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

[John Stultz]

On Fri, Feb 5, 2016 at 2:35 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> On Fri, Feb 5, 2016 at 12:50 PM, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>> On Fri, 5 Feb 2016 12:44:04 -0800 Kees Cook <keescook@xxxxxxxxxxxx> 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?

thanks
-john