3.2.80-rc1 review patch. If anyone has any objections, please let me know.
------------------ From: Thomas Gleixner <[email protected]> commit e9532e69b8d1d1284e8ecf8d2586de34aec61244 upstream. 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 <[email protected]> Acked-by: Rik van Riel <[email protected]> Cc: Frederic Weisbecker <[email protected]> Cc: Glauber Costa <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Peter Zijlstra <[email protected]> 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 <[email protected]> [bwh: Backported to 3.2: adjust filenames] Signed-off-by: Ben Hutchings <[email protected]> --- --- a/kernel/sched.c +++ b/kernel/sched.c @@ -2084,6 +2084,19 @@ EXPORT_SYMBOL_GPL(account_system_vtime); #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 +} + #ifdef CONFIG_PARAVIRT static inline u64 steal_ticks(u64 steal) { @@ -6851,6 +6864,7 @@ migration_call(struct notifier_block *nf case CPU_UP_PREPARE: rq->calc_load_update = calc_load_update; + account_reset_rq(rq); break; case CPU_ONLINE:
