[PATCH 3/3] iommu/vt-d: Fix an IOMMU perfmon warning when CPU hotplug

Lu Baolu
2023-03-29T13:47:23Z

From: Kan Liang <kan.liang@linux.intel.com>

A warning can be triggered when hotplug CPU 0.
$ echo 0 > /sys/devices/system/cpu/cpu0/online

------------[ cut here ]------------
Voluntary context switch within RCU read-side critical section!
WARNING: CPU: 0 PID: 19 at kernel/rcu/tree_plugin.h:318
rcu_note_context_switch+0x4f4/0x580
RIP: 0010:rcu_note_context_switch+0x4f4/0x580
Call Trace:
<TASK>
? perf_event_update_userpage+0x104/0x150
__schedule+0x8d/0x960
? perf_event_set_state.part.82+0x11/0x50
schedule+0x44/0xb0
schedule_timeout+0x226/0x310
? __perf_event_disable+0x64/0x1a0
? _raw_spin_unlock+0x14/0x30
wait_for_completion+0x94/0x130
__wait_rcu_gp+0x108/0x130
synchronize_rcu+0x67/0x70
? invoke_rcu_core+0xb0/0xb0
? __bpf_trace_rcu_stall_warning+0x10/0x10
perf_pmu_migrate_context+0x121/0x370
iommu_pmu_cpu_offline+0x6a/0xa0
? iommu_pmu_del+0x1e0/0x1e0
cpuhp_invoke_callback+0x129/0x510
cpuhp_thread_fun+0x94/0x150
smpboot_thread_fn+0x183/0x220
? sort_range+0x20/0x20
kthread+0xe6/0x110
? kthread_complete_and_exit+0x20/0x20
ret_from_fork+0x1f/0x30
</TASK>
---[ end trace 0000000000000000 ]---

The synchronize_rcu() will be invoked in the perf_pmu_migrate_context(),
when migrating a PMU to a new CPU. However, the current for_each_iommu()
is within RCU read-side critical section.

Two methods were considered to fix the issue.
- Use the dmar_global_lock to replace the RCU read lock when going
through the drhd list. But it triggers a lockdep warning.
- Use the cpuhp_setup_state_multi() to set up a dedicated state for each
IOMMU PMU. The lock can be avoided.

The latter method is implemented in this patch. Since each IOMMU PMU has
a dedicated state, add cpuhp_node and cpu in struct iommu_pmu to track
the state. The state can be dynamically allocated now. Remove the
CPUHP_AP_PERF_X86_IOMMU_PERF_ONLINE.

Fixes: 46284c6ceb5e ("iommu/vt-d: Support cpumask for IOMMU perfmon")
Reported-by: Ammy Yi <ammy.yi@intel.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Link: https://lore.kernel.org/r/20230328182028.1366416-1-kan.liang@linux.intel.com
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
---
include/linux/cpuhotplug.h | 1 -
drivers/iommu/intel/iommu.h | 2 ++
drivers/iommu/intel/perfmon.c | 68 ++++++++++++++++++++++-------------
3 files changed, 46 insertions(+), 25 deletions(-)

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index c6fab004104a..5b2f8147d1ae 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -218,7 +218,6 @@ enum cpuhp_state {
CPUHP_AP_PERF_X86_CQM_ONLINE,
CPUHP_AP_PERF_X86_CSTATE_ONLINE,
CPUHP_AP_PERF_X86_IDXD_ONLINE,
- CPUHP_AP_PERF_X86_IOMMU_PERF_ONLINE,
CPUHP_AP_PERF_S390_CF_ONLINE,
CPUHP_AP_PERF_S390_SF_ONLINE,
CPUHP_AP_PERF_ARM_CCI_ONLINE,
diff --git a/drivers/iommu/intel/iommu.h b/drivers/iommu/intel/iommu.h
index d6df3b865812..694ab9b7d3e9 100644
--- a/drivers/iommu/intel/iommu.h
+++ b/drivers/iommu/intel/iommu.h
@@ -641,6 +641,8 @@ struct iommu_pmu {
DECLARE_BITMAP(used_mask, IOMMU_PMU_IDX_MAX);
struct perf_event *event_list[IOMMU_PMU_IDX_MAX];
unsigned char irq_name[16];
+ struct hlist_node cpuhp_node;
+ int cpu;
};

#define IOMMU_IRQ_ID_OFFSET_PRQ (DMAR_UNITS_SUPPORTED)
diff --git a/drivers/iommu/intel/perfmon.c b/drivers/iommu/intel/perfmon.c
index e17d9743a0d8..cf43e798eca4 100644
--- a/drivers/iommu/intel/perfmon.c
+++ b/drivers/iommu/intel/perfmon.c
@@ -773,19 +773,34 @@ static void iommu_pmu_unset_interrupt(struct intel_iommu *iommu)
iommu->perf_irq = 0;
}

-static int iommu_pmu_cpu_online(unsigned int cpu)
+static int iommu_pmu_cpu_online(unsigned int cpu, struct hlist_node *node)
{
+ struct iommu_pmu *iommu_pmu = hlist_entry_safe(node, typeof(*iommu_pmu), cpuhp_node);
+
if (cpumask_empty(&iommu_pmu_cpu_mask))
cpumask_set_cpu(cpu, &iommu_pmu_cpu_mask);

+ if (cpumask_test_cpu(cpu, &iommu_pmu_cpu_mask))
+ iommu_pmu->cpu = cpu;
+
return 0;
}

-static int iommu_pmu_cpu_offline(unsigned int cpu)
+static int iommu_pmu_cpu_offline(unsigned int cpu, struct hlist_node *node)
{
- struct dmar_drhd_unit *drhd;
- struct intel_iommu *iommu;
- int target;
+ struct iommu_pmu *iommu_pmu = hlist_entry_safe(node, typeof(*iommu_pmu), cpuhp_node);
+ int target = cpumask_first(&iommu_pmu_cpu_mask);
+
+ /*
+ * The iommu_pmu_cpu_mask has been updated when offline the CPU
+ * for the first iommu_pmu. Migrate the other iommu_pmu to the
+ * new target.
+ */
+ if (target < nr_cpu_ids && target != iommu_pmu->cpu) {
+ perf_pmu_migrate_context(&iommu_pmu->pmu, cpu, target);
+ iommu_pmu->cpu = target;
+ return 0;
+ }

if (!cpumask_test_and_clear_cpu(cpu, &iommu_pmu_cpu_mask))
return 0;
@@ -795,45 +810,50 @@ static int iommu_pmu_cpu_offline(unsigned int cpu)
if (target < nr_cpu_ids)
cpumask_set_cpu(target, &iommu_pmu_cpu_mask);
else
- target = -1;
+ return 0;

- rcu_read_lock();
-
- for_each_iommu(iommu, drhd) {
- if (!iommu->pmu)
- continue;
- perf_pmu_migrate_context(&iommu->pmu->pmu, cpu, target);
- }
- rcu_read_unlock();
+ perf_pmu_migrate_context(&iommu_pmu->pmu, cpu, target);
+ iommu_pmu->cpu = target;

return 0;
}

static int nr_iommu_pmu;
+static enum cpuhp_state iommu_cpuhp_slot;

static int iommu_pmu_cpuhp_setup(struct iommu_pmu *iommu_pmu)
{
int ret;

- if (nr_iommu_pmu++)
- return 0;
+ if (!nr_iommu_pmu) {
+ ret = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN,
+ "driver/iommu/intel/perfmon:online",
+ iommu_pmu_cpu_online,
+ iommu_pmu_cpu_offline);
+ if (ret < 0)
+ return ret;
+ iommu_cpuhp_slot = ret;
+ }

- ret = cpuhp_setup_state(CPUHP_AP_PERF_X86_IOMMU_PERF_ONLINE,
- "driver/iommu/intel/perfmon:online",
- iommu_pmu_cpu_online,
- iommu_pmu_cpu_offline);
- if (ret)
- nr_iommu_pmu = 0;
+ ret = cpuhp_state_add_instance(iommu_cpuhp_slot, &iommu_pmu->cpuhp_node);
+ if (ret) {
+ if (!nr_iommu_pmu)
+ cpuhp_remove_multi_state(iommu_cpuhp_slot);
+ return ret;
+ }
+ nr_iommu_pmu++;

- return ret;
+ return 0;
}

static void iommu_pmu_cpuhp_free(struct iommu_pmu *iommu_pmu)
{
+ cpuhp_state_remove_instance(iommu_cpuhp_slot, &iommu_pmu->cpuhp_node);
+
if (--nr_iommu_pmu)
return;

- cpuhp_remove_state(CPUHP_AP_PERF_X86_IOMMU_PERF_ONLINE);
+ cpuhp_remove_multi_state(iommu_cpuhp_slot);
}

void iommu_pmu_register(struct intel_iommu *iommu)
--
2.34.1