Subject: [PATCH v2] powerpc/pseries: Only wait for dying CPU after call to rtas_stop_self()

From: Thiago Jung Bauermann <[email-masked]>
Date: Fri, 22 Feb 2019 19:57:52 -0300

When testing DLPAR CPU add/remove on a system under stress,
pseries_cpu_die() doesn't wait long enough for a CPU to die:

[  446.983944] cpu 148 (hwid 148) Ready to die...
[  446.984062] cpu 149 (hwid 149) Ready to die...
[  446.993518] cpu 150 (hwid 150) Ready to die...
[  446.993543] Querying DEAD? cpu 150 (150) shows 2
[  446.994098] cpu 151 (hwid 151) Ready to die...
[  447.133726] cpu 136 (hwid 136) Ready to die...
[  447.403532] cpu 137 (hwid 137) Ready to die...
[  447.403772] cpu 138 (hwid 138) Ready to die...
[  447.403839] cpu 139 (hwid 139) Ready to die...
[  447.403887] cpu 140 (hwid 140) Ready to die...
[  447.403937] cpu 141 (hwid 141) Ready to die...
[  447.403979] cpu 142 (hwid 142) Ready to die...
[  447.404038] cpu 143 (hwid 143) Ready to die...
[  447.513546] cpu 128 (hwid 128) Ready to die...
[  447.693533] cpu 129 (hwid 129) Ready to die...
[  447.693999] cpu 130 (hwid 130) Ready to die...
[  447.703530] cpu 131 (hwid 131) Ready to die...
[  447.704087] Querying DEAD? cpu 132 (132) shows 2
[  447.704102] cpu 132 (hwid 132) Ready to die...
[  447.713534] cpu 133 (hwid 133) Ready to die...
[  447.714064] Querying DEAD? cpu 134 (134) shows 2

This is a race between one CPU stopping and another one calling
pseries_cpu_die() to wait for it to stop. That function does a short busy
loop calling RTAS query-cpu-stopped-state on the stopping CPU to verify
that it is stopped, but I think there's a lot for the stopping CPU to do
which may take longer than this loop allows.

As can be seen in the dmesg right before or after the "Querying DEAD?"
messages, if pseries_cpu_die() waited a little longer it would have seen
the CPU in the stopped state.

I see two cases that can be causing this race:

1. It's possible that CPU 134 was inactive at the time it was unplugged. In
   that case, dlpar_offline_cpu() calls H_PROD on that CPU and immediately
   calls pseries_cpu_die(). Meanwhile, the prodded CPU activates and start
   the process of stopping itself. It's possible that the busy loop is not
   long enough to allow for the CPU to wake up and complete the stopping
   process.

2. If CPU 134 was online at the time it was unplugged, it would have gone
   through the new CPU hotplug state machine in kernel/cpu.c that was
   introduced in v4.6 to get itself stopped. It's possible that the busy
   loop in pseries_cpu_die() was long enough for the older hotplug code but
   not for the new hotplug state machine.

I don't know if this race condition has any ill effects, but we can make
the race a lot more even if we only start querying if the CPU is stopped
when the stopping CPU is close to call rtas_stop_self().

Since pseries_mach_cpu_die() sets the CPU current state to offline almost
immediately before calling rtas_stop_self(), we use that as a signal that
it is either already stopped or very close to that point, and we can start
the busy loop.

As suggested by Michael Ellerman, this patch also changes the busy loop to
wait for a fixed amount of wall time.

Signed-off-by: Thiago Jung Bauermann <[email-masked]>
---
 arch/powerpc/platforms/pseries/hotplug-cpu.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

I tried to estimate good amounts for the timeout and loop delays, but
I'm not sure how reasonable my numbers are. The busy loops will wait for
100 µs between each try, and spin_event_timeout() will timeout after
100 ms. I'll be happy to change these values if you have better
suggestions.

Gautham was able to test this patch and it solved the race condition.

v1 was a cruder patch which just increased the number of loops:
https://lists.ozlabs.org/pipermail/linuxppc-dev/2017-February/153734.html

v1 also mentioned a kernel crash but Gautham narrowed it down to a bug
in RTAS, which is in the process of being fixed.

diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index 97feb6e79f1a..424146cc752e 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -214,13 +214,21 @@ static void pseries_cpu_die(unsigned int cpu)
 			msleep(1);
 		}
 	} else if (get_preferred_offline_state(cpu) == CPU_STATE_OFFLINE) {
+		/*
+		 * If the current state is not offline yet, it means that the
+		 * dying CPU (which is in pseries_mach_cpu_die) didn't have a
+		 * chance to call rtas_stop_self yet and therefore it's too
+		 * early to query if the CPU is stopped.
+		 */
+		spin_event_timeout(get_cpu_current_state(cpu) == CPU_STATE_OFFLINE,
+				   100000, 100);
 
 		for (tries = 0; tries < 25; tries++) {
 			cpu_status = smp_query_cpu_stopped(pcpu);
 			if (cpu_status == QCSS_STOPPED ||
 			    cpu_status == QCSS_HARDWARE_ERROR)
 				break;
-			cpu_relax();
+			udelay(100);
 		}
 	}