linux-kernel - Re: [PATCH v2) posix-timers: make it configurable

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Date:   Wed, 14 Sep 2016 17:09:53 -0400 (EDT)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Josh Triplett <josh@...htriplett.org>
cc:     Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>,
        Richard Cochran <richardcochran@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2) posix-timers: make it configurable

On Wed, 14 Sep 2016, Josh Triplett wrote:

> On Wed, Sep 14, 2016 at 04:46:54PM -0400, Nicolas Pitre wrote:
> > Many embedded systems typically don't need them.  This removes about
> > 22KB from the kernel binary size on ARM when configured out.
> > 
> > Corresponding syscalls are routed to a stub logging the attempt to
> > use those syscalls which should be enough of a clue if they were
> > disabled without proper consideration. They are: timer_create,
> > timer_gettime: timer_getoverrun, timer_settime, timer_delete,
> > clock_adjtime.
> > 
> > The clock_settime, clock_gettime, clock_getres and clock_nanosleep syscalls
> > are replaced by simple wrappers compatible with CLOCK_REALTIME,
> > CLOCK_MONOTONIC and CLOCK_BOOTTIME only.
> > 
> > Signed-off-by: Nicolas Pitre <nico@...aro.org>
> > ---
> > 
> > Changes from RFC/v1:
> > 
> > - Stubbed-out functions moved to static inlines.
> > - The timer signal handling code is now removed.
> > - The list of removed syscalls is explicitly documented.
> > - The clock_settime, clock_gettime, clock_getres and clock_nanosleep 
> >   syscalls are minimally preserved as this required very little code.
> > 
> > I'm now able to boot a copy of Fedora 21 with this patch and 
> > CONFIG_POSIX_TIMERS=n with no apparent issues.
> 
> This looks quite reasonable.
> 
> Does booting a standard distro really require providing clock_settime?

I don't know.  Maybe some date(1) implementation uses it. It is however 
so small that there is no real advantage in explicitly removing it.

> I'd still prefer to see the special-case sys_ni support dropped,
> especially since the most common syscalls now remain.  However, if
> others want to see it kept, I won't object too strongly; whoever ends up
> implementing a common (and optional) version of that infrastructure for
> all syscalls can fold this into it.

Personally, I'd prefer to see more of those configurable syscalls before 
introducing such infrastructure.


Nicolas