Re: [PATCH RFC v3 0/7] epoll: Introduce new syscalls, epoll_ctl_batch and epoll_pwait1

Fam Zheng
2015-02-25T03:31:22Z

On Wed, 02/18 19:49, Ingo Molnar wrote:
>
> * Fam Zheng <famz@redhat.com> wrote:
>
> > On Sun, 02/15 15:00, Jonathan Corbet wrote:
> > > On Fri, 13 Feb 2015 17:03:56 +0800
> > > Fam Zheng <famz@redhat.com> wrote:
> > >
> > > > SYNOPSIS
> > > >
> > > > #include <sys/epoll.h>
> > > >
> > > > int epoll_pwait1(int epfd, int flags,
> > > > struct epoll_event *events,
> > > > int maxevents,
> > > > struct epoll_wait_params *params);
> > >
> > > Quick, possibly dumb question: might it make sense to also pass in
> > > sizeof(struct epoll_wait_params)? That way, when somebody wants to add
> > > another parameter in the future, the kernel can tell which version is in
> > > use and they won't have to do an epoll_pwait2()?
> > >
> >
> > Flags can be used for that, if the change is not
> > radically different.
>
> Passing in size is generally better than flags, because
> that way an extension of the ABI (new field[s])
> automatically signals towards the kernel what to do with
> old binaries - while extending the functionality of new
> binaries, without sacrificing functionality.
>
> With flags you are either limited to the same structure
> size - or have to decode a 'size' value from the flags
> value - which is fragile (and in which case a real 'size'
> parameter is better).
>
> in the perf ABI we use something like that: there's a
> perf_attr.size parameter that iterates the ABI forward,
> while still being binary compatible with older software.
>
> If old binaries pass in a smaller structure to a newer
> kernel then the kernel pads the new fields with zero by
> default - that way the kernel internals are never burdened
> with compatibility details and data format versions.
>
> If new user-space passes in a large structure than the
> kernel can handle then the kernel returns an error - this
> way user-space can transparently support conditional
> features and fallback logic.
>
> It works really well, we've done literally a hundred perf
> ABI extensions this way in the last 4+ years, in a pretty
> natural fashion, without littering the kernel (or
> user-space) with version legacies and without breaking
> existing perf tooling.
>
> Other syscall ABIs already get painful when trying to
> handle 2-3 data structure versions, so people either give
> up, or add flags kludges or go to new syscall entries:
> which is painful in its own fashion and adds unnecessary
> latency to feature introduction as well.
>

Excellent. This now makes a lot of sense to me, thanks to your explanations,
Ingo.

I'll add the "size" field in the next revision.

Thanks,
Fam