Re: [PATCH] prctl: Add PR_SET_TIMERSLACK_PID for setting timer slack of an arbitrary thread.

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 5 Feb 2016 16:51:02 -0800 John Stultz <john.stultz@linaro.org> wrote:

> On Fri, Feb 5, 2016 at 2:35 PM, John Stultz <john.stultz@linaro.org> wrote:
> > On Fri, Feb 5, 2016 at 12:50 PM, Andrew Morton
> > <akpm@linux-foundation.org> wrote:
> >> On Fri, 5 Feb 2016 12:44:04 -0800 Kees Cook <keescook@chromium.org> wrote:
> >>> Could this be exposed as a writable /proc entry instead? Like the oom_* stuff?
> >>
> >> /proc/<pid>/timer_slack_ns, guarded by ptrace_may_access(), documented
> >> under Documentation/?  Yup, that would work.  It's there for all
> >> architectures from day one and there is precedent.  It's not as nice,
> >> but /proc nasties will always be with us.
> >
> > Ok. I'll start working on that.
> 
> Arjan/Thomas:  One curious thing I noticed here while writing some
> documentation. The timer_slack_ns value in the task struct is a
> unsigned long.
> 
> So this means PR_SET_TIMERSLACK limits the maximum slack on 32 bit
> machines to ~4 seconds. Where on 64bit machines it can be quite a bit
> longer (unreasonably long, really :).
> 
> While 4 seconds is probably a reasonable interactivity limit, testing
> w/ 10 second slack values on a VM showed those timers pushed back to
> almost 10 seconds. So it may be useful to have > 4 second slack values
> generally. Thus left alone this seems like an unfair disadvantage to
> 32bit machines.
> 
> We can't do too much about the PR_GET_TIMERSLACK/PR_SET_TIMERSLACK
> interfaces, since its ABI and specifies a long, so one option there
> would be to make sure the value specified is capped to UINT_MAX which
> would keep the max value to ~4 seconds on all architectures.
> 
> Alternatively, with the /proc/pid/timerslack_ns interface I'm working
> on, we can make the backing storage a long long and support 64bits of
> nanoseconds on all architectures. (But again, we can't really change
> PR_SET/GET_TIMERSLACK, so 32bit systems might see strange values from
> that with larger then uint slack values).
> 
> Or I can just leave it as ULONG_MAX on all interfaces.
> 
> Thoughts or preferences?

/proc/<pid>/timer_slack_us?