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Date: Fri, 5 Feb 2016 16:51:02 -0800 From: John Stultz <john.stultz@...aro.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Kees Cook <keescook@...omium.org>, lkml <linux-kernel@...r.kernel.org>, Ruchi Kandoi <kandoiruchi@...gle.com>, Arjan van de Ven <arjan@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>, Oren Laadan <orenl@...lrox.com>, Rom Lemarchand <romlem@...roid.com>, Android Kernel Team <kernel-team@...roid.com> Subject: Re: [PATCH] prctl: Add PR_SET_TIMERSLACK_PID for setting timer slack of an arbitrary thread. On Fri, Feb 5, 2016 at 2:35 PM, John Stultz <john.stultz@...aro.org> wrote: > On Fri, Feb 5, 2016 at 12:50 PM, Andrew Morton > <akpm@...ux-foundation.org> wrote: >> On Fri, 5 Feb 2016 12:44:04 -0800 Kees Cook <keescook@...omium.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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