linux-kernel - Re: [PATCH 6/6] eventpoll: add support for min-wait

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Date:   Tue, 8 Nov 2022 17:44:58 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [PATCH 6/6] eventpoll: add support for min-wait

On Tue, Nov 8, 2022 at 5:30 PM Jens Axboe <axboe@...nel.dk> wrote:
>
> On 11/8/22 3:25 PM, Willem de Bruijn wrote:
> >>> This would be similar to the approach that willemb@...gle.com used
> >>> when introducing epoll_pwait2.
> >>
> >> I have, see other replies in this thread, notably the ones with Stefan
> >> today. Happy to do that, and my current branch does split out the ctl
> >> addition from the meat of the min_wait support for this reason. Can't
> >> seem to find a great way to do it, as we'd need to move to a struct
> >> argument for this as epoll_pwait2() is already at max arguments for a
> >> syscall. Suggestions more than welcome.
> >
> > Expect an array of two timespecs as fourth argument?
>
> Unfortunately even epoll_pwait2() doesn't have any kind of flags
> argument to be able to do tricks like that... But I guess we could do
> that with epoll_pwait3(), but it'd be an extra indirection for the copy
> at that point (copy array of pointers, copy pointer if not NULL), which
> would be unfortunate. I'd hate to have to argue that API to anyone, let
> alone Linus, when pushing the series.

I did mean for a new syscall epoll_pwait3. But not an array of
pointers, an array of structs. The second arg is then mandatory for
this epoll_pwait_minwait variant of the syscall.

It would indeed have been nicer to be able to do this in epoll_pwait2
based on a flag. It's just doubling the size in copy_from_user in
get_timespec64.

Btw, when I added epoll_pwait2, there was a reasonable request to
also update the manpages and add a basic test to
tools/testing/selftests/filesystems/epoll. That is some extra work with
a syscall based approach.