy sendfile", 
> see the constants for setsockopt and sock_diag.
> ss should just print  whatever is exposed via sock_diag as is. IMO,
> inventing new names for it would cause confusion. Calling the feature
> by the same name everywhere looks clearer to me.

Sure, there discrepancy is a little annoying. Do you want to send 
the kernel rename patch, or should I?

> > the term "zero-copy"
> > is particularly confusing in TLS where we call decrypt/encrypt
> > directly from user space a zero-copy as well.  
> 
> I don't think "zerocopy_sendfile" is confusing. There is no second 
> zerocopy sendfile, and the zero-copy you are talking about is neither 
> related to sendfile nor exposed to the userspace, as far as I see.

What is your thinking based on? I spent the last 8 months in meetings
about TLS and I had to explain over and over that TLS zero-copy is not
zero-copy. Granted that's the SW path that's to blame as it moves data
from one place to another and still calls that zero-copy. But the term
zero-copy is tainted for all of kernel TLS at this point.

Unless we report a matrix with the number of copies per syscall I'd
prefer to avoid calling random ones zero-copy again.

> What is confusing is calling a feature not by its name, but by one of 
> its implications, and picking a name that doesn't have any references 
> elsewhere.

The sockopt is a promise from the user space to the kernel that it will
not modify the data in the file. So I'd prefer to call it sendfile_ro. 
I have a similar (in spirit) optimization I'll send out for the Rx path
which saves the SW path from making a copy when the user knows that
there will be no TLS 1.3 padding. I want to call it expect_nopad or
such, not tls13_zc or tls13_onefewercopy or IDK what.

> I believe, we are going to have more and more zerocopy features in
> the kernel, and it's OK to distinguish them by "zerocopy TLS
> sendfile", "zerocopy AF_XDP", etc. This is why my feature isn't
> called just "zerocopy".
> 
>  > > I suggest mentioning the purpose of this optimization: a huge
>  > > performance boost of up to 2.4 times compared to non-zerocopy
>  > > device offload. See the performance numbers from my commit
>  > > message:  
> 
>  > That reads like and ad to me.  
> 
> My intention was to emphasize some positive points and give the
> readers understanding why they may want to enable this feature.
> "Zero-copy behavior" sounds neutral to me, and the following
> paragraphs describe the limitations only, so I wanted to add some
> positive phrasing like "improved performance" or "reduced CPU cycles
> spent on extra copies". "Transmitting data directly to the NIC
> without making an in-kernel copy" implies these points, but it's not
> explicit. If you think it's obvious enough for the target audience,
> I'm fine with the current version.

Everyone wants zero-copy, if that's what was being declared here
we should just default it to enabled and not bother.

Developers need to read the fine print first.

>  > Avoid "salesman speak", the term "zero-copy"  
> 
> In the documentation you wrote, "true zero-copy behavior" was an
> acceptable term, and the "ad" was the performance numbers. 

I don't understand why you care about the numbers. They will be
meaningless for other platforms (AMD), other versions of your NICs, and
under real application loads. It seems like a declaration of a
performance boost which can't be accurately delivered on. Do you really
want the users to call you and say "your numbers show 40% improvement
but we only see 20%"? I'm not dead-set on excluding the numbers, I just
can't recall ever seeing numbers for a particular NIC included in the
documentation for a feature or an API.

> However, in the context of this patch, you call "zerocopy" a
> "salesman speak". What is different in this context that "zerocopy"
> became an unwanted term?

I put that sentence in there because I thought you'd appreciate it.
I can remove it if it makes my opinion look inconsistent.
Trying to be nice always backfires for me, eh.