Messages in this thread | | | Date | Fri, 5 Feb 2016 16:51:02 -0800 | Subject | Re: [PATCH] prctl: Add PR_SET_TIMERSLACK_PID for setting timer slack of an arbitrary thread. | From | John Stultz <> |
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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?
thanks -john
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