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    • Heiko Carstens's avatar
      tick-sched: Fix idle and iowait sleeptime accounting vs CPU hotplug · 71fee48f
      Heiko Carstens authored
      When offlining and onlining CPUs the overall reported idle and iowait
      times as reported by /proc/stat jump backward and forward:
      
      cpu  132 0 176 225249 47 6 6 21 0 0
      cpu0 80 0 115 112575 33 3 4 18 0 0
      cpu1 52 0 60 112673 13 3 1 2 0 0
      
      cpu  133 0 177 226681 47 6 6 21 0 0
      cpu0 80 0 116 113387 33 3 4 18 0 0
      
      cpu  133 0 178 114431 33 6 6 21 0 0 <---- jump backward
      cpu0 80 0 116 114247 33 3 4 18 0 0
      cpu1 52 0 61 183 0 3 1 2 0 0        <---- idle + iowait start with 0
      
      cpu  133 0 178 228956 47 6 6 21 0 0 <---- jump forward
      cpu0 81 0 117 114929 33 3 4 18 0 0
      
      Reason for this is that get_idle_time() in fs/proc/stat.c has different
      sources for both values depending on if a CPU is online or offline:
      
      - if a CPU is online the values may be taken from its per cpu
        tick_cpu_sched structure
      
      - if a CPU is offline the values are taken from its per cpu cpustat
        structure
      
      The problem is that the per cpu tick_cpu_sched structure is set to zero on
      CPU offline. See tick_cancel_sched_timer() in kernel/time/tick-sched.c.
      
      Therefore when a CPU is brought offline and online afterwards both its idle
      and iowait sleeptime will be zero, causing a jump backward in total system
      idle and iowait sleeptime. In a similar way if a CPU is then brought
      offline again the total idle and iowait sleeptimes will jump forward.
      
      It looks like this behavior was introduced with commit 4b0c0f29
      ("tick: Cleanup NOHZ per cpu data on cpu down").
      
      This was only noticed now on s390, since we switched to generic idle time
      reporting with commit be76ea61 ("s390/idle: remove arch_cpu_idle_time()
      and corresponding code").
      
      Fix this by preserving the values of idle_sleeptime and iowait_sleeptime
      members of the per-cpu tick_sched structure on CPU hotplug.
      
      Fixes: 4b0c0f29 ("tick: Cleanup NOHZ per cpu data on cpu down")
      Reported-by: default avatarGerald Schaefer <gerald.schaefer@linux.ibm.com>
      Signed-off-by: default avatarHeiko Carstens <hca@linux.ibm.com>
      Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
      Reviewed-by: default avatarFrederic Weisbecker <frederic@kernel.org>
      Link: https://lore.kernel.org/r/20240115163555.1004144-1-hca@linux.ibm.com
      71fee48f
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