[PATCH 2/2] s390/cpum_cf: Honor hwctr session cpu mask during hotplug

[Sumanth Korikkar <sumanthk@linux.ibm.com>]

cfset_online_cpu() and cfset_offline_cpu() cpu hotplug callbacks start
and stop counter sets for every active hwctr session without checking
the session cpu mask, and dynamically modify that mask on each hotplug
event.

For example, assume cpu0 and cpu1 are online and cpu2 is offline. Start
an hwctr session restricted to cpu0 and cpu1, and then bring cpu2 online:

lshwc -l 9999 -i 5 0-1:b
chcpu -e 2

Although cpu2 is not part of the requested mask, cfset_online_cpu()
starts the counter sets on cpu2 and adds it to the session mask. cpu2
can then appear in lshwc output (s390-tools) and is handled as part of
the session until it is stopped.

Keep the userspace provided cpu mask unchanged and update counter sets
only when the hotplugged cpu belongs to that mask. For all cpu requests,
lshwc should supply the possible cpu mask so that cpus coming online later
remain part of the session.

Note:
A corresponding lshwc (s390-tools) change is needed to supply the
possible cpu mask for allcpu sessions, ensuring that cpus coming online
later remain part of the session. Without it, newly online cpus would be
silently excluded from an active session.

Since this userspace change is not yet available, the patch is not
suitable for stable kernels and intentionally omits a Fixes: tag. The
prerequisite bug fix is in "s390/cpum_cf: Check session mask before cpu
offline".

Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
---
 arch/s390/kernel/perf_cpum_cf.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/s390/kernel/perf_cpum_cf.c b/arch/s390/kernel/perf_cpum_cf.c
index 1d63dc3b0ab9..a2334af51544 100644
--- a/arch/s390/kernel/perf_cpum_cf.c
+++ b/arch/s390/kernel/perf_cpum_cf.c
@@ -1768,8 +1768,9 @@ static struct miscdevice cfset_dev = {
 	.mode	= 0666,
 };
 
-/* Hotplug add of a CPU. Scan through all active processes and add
- * that CPU to the list of CPUs supplied with ioctl(..., START, ...).
+/*
+ * Start counter sets on a newly online CPU for each active hwctr session that
+ * includes this CPU in its requested mask.
  */
 static int cfset_online_cpu(unsigned int cpu)
 {
@@ -1778,17 +1779,19 @@ static int cfset_online_cpu(unsigned int cpu)
 
 	if (!list_empty(&cfset_session.head)) {
 		list_for_each_entry(rp, &cfset_session.head, node) {
+			if (!cpumask_test_cpu(cpu, &rp->mask))
+				continue;
+
 			p.sets = rp->ctrset;
 			cfset_ioctl_on(&p);
-			cpumask_set_cpu(cpu, &rp->mask);
 		}
 	}
 	return 0;
 }
 
-/* Hotplug remove of a CPU. Scan through all active processes and clear
- * that CPU from the list of CPUs supplied with ioctl(..., START, ...).
- * Adjust reference counts.
+/*
+ * Stop counter sets on an outgoing CPU for each active hwctr session that
+ * includes this CPU in its requested mask.
  */
 static int cfset_offline_cpu(unsigned int cpu)
 {
@@ -1802,7 +1805,6 @@ static int cfset_offline_cpu(unsigned int cpu)
 
 			p.sets = rp->ctrset;
 			cfset_ioctl_off(&p);
-			cpumask_clear_cpu(cpu, &rp->mask);
 		}
 	}
 	return 0;
-- 
2.53.0