• Thomas Gleixner's avatar
    sched/cputime: Fix steal time accounting vs. CPU hotplug · 7aa5e217
    Thomas Gleixner authored
    commit e9532e69 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: default avatarThomas Gleixner <tglx@linutronix.de>
    Acked-by: default avatarRik van Riel <riel@redhat.com>
    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 095c0aa8 "sched: adjust scheduler cpu power for stolen time"
    Fixes: commit aa483808 "sched: Remove irq time from available CPU power"
    Fixes: commit e6e6685a "KVM guest: Steal time accounting"
    Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1603041539490.3686@nanosSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
    7aa5e217
core.c 192 KB