Messages in this thread | | | Date | Tue, 31 Jan 2023 11:21:17 +0800 | From | Chen Yu <> | Subject | Re: [bug-report] possible s64 overflow in max_vruntime() |
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On 2023-01-27 at 17:18:56 +0100, Vincent Guittot wrote: > On Fri, 27 Jan 2023 at 12:44, Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Thu, Jan 26, 2023 at 07:31:02PM +0100, Roman Kagan wrote: > > > > > > All that only matters for small sleeps anyway. > > > > > > > > Something like: > > > > > > > > sleep_time = U64_MAX; > > > > if (se->avg.last_update_time) > > > > sleep_time = cfs_rq_clock_pelt(cfs_rq) - se->avg.last_update_time; > > > > > > Interesting, why not rq_clock_task(rq_of(cfs_rq)) - se->exec_start, as > > > others were suggesting? It appears to better match the notion of sleep > > > wall-time, no? > > > > Should also work I suppose. cfs_rq_clock takes throttling into account, > > but that should hopefully also not be *that* long, so either should > > work. > > yes rq_clock_task(rq_of(cfs_rq)) should be fine too > > Another thing to take into account is the sleeper credit that the > waking task deserves so the detection should be done once it has been > subtracted from vruntime. > > Last point, when a nice -20 task runs on a rq, it will take a bit more > than 2 seconds for the vruntime to be increased by more than 24ms (the > maximum credit that a waking task can get) so threshold must be > significantly higher than 2 sec. On the opposite side, the lowest > possible weight of a cfs rq is 2 which means that the problem appears > for a sleep longer or equal to 2^54 = 2^63*2/1024. We should use this > value instead of an arbitrary 200 days Does it mean any threshold between 2 sec and 2^54 nsec should be fine? Because 1. Any task sleeps longer than 2 sec will get at most 24 ms(sysctl_sched_latency) 'vruntime bonus' when enqueued. 2. Although a low weight cfs rq run for 2^54 nsec could trigger the overflow, we can choose threshold lower than 2^54 to avoid any overflow.
thanks, Chenyu
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