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Date: Tue, 13 May 2014 11:57:49 +0200 From: Juri Lelli <juri.lelli@...il.com> To: Peter Zijlstra <peterz@...radead.org> Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>, Dario Faggioli <raistlin@...ux.it>, Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>, Dave Jones <davej@...hat.com> Subject: Re: [BUG] sched_setattr() SCHED_DEADLINE hangs system Hi all, On Mon, 12 May 2014 14:30:32 +0200 Peter Zijlstra <peterz@...radead.org> wrote: > On Mon, May 12, 2014 at 11:19:39AM +0200, Michael Kerrisk (man-pages) wrote: > > Hi Peter, > > > > On Mon, May 12, 2014 at 10:47 AM, Peter Zijlstra <peterz@...radead.org> wrote: > > > On Mon, May 12, 2014 at 08:53:59AM +0200, Michael Kerrisk (man-pages) wrote: > > >> On 05/11/2014 04:54 PM, Michael Kerrisk (man-pages) wrote: > > > > > >> > $ time sudo ./t_sched_setattr d 18446744072 18446744072 18446744073 > > >> > > >> I realize my speculation was completely off the mark. time(2) really > > >> is reporting the truth, and the sched_setattr() call returns immediately. > > >> But it looks like with these settings the deadline scheduler gets itself > > >> into a confused state. The process chews up a vast amount of CPU time > > >> for the few actions (including process teardown) that occur after > > >> the sched_setattr() call, and since the SCHED_DEADLINE process has > > >> priority over everything else, the system locks up. > > > > > > Yeah, its doing something weird alright.. let me see if I can get > > > something useful out. > > > > Thanks! > > So I think its because the way we check wrapping > > (s64)(a - b) < 0 > > This means that its impossible to tell if time went fwd or bwd with > 64bit increments. I've not entirely pinpointed where this is wrecking > things, but it seems like a fair bet this is what's going wrong. > > So I'm tempted to put a sanity check on all these values to make sure <= > 2^63. That way the wrapping logic in the kernel keeps working. > > And 2^63 [ns] should be plenty large enough for everyone (famous last > words of course). > Does the following fix the thing? Thanks, - Juri --- >From 90a7603a0b6b620c9d07e3f375906b436dcc2230 Mon Sep 17 00:00:00 2001 From: Juri Lelli <juri.lelli@...il.com> Date: Tue, 13 May 2014 10:15:59 +0200 Subject: [PATCH] sched/deadline: restrict user params max value to 2^63 ns Michael Kerrisk noticed that creating SCHED_DEADLINE reservations with certain parameters (e.g, a runtime of something near 2^64 ns) can cause a system freeze for some amount of time. The problem is that in the interface we have u64 sched_runtime; while internally we need to have a signed runtime (to cope with budget overruns) s64 runtime; At the time we setup a new dl_entity we copy the first value in the second. The cast turns out with negative values when sched_runtime is too big, and this causes the scheduler to go crazy right from the start. Moreover, considering how we deal with deadlines wraparound (s64)(a - b) < 0 we also have to restrict acceptable values for sched_{deadline,period}. This patch fixes the thing checking that user parameters are always below 2^63 ns (still large enough for everyone). It also rewrites other conditions that we check, since in __checkparam_dl we don't have to deal with deadline wraparounds and what we have now erroneously fails when the difference between values is too big. Reported-by: Michael Kerrisk <mtk.manpages@...il.com> Suggested-by: Peter Zijlstra <peterz@...radead.org> Signed-off-by: Juri Lelli <juri.lelli@...il.com> --- kernel/sched/core.c | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index d9d8ece..96ba59d 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3188,17 +3188,21 @@ __getparam_dl(struct task_struct *p, struct sched_attr *attr) * We ask for the deadline not being zero, and greater or equal * than the runtime, as well as the period of being zero or * greater than deadline. Furthermore, we have to be sure that - * user parameters are above the internal resolution (1us); we - * check sched_runtime only since it is always the smaller one. + * user parameters are above the internal resolution of 1us (we + * check sched_runtime only since it is always the smaller one) and + * below 2^63 ns (we have to check both sched_deadline and + * sched_period, as the latter can be zero). */ static bool __checkparam_dl(const struct sched_attr *attr) { return attr && attr->sched_deadline != 0 && (attr->sched_period == 0 || - (s64)(attr->sched_period - attr->sched_deadline) >= 0) && - (s64)(attr->sched_deadline - attr->sched_runtime ) >= 0 && - attr->sched_runtime >= (2 << (DL_SCALE - 1)); + (attr->sched_period >= attr->sched_deadline)) && + (attr->sched_deadline >= attr->sched_runtime) && + attr->sched_runtime >= (1ULL << DL_SCALE) && + (attr->sched_deadline < (1ULL << 63) && + attr->sched_period < (1ULL << 63)); } /* -- 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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