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

Ingo Molnar
2015-02-18T18:49:43Z

* 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.

Thanks,

Ingo