Re: [PATCH] iommu/arm-smmu-v3: add nr_ats_masters to avoid unnecessary operations

[John Garry]

On 01/08/2019 13:20, Zhen Lei wrote:
> When (smmu_domain->smmu->features & ARM_SMMU_FEAT_ATS) is true, even if a
> smmu domain does not contain any ats master, the operations of
> arm_smmu_atc_inv_to_cmd() and lock protection in arm_smmu_atc_inv_domain()
> are always executed. This will impact performance, especially in
> multi-core and stress scenarios. For my FIO test scenario, about 8%
> performance reduced.
>
> In fact, we can use a atomic member to record how many ats masters the
> smmu contains. And check that without traverse the list and check all
> masters one by one in the lock protection.

Hi Will, Robin, Jean-Philippe,

Can you kindly check this issue? We have seen a signifigant performance 
regression here.

Thanks!

> Fixes: 9ce27afc0830 ("iommu/arm-smmu-v3: Add support for PCI ATS")
> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> ---
>  drivers/iommu/arm-smmu-v3.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index a9a9fabd396804a..1b370d9aca95f94 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -631,6 +631,7 @@ struct arm_smmu_domain {
>
>  	struct io_pgtable_ops		*pgtbl_ops;
>  	bool				non_strict;
> +	atomic_t			nr_ats_masters;


It's not ideal to keep a separate count of ats masters...hmmm

>  	enum arm_smmu_domain_stage	stage;
>  	union {
> @@ -1531,7 +1532,7 @@ static int arm_smmu_atc_inv_domain(struct arm_smmu_domain *smmu_domain,
>  	struct arm_smmu_cmdq_ent cmd;
>  	struct arm_smmu_master *master;
>
> -	if (!(smmu_domain->smmu->features & ARM_SMMU_FEAT_ATS))
> +	if (!atomic_read(&smmu_domain->nr_ats_masters))
>  		return 0;

The rest of the code is here:

	arm_smmu_atc_inv_to_cmd(ssid, iova, size, &cmd);

	spin_lock_irqsave(&smmu_domain->devices_lock, flags);
	list_for_each_entry(master, &smmu_domain->devices, domain_head)
		ret |= arm_smmu_atc_inv_master(master, &cmd);
	spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);

	return ret ? -ETIMEDOUT : 0;
}

Not directly related to leizhen's issue: Could RCU protection be used 
for this list iteration? I can't imagine that the devices list changes 
often. And also we already protect the cmdq in arm_smmu_atc_inv_master().

>  	arm_smmu_atc_inv_to_cmd(ssid, iova, size, &cmd);
> @@ -1869,6 +1870,7 @@ static int arm_smmu_enable_ats(struct arm_smmu_master *master)
>  	size_t stu;
>  	struct pci_dev *pdev;
>  	struct arm_smmu_device *smmu = master->smmu;
> +	struct arm_smmu_domain *smmu_domain = master->domain;
>  	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(master->dev);
>
>  	if (!(smmu->features & ARM_SMMU_FEAT_ATS) || !dev_is_pci(master->dev) ||
> @@ -1887,12 +1889,15 @@ static int arm_smmu_enable_ats(struct arm_smmu_master *master)
>  		return ret;
>
>  	master->ats_enabled = true;
> +	atomic_inc(&smmu_domain->nr_ats_masters);
> +
>  	return 0;
>  }
>
>  static void arm_smmu_disable_ats(struct arm_smmu_master *master)
>  {
>  	struct arm_smmu_cmdq_ent cmd;
> +	struct arm_smmu_domain *smmu_domain = master->domain;
>
>  	if (!master->ats_enabled || !dev_is_pci(master->dev))
>  		return;
> @@ -1901,6 +1906,7 @@ static void arm_smmu_disable_ats(struct arm_smmu_master *master)
>  	arm_smmu_atc_inv_master(master, &cmd);
>  	pci_disable_ats(to_pci_dev(master->dev));
>  	master->ats_enabled = false;
> +	atomic_dec(&smmu_domain->nr_ats_masters);
>  }
>
>  static void arm_smmu_detach_dev(struct arm_smmu_master *master)
> @@ -1915,10 +1921,10 @@ static void arm_smmu_detach_dev(struct arm_smmu_master *master)
>  	list_del(&master->domain_head);
>  	spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);
>
> -	master->domain = NULL;
>  	arm_smmu_install_ste_for_dev(master);
>
>  	arm_smmu_disable_ats(master);
> +	master->domain = NULL;
>  }
>
>  static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)