Re: [PATCH V2] iommu/vt-d: Fix a IOMMU perfmon warning when CPU hotplug

[Liang, Kan]

On 2023-03-28 2:20 p.m., kan.liang@xxxxxxxxxxxxxxx wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> 
> A warning can be triggered when hotplug CPU 0.
> $ echo 0 > /sys/devices/system/cpu/cpu0/online
> 
> [11958.737635] ------------[ cut here ]------------
> [11958.742882] Voluntary context switch within RCU read-side critical
> section!
> [11958.742891] WARNING: CPU: 0 PID: 19 at kernel/rcu/tree_plugin.h:318
> rcu_note_context_switch+0x4f4/0x580
> [11958.860095] RIP: 0010:rcu_note_context_switch+0x4f4/0x580
> [11958.960360] Call Trace:
> [11958.963161]  <TASK>
> [11958.965565]  ? perf_event_update_userpage+0x104/0x150
> [11958.971293]  __schedule+0x8d/0x960
> [11958.975165]  ? perf_event_set_state.part.82+0x11/0x50
> [11958.980891]  schedule+0x44/0xb0
> [11958.984464]  schedule_timeout+0x226/0x310
> [11958.989017]  ? __perf_event_disable+0x64/0x1a0
> [11958.994054]  ? _raw_spin_unlock+0x14/0x30
> [11958.998605]  wait_for_completion+0x94/0x130
> [11959.003352]  __wait_rcu_gp+0x108/0x130
> [11959.007616]  synchronize_rcu+0x67/0x70
> [11959.011876]  ? invoke_rcu_core+0xb0/0xb0
> [11959.016333]  ? __bpf_trace_rcu_stall_warning+0x10/0x10
> [11959.022147]  perf_pmu_migrate_context+0x121/0x370
> [11959.027478]  iommu_pmu_cpu_offline+0x6a/0xa0
> [11959.032325]  ? iommu_pmu_del+0x1e0/0x1e0
> [11959.036782]  cpuhp_invoke_callback+0x129/0x510
> [11959.041825]  cpuhp_thread_fun+0x94/0x150
> [11959.046283]  smpboot_thread_fn+0x183/0x220
> [11959.050933]  ? sort_range+0x20/0x20
> [11959.054902]  kthread+0xe6/0x110
> [11959.058479]  ? kthread_complete_and_exit+0x20/0x20
> [11959.063911]  ret_from_fork+0x1f/0x30
> [11959.067982]  </TASK>
> [11959.070489] ---[ 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")

Forgot to add a tag. If the patch looks good, please add the below tag
when you pull the patch.

Reported-by: Ammy Yi <ammy.yi@xxxxxxxxx>

Thanks,
Kan

> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> ---
>  drivers/iommu/intel/iommu.h   |  2 ++
>  drivers/iommu/intel/perfmon.c | 68 ++++++++++++++++++++++-------------
>  include/linux/cpuhotplug.h    |  1 -
>  3 files changed, 46 insertions(+), 25 deletions(-)
> 
> 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..e27bc954e866 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)
> 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,