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

[Ingo Molnar <mingo@kernel.org>]

* 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