perf: Generic hotplug/cpumask for system PMUs

v1: https://lore.kernel.org/linux-perf-users/[email protected]/

Hi all,

Here's my take on finally trying to clean up the longstanding PMUs vs. CPU hotplug can of worms. Patch #1 is the thing to look at; patch #2 is included as an example of how it simplifies my "favourite" driver (in fact arm-cmn actually gets one of the least-negative diffstats on offer, but it's the one I can test on a box under my desk). I'll save the remaining bulk-conversion noise for a follow-up if and when.

For simplicity I've framed this around the system PMU use-case, since consolidating their whole cpumask/migration business is the big win here, and isn't all that easy to do in separate stages. However, the design does also allow for any driver to now benefit from implementing the init_cpu/exit_cpu ops in place of managing their own cpuhp state; only the automatic filtering and context migration is tied to the new cpumask-specific scope.

v2 fixes the definite functional mishaps from v1; as for the comments and other subtleties I'd rather know what real human maintainers think. I'm still not entirely sure about trying to put ->exit_cpu() calls in perf_pmu_free() for the sake of symmetry - it looks like it would be a little fiddly to get right, and as far as the system PMU drivers go, the ones which aren't already using _nocalls variants for remove/ probe-failure teardown anyway look to be just wrong and making exactly the mistakes I'm trying to clean up: doing a bunch of busywork chasing around the affinity of an IRQ that's about to be freed, plus calling perf_pmu_migrate_context() with an unregistered PMU! I do see at least arm-spe, coresight-trbe and riscv-sbi actually doing some kind of vaguely meaningful per-CPU disable, so perhaps the question of whether it's common enough to be generic or simple enough to open-code can wait for a future generic hotplug conversion of those?

Thanks,
Robin.

Robin Murphy (2):
perf: Generic hotplug/cpumask for system PMUs
perf/arm-cmn: Switch to generic cpumask

 drivers/perf/arm-cmn.c     | 96 +++++++++-----------------------------
 include/linux/perf_event.h | 19 ++++++++
 kernel/events/core.c       | 78 +++++++++++++++++++++++++++----
 3 files changed, 110 insertions(+), 83 deletions(-)

2.54.0.dirty