FAILED: patch "[PATCH] s390/pai: Support CPU hotplug for PMU PAI" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 9ecc4d033879f7761f2df07e20cd2fbec00fd90b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026091645-securely-immobile-9a05@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 9ecc4d033879f7761f2df07e20cd2fbec00fd90b Mon Sep 17 00:00:00 2001
From: Thomas Richter <[email protected]>
Date: Tue, 1 Sep 2026 14:33:20 +0200
Subject: [PATCH] s390/pai: Support CPU hotplug for PMU PAI

The command 'perf stat -e pai_crypto/CRYPTO_ALL/ -- <command>' crashes the kernel when CPUs are hotplug added during that run.

Root cause is the missing allocation of per-CPU data structures for that new CPU. The allocation is dynamic and the first event that has task context creates such a structure for each online CPU. This is not sufficient. CPUs may be offline during event creation and can be set online during the perf run time. For example commands

 # echo 0 > /sys/devices/system/cpu/cpu1/online
 # perf stat -e cycles -i -- stress-ng -t10s --matrix X
 # sleep 1
 # echo 1 > /sys/devices/system/cpu/cpu1/online

Currently without a CPU hotplug handler, that new CPU has no per-CPU data infrastructure. The scheduler runs PMU call back function pai_add() to install the PMU support for that CPU before the task is being scheduled on that new CPU.
In pai_add() instructions

    mp = this_cpu_ptr(pai_root[idx].mapptr);
    cpump = mp->mapptr;

return a NULL pointer and the result is a kernel panic as variable cpump is used inside that function.

Add CPU hotplug support for CPU add and delete and create the necessary per-CPU data infrastructure during CPU hotplug add processing. Same for CPU hotplug remove.
This is done when the CPU is offline to ensure the data structures are available when CPU is made online and tasks are scheduled on it.

[[email protected]: fixup error path in pai_init()]

index 534d0320e2aa..a3456a36aaa7 100644
@@ -76,7 +76,6 @@ static __always_inline void pai_kernel_exit(struct pt_regs *regs)
 }
 
 #define PAI_SAVE_AREA(x)	((x)->hw.event_base)
-#define PAI_CPU_MASK(x)		((x)->hw.addr_filters)
 #define PAI_PMU_IDX(x)		((x)->hw.last_tag)
 #define PAI_SWLIST(x)		(&(x)->hw.tp_list)