4.19-stable review patch. If anyone has any objections, please let me know.
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From: Heiko Carstens hca@linux.ibm.com
commit 71fee48fb772ac4f6cfa63dbebc5629de8b4cc09 upstream.
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 4b0c0f294f60 ("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 be76ea614460 ("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: 4b0c0f294f60 ("tick: Cleanup NOHZ per cpu data on cpu down") Reported-by: Gerald Schaefer gerald.schaefer@linux.ibm.com Signed-off-by: Heiko Carstens hca@linux.ibm.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Reviewed-by: Frederic Weisbecker frederic@kernel.org Link: https://lore.kernel.org/r/20240115163555.1004144-1-hca@linux.ibm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/time/tick-sched.c | 5 +++++ 1 file changed, 5 insertions(+)
--- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -1345,13 +1345,18 @@ void tick_setup_sched_timer(void) void tick_cancel_sched_timer(int cpu) { struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu); + ktime_t idle_sleeptime, iowait_sleeptime;
# ifdef CONFIG_HIGH_RES_TIMERS if (ts->sched_timer.base) hrtimer_cancel(&ts->sched_timer); # endif
+ idle_sleeptime = ts->idle_sleeptime; + iowait_sleeptime = ts->iowait_sleeptime; memset(ts, 0, sizeof(*ts)); + ts->idle_sleeptime = idle_sleeptime; + ts->iowait_sleeptime = iowait_sleeptime; } #endif