powerpc/vtime: Initialize starttime at boot for native accounting

It was observed that /proc/stat had very large value for one ore more
CPUs. It was more visible after recent code simplifications around
cpustats.

System has 240 CPUs.

cat /proc/uptime;
194.18 46500.55
cat /proc/stat
cpu  5966 39 837032887 4650070 164 185 100 0 0 0
cpu0 108 0 837030890 19109 24 4 23 0 0 0

Since uptime is 194s, system time of each CPU can't be more than 19400.
Sum of system time  of all CPUs can't be more than 19400*240 4656000.
In fact huge value is close to mftb(). Note mftb doesn't reset on powerVM
when the LPAR restart. It only resets when whole system resets. The same
issue exists for kexec too.

This happens since starttime is not setup at init time. Once it is set
then subsequent vtime_delta will return the right delta.

Fix it by initializing the starttime during CPU initialization. This
fixes the large times seen.

cat /proc/uptime; cat /proc/stat
15.78 3694.63
cpu  6035 35 1347 369479 23 144 49 0 0 0
cpu0 19 0 38 1508 0 1 14 0 0 0

Now, system time is reported as expected.

Fixes: cf9efce0ce31 ("powerpc: Account time using timebase rather  than PURR")
Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Suggested-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260605124329.377533-1-sshegde@linux.ibm.com

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 3460d1a5a97c2abfce40404f53d835110b2fe789..11145c40183dd1218fbbb170eb71831b4a272d32 100644 (file)
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -377,7 +377,6 @@ void vtime_task_switch(struct task_struct *prev)
        }
 }
 
-#ifdef CONFIG_NO_HZ_COMMON
 /**
  * vtime_reset - Fast forward vtime entry clocks
  *
@@ -394,6 +393,7 @@ void vtime_reset(void)
 #endif
 }
 
+#ifdef CONFIG_NO_HZ_COMMON
 /**
  * vtime_dyntick_start - Inform vtime about entry to idle-dynticks
  *
@@ -933,6 +933,7 @@ static void __init set_decrementer_max(void)
 static void __init init_decrementer_clockevent(void)
 {
        register_decrementer_clockevent(smp_processor_id());
+       vtime_reset();
 }
 
 void secondary_cpu_time_init(void)
@@ -948,6 +949,7 @@ void secondary_cpu_time_init(void)
        /* FIME: Should make unrelated change to move snapshot_timebase
         * call here ! */
        register_decrementer_clockevent(smp_processor_id());
+       vtime_reset();
 }
 
 /*