Re: Regression 5.12.0-rc4 net: ice: significant throughput drop

[Robin Murphy <robin.murphy@arm.com>]

On 2021-06-01 07:57, Daniel Borkmann wrote:
> [ ping Robin / Joerg, +Cc Christoph ]

Sorry, I was off on Friday on top of the Bank Holiday yesterday.

> On 5/28/21 10:34 AM, Jussi Maki wrote:
>> Hi all,
>>
>> While measuring the impact of a kernel patch on our lab machines I 
>> stumbled upon
>> a performance regression affecting the 100Gbit ICE nic and bisected it
>> from range v5.11.1..v5.13-rc3 to the commit:
>> a250c23f15c2 iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE
>>
>> Both recent bpf-next (d6a6a55518) and linux-stable (c4681547bc) are
>> affected by the issue.
>>
>> The regression shows as a significant drop in throughput as measured
>> with "super_netperf" [0],
>> with measured bandwidth of ~95Gbps before and ~35Gbps after:

I guess that must be the difference between using the flush queue
vs. strict invalidation. On closer inspection, it seems to me that
there's a subtle pre-existing bug in the AMD IOMMU driver, in that
amd_iommu_init_dma_ops() actually runs *after* amd_iommu_init_api()
has called bus_set_iommu(). Does the patch below work?

Robin.

----->8-----

Subject: [PATCH] iommu/amd: Tidy up DMA ops init

Now that DMA ops are part of the core API via iommu-dma, fold the
vestigial remains of the IOMMU_DMA_OPS init state into the IOMMU API
phase, and clean up a few other leftovers. This should also close the
race window wherein bus_set_iommu() effectively makes the DMA ops state
visible before its nominal initialisation, which since commit
a250c23f15c2 ("iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE") can now
lead to the wrong flush queue policy being picked.

Reported-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
---
  drivers/iommu/amd/amd_iommu.h |  2 --
  drivers/iommu/amd/init.c      |  5 -----
  drivers/iommu/amd/iommu.c     | 29 ++++++++++++-----------------
  3 files changed, 12 insertions(+), 24 deletions(-)

diff --git a/drivers/iommu/amd/amd_iommu.h b/drivers/iommu/amd/amd_iommu.h
index 55dd38d814d9..416815a525d6 100644
--- a/drivers/iommu/amd/amd_iommu.h
+++ b/drivers/iommu/amd/amd_iommu.h
@@ -11,8 +11,6 @@
  
  #include "amd_iommu_types.h"
  
-extern int amd_iommu_init_dma_ops(void);
-extern int amd_iommu_init_passthrough(void);
  extern irqreturn_t amd_iommu_int_thread(int irq, void *data);
  extern irqreturn_t amd_iommu_int_handler(int irq, void *data);
  extern void amd_iommu_apply_erratum_63(u16 devid);
diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index d006724f4dc2..a418bf560a4b 100644
--- a/drivers/iommu/amd/init.c
+++ b/drivers/iommu/amd/init.c
@@ -231,7 +231,6 @@ enum iommu_init_state {
  	IOMMU_ENABLED,
  	IOMMU_PCI_INIT,
  	IOMMU_INTERRUPTS_EN,
-	IOMMU_DMA_OPS,
  	IOMMU_INITIALIZED,
  	IOMMU_NOT_FOUND,
  	IOMMU_INIT_ERROR,
@@ -2895,10 +2894,6 @@ static int __init state_next(void)
  		init_state = ret ? IOMMU_INIT_ERROR : IOMMU_INTERRUPTS_EN;
  		break;
  	case IOMMU_INTERRUPTS_EN:
-		ret = amd_iommu_init_dma_ops();
-		init_state = ret ? IOMMU_INIT_ERROR : IOMMU_DMA_OPS;
-		break;
-	case IOMMU_DMA_OPS:
  		init_state = IOMMU_INITIALIZED;
  		break;
  	case IOMMU_INITIALIZED:
diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
index 80e8e1916dd1..20f7d141ea53 100644
--- a/drivers/iommu/amd/iommu.c
+++ b/drivers/iommu/amd/iommu.c
@@ -30,7 +30,6 @@
  #include <linux/msi.h>
  #include <linux/irqdomain.h>
  #include <linux/percpu.h>
-#include <linux/iova.h>
  #include <linux/io-pgtable.h>
  #include <asm/irq_remapping.h>
  #include <asm/io_apic.h>
@@ -1771,13 +1770,22 @@ void amd_iommu_domain_update(struct protection_domain *domain)
  	amd_iommu_domain_flush_complete(domain);
  }
  
+static void __init amd_iommu_init_dma_ops(void)
+{
+	swiotlb = (iommu_default_passthrough() || sme_me_mask) ? 1 : 0;
+
+	if (amd_iommu_unmap_flush)
+		pr_info("IO/TLB flush on unmap enabled\n");
+	else
+		pr_info("Lazy IO/TLB flushing enabled\n");
+	iommu_set_dma_strict(amd_iommu_unmap_flush);
+}
+
  int __init amd_iommu_init_api(void)
  {
  	int ret, err = 0;
  
-	ret = iova_cache_get();
-	if (ret)
-		return ret;
+	amd_iommu_init_dma_ops();
  
  	err = bus_set_iommu(&pci_bus_type, &amd_iommu_ops);
  	if (err)
@@ -1794,19 +1802,6 @@ int __init amd_iommu_init_api(void)
  	return 0;
  }
  
-int __init amd_iommu_init_dma_ops(void)
-{
-	swiotlb        = (iommu_default_passthrough() || sme_me_mask) ? 1 : 0;
-
-	if (amd_iommu_unmap_flush)
-		pr_info("IO/TLB flush on unmap enabled\n");
-	else
-		pr_info("Lazy IO/TLB flushing enabled\n");
-	iommu_set_dma_strict(amd_iommu_unmap_flush);
-	return 0;
-
-}
-
  /*****************************************************************************
   *
   * The following functions belong to the exported interface of AMD IOMMU
-- 
2.21.0.dirty

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