Nicholas Piggin [off-list ref] writes:

Excerpts from Athira Rajeev's message of October 29, 2021 1:05 pm:

quoted

During Live Partition Migration (LPM), it is observed that perf
counter values reports zero post migration completion. However
'perf stat' with workload continues to show counts post migration
since PMU gets disabled/enabled during sched switches. But incase
of system/cpu wide monitoring, zero counts were reported with 'perf
stat' after migration completion.

Example:
 ./perf stat -e r1001e -I 1000
           time             counts unit events
     1.001010437         22,137,414      r1001e
     2.002495447         15,455,821      r1001e
<<>> As seen in next below logs, the counter values shows zero
        after migration is completed.
<<>>
    86.142535370    129,392,333,440      r1001e
    87.144714617                  0      r1001e
    88.146526636                  0      r1001e
    89.148085029                  0      r1001e

This is the output without the patch? After the patch it keeps counting 
I suppose? And does the very large count go away too?

quoted

Here PMU is enabled during start of perf session and counter
values are read at intervals. Counters are only disabled at the
end of session. The powerpc mobility code presently does not handle
disabling and enabling back of PMU counters during partition
migration. Also since the PMU register values are not saved/restored
during migration, PMU registers like Monitor Mode Control Register 0
(MMCR0), Monitor Mode Control Register 1 (MMCR1) will not contain
the value it was programmed with. Hence PMU counters will not be
enabled correctly post migration.

Fix this in mobility code by handling disabling and enabling of
PMU in all cpu's before and after migration. Patch introduces two
functions 'mobility_pmu_disable' and 'mobility_pmu_enable'.
mobility_pmu_disable() is called before the processor threads goes
to suspend state so as to disable the PMU counters. And disable is
done only if there are any active events running on that cpu.
mobility_pmu_enable() is called after the migrate is done to enable
back the PMU counters.

Since the performance Monitor counters ( PMCs) are not
saved/restored during LPM, results in PMC value being zero and the
'event->hw.prev_count' being non-zero value. This causes problem
during updation of event->count since we always accumulate
(event->hw.prev_count - PMC value) in event->count.  If
event->hw.prev_count is greater PMC value, event->count becomes
negative. To fix this, 'prev_count' also needs to be re-initialised
for all events while enabling back the events. Hence read the
existing events and clear the PMC index (stored in event->hw.idx)
for all events im mobility_pmu_disable. By this way, event count
settings will get re-initialised correctly in power_pmu_enable.

Signed-off-by: Athira Rajeev <redacted>
[ Fixed compilation error reported by kernel test robot ]
Reported-by: kernel test robot <redacted>
---
Changelog:
Change from v2 -> v3:
Addressed review comments from Nicholas Piggin.
 - Removed the "migrate" field which was added in initial
   patch to address updation of event count settings correctly
   in power_pmu_enable. Instead read off existing events in
   mobility_pmu_disable before power_pmu_enable.
 - Moved the mobility_pmu_disable/enable declaration from
   rtas.h to perf event header file.

Addressed review comments from Nathan.
 - Moved the mobility function calls from stop_machine
   context out to pseries_migrate_partition. Also now this
   is a per cpu invocation.

Change from v1 -> v2:
 - Moved the mobility_pmu_enable and mobility_pmu_disable
   declarations under CONFIG_PPC_PERF_CTRS in rtas.h.
   Also included 'asm/rtas.h' in core-book3s to fix the
   compilation warning reported by kernel test robot.

 arch/powerpc/include/asm/perf_event.h     |  8 +++++
 arch/powerpc/perf/core-book3s.c           | 39 +++++++++++++++++++++++
 arch/powerpc/platforms/pseries/mobility.c |  7 ++++
 3 files changed, 54 insertions(+)

diff --git a/arch/powerpc/include/asm/perf_event.h b/arch/powerpc/include/asm/perf_event.h
index 164e910bf654..88aab6cf840c 100644
--- a/arch/powerpc/include/asm/perf_event.h
+++ b/arch/powerpc/include/asm/perf_event.h

@@ -17,6 +17,14 @@ static inline bool is_sier_available(void) { return false; }
 static inline unsigned long get_pmcs_ext_regs(int idx) { return 0; }
 #endif
 
+#ifdef CONFIG_PPC_PERF_CTRS
+void mobility_pmu_disable(void *unused);
+void mobility_pmu_enable(void *unused);
+#else
+static inline void mobility_pmu_disable(void *unused) { }
+static inline void mobility_pmu_enable(void *unused) { }
+#endif
+
 #ifdef CONFIG_FSL_EMB_PERF_EVENT
 #include <asm/perf_event_fsl_emb.h>
 #endif

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 73e62e9b179b..2e8c4c668fa3 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c

@@ -1343,6 +1343,33 @@ static void power_pmu_disable(struct pmu *pmu)
 	local_irq_restore(flags);
 }
 
+/*
+ * Called from pseries_migrate_partition() function
+ * before migration, from powerpc/mobility code.
+ */

These are only needed if pseries is built, so should be inside a PSERIES
ifdef.

This function should handle iterating over CPUs, that shouldn't be left
up to the mobility.c code.

And the names should be something like pmu_start_migration(),
pmu_finish_migration().

quoted

+void mobility_pmu_disable(void *unused)
+{
+	struct cpu_hw_events *cpuhw = this_cpu_ptr(&cpu_hw_events);
+	struct perf_event *event;
+
+	if (cpuhw->n_events != 0) {
+		int i;
+
+		power_pmu_disable(NULL);
+		/*
+		 * Read off any pre-existing events because the register
+		 * values may not be migrated.
+		 */
+		for (i = 0; i < cpuhw->n_events; ++i) {
+			event = cpuhw->event[i];
+			if (event->hw.idx) {
+				power_pmu_read(event);
+				event->hw.idx = 0;
+			}
+		}
+	}
+}
+
 /*
  * Re-enable all events if disable == 0.
  * If we were previously disabled and events were added, then

@@ -1515,6 +1542,18 @@ static void power_pmu_enable(struct pmu *pmu)
 	local_irq_restore(flags);
 }
 
+/*
+ * Called from pseries_migrate_partition() function
+ * after migration, from powerpc/mobility code.
+ */
+void mobility_pmu_enable(void *unused)
+{
+	struct cpu_hw_events *cpuhw = this_cpu_ptr(&cpu_hw_events);
+
+	cpuhw->n_added = cpuhw->n_events;
+	power_pmu_enable(NULL);
+}
+
 static int collect_events(struct perf_event *group, int max_count,
 			  struct perf_event *ctrs[], u64 *events,
 			  unsigned int *flags)

diff --git a/arch/powerpc/platforms/pseries/mobility.c b/arch/powerpc/platforms/pseries/mobility.c
index e83e0891272d..3e96485ccba4 100644
--- a/arch/powerpc/platforms/pseries/mobility.c
+++ b/arch/powerpc/platforms/pseries/mobility.c

@@ -22,6 +22,7 @@
 #include <linux/delay.h>
 #include <linux/slab.h>
 #include <linux/stringify.h>
+#include <linux/perf_event.h>
 
 #include <asm/machdep.h>
 #include <asm/rtas.h>

@@ -631,12 +632,18 @@ static int pseries_migrate_partition(u64 handle)
 	if (ret)
 		return ret;
 
+	/* Disable PMU before suspend */
+	on_each_cpu(&mobility_pmu_disable, NULL, 0);

Why was this moved out of stop machine and to an IPI?

My concern would be, what are the other CPUs doing at this time? Is it 
possible they could take interrupts and schedule? Could that mess up the
perf state here?

pseries_migrate_partition() is called directly from migration_store(),
which is the sysfs store function, which can be called concurrently by
different CPUs.

It's also potentially called from rtas_syscall_dispatch_ibm_suspend_me(),
from sys_rtas(), again with no locking.

So we could have two CPUs calling into here at the same time, which
might not crash, but is unlikely to work well.

I think the lack of locking might have been OK in the past because only
one CPU will successfully get the other CPUs to call do_join() in
pseries_suspend(). But I could be wrong.

Anyway, now that we're mutating the PMU state before suspending we need
to be more careful. So I think we need a lock around the whole sequence.

cheers