| Message ID | 20190801122040.26024-1-thunder.leizhen@huawei.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | iommu/arm-smmu-v3: add nr_ats_masters to avoid unnecessary operations | expand |
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@huawei.com> > --- > 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) >
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
On 2019/8/14 1:10, Will Deacon wrote: > 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. Hi, all: I found my patch have some mistake, see below. I'm sorry I didn't see this coupling. I'm preparing v2. > @@ -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); "master->domain = NULL" is needed in arm_smmu_install_ste_for_dev(). > > arm_smmu_disable_ats(master); > + master->domain = NULL; > } > > Will > > . >
Hi, I've been struggling with the memory ordering requirements here. More below. On Thu, Aug 01, 2019 at 08:20:40PM +0800, 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. > > Fixes: 9ce27afc0830 ("iommu/arm-smmu-v3: Add support for PCI ATS") > Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com> > --- > 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; > > 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; This feels wrong to me: the CPU can speculate ahead of time that 'nr_ats_masters' is 0, but we could have a concurrent call to '->attach()' for an ATS-enabled device. Wouldn't it then be possible for the new device to populate its ATC as a result of speculative accesses for the mapping that we're tearing down? The devices lock solves this problem by serialising invalidation with '->attach()/->detach()' operations. John's suggestion of RCU might work better, but I think you'll need to call synchronize_rcu() between adding yourself to the 'devices' list and enabling ATS. What do you think? > 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); Here, we need to make sure that concurrent invalidation sees the updated 'nr_ats_masters' value before ATS is enabled for the device, otherwise we could miss an ATC invalidation. I think the code above gets this guarantee because of the way that ATS is enabled in the STE, which ensures that we issue invalidation commands before making the STE 'live'; this has the side-effect of a write barrier before updating PROD, which I think we also rely on for installing the CD pointer. Put another way: writes are ordered before a subsequent command insertion. Do you agree? If so, I'll add a comment because this is subtle and easily overlooked. > 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); This part is the other way around: now we need to ensure that we don't decrement 'nr_ats_masters' until we've disabled ATS. This works for a number of reasons, none of which are obvious: - The control dependency from completing the prior CMD_SYNCs for tearing down the STE and invalidating the ATC - The spinlock handover from the CMD_SYNCs above - The writel() when poking PCI configuration space in pci_disable_ats() happens to be implemented with a write-write barrier I suppose the control dependency is the most compelling one: we can't let stores out whilst we're awaiting completion of a CMD_SYNC. Put another way: writes are ordered after the completion of a prior CMD_SYNC. But yeah, I need to write this down. > 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; As you mentioned, this is broken. Can you simply drop this hunk completely? Will
On 2019/8/14 19:14, Will Deacon wrote: > Hi, > > I've been struggling with the memory ordering requirements here. More below. > > On Thu, Aug 01, 2019 at 08:20:40PM +0800, 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. >> >> Fixes: 9ce27afc0830 ("iommu/arm-smmu-v3: Add support for PCI ATS") >> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com> >> --- >> 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; >> >> 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; > > This feels wrong to me: the CPU can speculate ahead of time that > 'nr_ats_masters' is 0, but we could have a concurrent call to '->attach()' > for an ATS-enabled device. Wouldn't it then be possible for the new device > to populate its ATC as a result of speculative accesses for the mapping that > we're tearing down? > > The devices lock solves this problem by serialising invalidation with > '->attach()/->detach()' operations. > > John's suggestion of RCU might work better, but I think you'll need to call > synchronize_rcu() between adding yourself to the 'devices' list and enabling > ATS. > > What do you think? I have updated my patch and just sent, below it's my thoughts. - if (!(smmu_domain->smmu->features & ARM_SMMU_FEAT_ATS)) + /* + * The protectiom of spinlock(&iommu_domain->devices_lock) is omitted. + * Because for a given master, its map/unmap operations should only be + * happened after it has been attached and before it has been detached. + * So that, if at least one master need to be atc invalidated, the + * value of smmu_domain->nr_ats_masters can not be zero. + * + * This can alleviate performance loss in multi-core scenarios. + */ + if (!smmu_domain->nr_ats_masters) > >> 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); > > Here, we need to make sure that concurrent invalidation sees the updated > 'nr_ats_masters' value before ATS is enabled for the device, otherwise we > could miss an ATC invalidation. > > I think the code above gets this guarantee because of the way that ATS is > enabled in the STE, which ensures that we issue invalidation commands before > making the STE 'live'; this has the side-effect of a write barrier before > updating PROD, which I think we also rely on for installing the CD pointer. > > Put another way: writes are ordered before a subsequent command insertion. > > Do you agree? If so, I'll add a comment because this is subtle and easily > overlooked. > >> 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); > > This part is the other way around: now we need to ensure that we don't > decrement 'nr_ats_masters' until we've disabled ATS. This works for a > number of reasons, none of which are obvious: > > - The control dependency from completing the prior CMD_SYNCs for tearing > down the STE and invalidating the ATC > > - The spinlock handover from the CMD_SYNCs above > > - The writel() when poking PCI configuration space in pci_disable_ats() > happens to be implemented with a write-write barrier > > I suppose the control dependency is the most compelling one: we can't let > stores out whilst we're awaiting completion of a CMD_SYNC. > > Put another way: writes are ordered after the completion of a prior CMD_SYNC. > > But yeah, I need to write this down. > >> 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; > > As you mentioned, this is broken. Can you simply drop this hunk completely? > > Will > > . >
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; 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; 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)
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. Fixes: 9ce27afc0830 ("iommu/arm-smmu-v3: Add support for PCI ATS") Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com> --- drivers/iommu/arm-smmu-v3.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)