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authorThomas Gleixner <tglx@linutronix.de>2016-03-04 15:59:42 +0100
committerIngo Molnar <mingo@kernel.org>2016-03-05 09:17:20 +0100
commite9532e69b8d1d1284e8ecf8d2586de34aec61244 (patch)
treee63d14fe6e3451ca95b4c16b933fa9d13d6eee9e /arch/x86/include
parentsched/deadline: Always calculate end of period on sched_yield() (diff)
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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 <tglx@linutronix.de> Acked-by: Rik van Riel <riel@redhat.com> Cc: <stable@vger.kernel.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Glauber Costa <glommer@parallels.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> 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 <mingo@kernel.org>
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