Date: Tue, 30 Jan 2024 15:33:33 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Joe Damato <jdamato@...tly.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
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Subject: Re: [PATCH net-next v3 0/3] Per epoll context busy poll support

Joe Damato wrote:
> On Sat, Jan 27, 2024 at 11:20:51AM -0500, Willem de Bruijn wrote:
> > Joe Damato wrote:
> > > Greetings:
> > > 
> > > Welcome to v3. Cover letter updated from v2 to explain why ioctl and
> > > adjusted my cc_cmd to try to get the correct people in addition to folks
> > > who were added in v1 & v2. Labeled as net-next because it seems networking
> > > related to me even though it is fs code.
> > > 
> > > TL;DR This builds on commit bf3b9f6372c4 ("epoll: Add busy poll support to
> > > epoll with socket fds.") by allowing user applications to enable
> > > epoll-based busy polling and set a busy poll packet budget on a per epoll
> > > context basis.
> > > 
> > > This makes epoll-based busy polling much more usable for user
> > > applications than the current system-wide sysctl and hardcoded budget.
> > > 
> > > To allow for this, two ioctls have been added for epoll contexts for
> > > getting and setting a new struct, struct epoll_params.
> > > 
> > > ioctl was chosen vs a new syscall after reviewing a suggestion by Willem
> > > de Bruijn [1]. I am open to using a new syscall instead of an ioctl, but it
> > > seemed that: 
> > >   - Busy poll affects all existing epoll_wait and epoll_pwait variants in
> > >     the same way, so new verions of many syscalls might be needed. It
> > 
> > There is no need to support a new feature on legacy calls. Applications have
> > to be upgraded to the new ioctl, so they can also be upgraded to the latest
> > epoll_wait variant.
> 
> Sure, that's a fair point. I think we could probably make reasonable
> arguments in both directions about the pros/cons of each approach.
> 
> It's still not clear to me that a new syscall is the best way to go on
> this, and IMO it does not offer a clear advantage. I understand that part
> of the premise of your argument is that ioctls are not recommended, but in
> this particular case it seems like a good use case and there have been
> new ioctls added recently (at least according to git log).
> 
> This makes me think that while their use is not recommended, they can serve
> a purpose in specific use cases. To me, this use case seems very fitting.
> 
> More of a joke and I hate to mention this, but this setting is changing how
> io is done and it seems fitting that this done via an ioctl ;)
> 
> > epoll_pwait extends epoll_wait with a sigmask.
> > epoll_pwait2 extends extends epoll_pwait with nsec resolution timespec.
> > Since they are supersets, nothing is lots by limiting to the most recent API.
> > 
> > In the discussion of epoll_pwait2 the addition of a forward looking flags
> > argument was discussed, but eventually dropped. Based on the argument that
> > adding a syscall is not a big task and does not warrant preemptive code.
> > This decision did receive a suitably snarky comment from Jonathan Corbet [1].
> > 
> > It is definitely more boilerplate, but essentially it is as feasible to add an
> > epoll_pwait3 that takes an optional busy poll argument. In which case, I also
> > believe that it makes more sense to configure the behavior of the syscall
> > directly, than through another syscall and state stored in the kernel.
> 
> I definitely hear what you are saying; I think I'm still not convinced, but
> I am thinking it through.
> 
> In my mind, all of the other busy poll settings are configured by setting
> options on the sockets using various SO_* options, which modify some state
> in the kernel. The existing system-wide busy poll sysctl also does this. It
> feels strange to me to diverge from that pattern just for epoll.

I think the stateful approach for read is because there we do want
to support all variants: read, readv, recv, recvfrom, recvmsg,
recvmmsg. So there is no way to pass it directly.

That said, I don't mean to argue strenously for this API or against
yours. Want to make sure the option space is explored. There does not
seem to be much other feedback. I don't hold a strong opinion either.

> In the case of epoll_pwait2 the addition of a new syscall is an approach
> that I think makes a lot of sense. The new system call is also probably
> better from an end-user usability perspective, as well. For busy poll, I
> don't see a clear reasoning why a new system call is better, but maybe I am
> still missing something.
>
> > I don't think that the usec fine grain busy poll argument is all that useful.
> > Documentation always suggests setting it to 50us or 100us, based on limited
> > data. Main point is to set it to exceed the round-trip delay of whatever the
> > process is trying to wait on. Overestimating is not costly, as the call
> > returns as soon as the condition is met. An epoll_pwait3 flag EPOLL_BUSY_POLL
> > with default 100us might be sufficient.
> > 
> > [1] https://lwn.net/Articles/837816/
> 
> Perhaps I am misunderstanding what you are suggesting, but I am opposed to
> hardcoding a value. If it is currently configurable system-wide and via
> SO_* options for other forms of busy poll, I think it should similarly be
> configurable for epoll busy poll.
> 
> I may yet be convinced by the new syscall argument, but I don't think I'd
> agree on imposing a default. The value can be modified by other forms of
> busy poll and the goal of my changes are to:
>   - make epoll-based busy poll per context
>   - allow applications to configure (within reason) how epoll-based busy
>     poll behaves, like they can do now with the existing SO_* options for
>     other busy poll methods.

Okay. I expected some push back. Was curious if people would come back
with examples of where the full range is actually being used.

> > >     seems much simpler for users to use the correct
> > >     epoll_wait/epoll_pwait for their app and add a call to ioctl to enable
> > >     or disable busy poll as needed. This also probably means less work to
> > >     get an existing epoll app using busy poll.
> > 

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