From e9532e69b8d1d1284e8ecf8d2586de34aec61244 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Fri, 4 Mar 2016 15:59:42 +0100 Subject: [PATCH] sched/cputime: Fix steal time accounting vs. CPU hotplug On CPU hotplug the steal time accounting can keep a stale rq->prev_steal_time value over CPU down and up. So after the CPU comes up again the delta calculation in steal_account_process_tick() wreckages itself due to the unsigned math: u64 steal = paravirt_steal_clock(smp_processor_id()); steal -= this_rq()->prev_steal_time; So if steal is smaller than rq->prev_steal_time we end up with an insane large value which then gets added to rq->prev_steal_time, resulting in a permanent wreckage of the accounting. As a consequence the per CPU stats in /proc/stat become stale. Nice trick to tell the world how idle the system is (100%) while the CPU is 100% busy running tasks. Though we prefer realistic numbers. None of the accounting values which use a previous value to account for fractions is reset at CPU hotplug time. update_rq_clock_task() has a sanity check for prev_irq_time and prev_steal_time_rq, but that sanity check solely deals with clock warps and limits the /proc/stat visible wreckage. The prev_time values are still wrong. Solution is simple: Reset rq->prev_*_time when the CPU is plugged in again. Signed-off-by: Thomas Gleixner Acked-by: Rik van Riel Cc: Cc: Frederic Weisbecker Cc: Glauber Costa Cc: Linus Torvalds Cc: Peter Zijlstra Fixes: commit 095c0aa83e52 "sched: adjust scheduler cpu power for stolen time" Fixes: commit aa483808516c "sched: Remove irq time from available CPU power" Fixes: commit e6e6685accfa "KVM guest: Steal time accounting" Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1603041539490.3686@nanos Signed-off-by: Ingo Molnar --- kernel/sched/core.c | 1 + kernel/sched/sched.h | 13 +++++++++++++ 2 files changed, 14 insertions(+) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index ab814bf100e1..406182af99ac 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5627,6 +5627,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu) case CPU_UP_PREPARE: rq->calc_load_update = calc_load_update; + account_reset_rq(rq); break; case CPU_ONLINE: diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 30ea2d871ba7..4f6598ae4c31 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1738,3 +1738,16 @@ static inline u64 irq_time_read(int cpu) } #endif /* CONFIG_64BIT */ #endif /* CONFIG_IRQ_TIME_ACCOUNTING */ + +static inline void account_reset_rq(struct rq *rq) +{ +#ifdef CONFIG_IRQ_TIME_ACCOUNTING + rq->prev_irq_time = 0; +#endif +#ifdef CONFIG_PARAVIRT + rq->prev_steal_time = 0; +#endif +#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING + rq->prev_steal_time_rq = 0; +#endif +}