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

[Will Deacon]

On Mon, Aug 12, 2019 at 11:42:17AM +0100, John Garry wrote:
> 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.

Sorry, John: Robin and Jean-Philippe are off at the moment and I've been
swamped dealing with the arm64 queue. I'll try to get to this tomorrow.

Will