linux-kernel - RE: [PATCHSET v3 0/5] Add support for epoll min

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Message-ID: <a31ac5f723f44deab35b93c31263f5b5@AcuMS.aculab.com>
Date:   Mon, 7 Nov 2022 10:10:45 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jens Axboe' <axboe@...nel.dk>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCHSET v3 0/5] Add support for epoll min_wait

From: Jens Axboe
> Sent: 05 November 2022 17:39
> 
> >> FWIW, when adding nsec resolution I initially opted for an init-based
> >> approach, passing a new flag to epoll_create1. Feedback then was that
> >> it was odd to have one syscall affect the behavior of another. The
> >> final version just added a new epoll_pwait2 with timespec.
> >
> > I'm fine with just doing a pure syscall variant too, it was my original
> > plan. Only changed it to allow for easier experimentation and adoption,
> > and based on the fact that most use cases would likely use a fixed value
> > per context anyway.
> >
> > I think it'd be a shame to drop the ctl, unless there's strong arguments
> > against it. I'm quite happy to add a syscall variant too, that's not a
> > big deal and would be a minor addition. Patch 6 should probably cut out
> > the ctl addition and leave that for a patch 7, and then a patch 8 for
> > adding a syscall.
>
> I split the ctl patch out from the core change, and then took a look at
> doing a syscall variant too. But there are a few complications there...
> It would seem to make the most sense to build this on top of the newest
> epoll wait syscall, epoll_pwait2(). But we're already at the max number
> of arguments there...
> 
> Arguably pwait2 should've been converted to use some kind of versioned
> struct instead. I'm going to take a stab at pwait3 with that kind of
> interface.

Adding an extra copy_from_user() adds a measurable overhead
to a system call - so you really don't want to do it unless
absolutely necessary.

I was wondering if you actually need two timeout parameters?
Could you just use a single bit (I presume one is available)
to request that the timeout be restarted when he first message
arrives and the syscall then return when either the timer
expires or the full number of events has been returned.

	David

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