[PATCH v4 net] net: mana: Optimize irq affinity for low vcpu configs

[PATCH v4 net] net: mana: Optimize irq affinity for low vcpu configs

[Shradha Gupta]

In mana driver, the number of IRQs allocated is capped by the
min(num_cpu + 1, queue count). In cases, where the IRQ count is greater
than the vcpu count, we want to utilize all the vCPUs, irrespective of
their NUMA/core bindings.

This is important, especially in the envs where number of vCPUs are so
few that the softIRQ handling overhead on two IRQs on the same vCPU is
much more than their overheads if they were spread across sibling vCPUs.

This behaviour is more evident with dynamic IRQ allocation. Since MANA
IRQs are assigned at a later stage compared to static allocation, other
device IRQs may already be affinitized to the vCPUs. As a result, IRQ
weights become imbalanced, causing multiple MANA IRQs to land on the
same vCPU, while some vCPUs have none.

In such cases when many parallel TCP connections are tested, the
throughput drops significantly.

We also studied the results of setting the affinity and hint to
NULL in these cases, and observed that, with this logic if there are
pre existing IRQs allocated on the VM(apart from MANA), during MANA
IRQs allocation, it leads to clustering of the MANA queue IRQs again.
These results can be seen through case 3 in the following data.

Test envs:
=======================================================
Case 1: without this patch
=======================================================
4 vcpu(2 cores), 5 MANA IRQs (1 HWC + 4 Queue)

	TYPE		effective vCPU aff
=======================================================
IRQ0:	HWC		0
IRQ1:	mana_q1		0
IRQ2:	mana_q2		2
IRQ3:	mana_q3		0
IRQ4:	mana_q4		3

%soft on each vCPU(mpstat -P ALL 1) on receiver
vCPU		0	1	2	3
=======================================================
pass 1:		38.85	0.03	24.89	24.65
pass 2:		39.15	0.03	24.57	25.28
pass 3:		40.36	0.03	23.20	23.17

=======================================================
Case 2: with this patch
=======================================================
4 vcpu(2 cores), 5 MANA IRQs (1 HWC + 4 Queue)

        TYPE            effective vCPU aff
=======================================================
IRQ0:   HWC             0
IRQ1:   mana_q1         0
IRQ2:   mana_q2         1
IRQ3:   mana_q3         2
IRQ4:   mana_q4         3

%soft on each vCPU(mpstat -P ALL 1) on receiver
vCPU            0       1       2       3
=======================================================
pass 1:         15.42	15.85	14.99	14.51
pass 2:         15.53	15.94	15.81	15.93
pass 3:         16.41	16.35	16.40	16.36

=======================================================
Case 3: with affinity set to NULL
=======================================================
4 vCPU(2 cores), 5 MANA IRQs (1 HWC + 4 Queue)

	TYPE		effective vCPU aff
=======================================================
IRQ0:	HWC			0
IRQ1:	mana_q1			2
IRQ2:	mana_q2			3
IRQ3:	mana_q3			2
IRQ4:	mana_q4			3

=======================================================
Throughput Impact(in Gbps, same env)
=======================================================
TCP conn	with patch	w/o patch	aff NULL
20480		15.65		7.73		5.25
10240		15.63		8.93		5.77
8192		15.64		9.69		7.16
6144		15.64		13.16		9.33
4096		15.69		15.75		13.50
2048		15.69		15.83		13.61
1024		15.71		15.28		13.60

Fixes: 755391121038 ("net: mana: Allocate MSI-X vectors dynamically")
Cc: stable@vger.kernel.org
Co-developed-by: Erni Sri Satya Vennela <ernis@linux.microsoft.com>
Signed-off-by: Erni Sri Satya Vennela <ernis@linux.microsoft.com>
Signed-off-by: Shradha Gupta <shradhagupta@linux.microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Simon Horman <horms@kernel.org>
---
Changes in v4
 * Add mana prefix on irq_affinity_*() in mana driver
 * Corrected grammar, comment for mana_irq_setup_linear()
 * added new line as per guidelines
 * added case 3 in commit message for when affinity is NULL
---
Changes in v3
 * Optimize the comments in mana_gd_setup_dyn_irqs()
 * add more details in the dev_dbg for extra IRQs
---
Changes in v2
 * Removed the unused skip_first_cpu variable
 * fixed exit condition in irq_setup_linear() with len == 0
 * changed return type of irq_setup_linear() as it will always be 0
 * removed the unnecessary rcu_read_lock() in irq_setup_linear()
 * added appropriate comments to indicate expected behaviour when
   IRQs are more than or equal to num_online_cpus()
---
 .../net/ethernet/microsoft/mana/gdma_main.c   | 78 +++++++++++++++----
 1 file changed, 64 insertions(+), 14 deletions(-)

diff --git a/drivers/net/ethernet/microsoft/mana/gdma_main.c b/drivers/net/ethernet/microsoft/mana/gdma_main.c
index a0fdd052d7f1..e8b7ffb47eb9 100644
--- a/drivers/net/ethernet/microsoft/mana/gdma_main.c
+++ b/drivers/net/ethernet/microsoft/mana/gdma_main.c
@@ -210,6 +210,8 @@ static int mana_gd_query_max_resources(struct pci_dev *pdev)
 	} else {
 		/* If dynamic allocation is enabled we have already allocated
 		 * hwc msi
+		 * Also, we make sure in this case the following is always true
+		 * (num_msix_usable - 1 HWC) <= num_online_cpus()
 		 */
 		gc->num_msix_usable = min(resp.max_msix, num_online_cpus() + 1);
 	}
@@ -1909,8 +1911,8 @@ void mana_gd_free_res_map(struct gdma_resource *r)
  * do the same thing.
  */
 
-static int irq_setup(unsigned int *irqs, unsigned int len, int node,
-		     bool skip_first_cpu)
+static int mana_irq_setup_numa_aware(unsigned int *irqs, unsigned int len,
+				     int node, bool skip_first_cpu)
 {
 	const struct cpumask *next, *prev = cpu_none_mask;
 	cpumask_var_t cpus __free(free_cpumask_var);
@@ -1946,11 +1948,24 @@ static int irq_setup(unsigned int *irqs, unsigned int len, int node,
 	return 0;
 }
 
+/* must be called with cpus_read_lock() held */
+static void mana_irq_setup_linear(unsigned int *irqs, unsigned int len)
+{
+	int cpu;
+
+	for_each_online_cpu(cpu) {
+		if (len == 0)
+			break;
+
+		irq_set_affinity_and_hint(*irqs++, cpumask_of(cpu));
+		len--;
+	}
+}
+
 static int mana_gd_setup_dyn_irqs(struct pci_dev *pdev, int nvec)
 {
 	struct gdma_context *gc = pci_get_drvdata(pdev);
 	struct gdma_irq_context *gic;
-	bool skip_first_cpu = false;
 	int *irqs, err, i, msi;
 
 	irqs = kmalloc_objs(int, nvec);
@@ -1958,10 +1973,12 @@ static int mana_gd_setup_dyn_irqs(struct pci_dev *pdev, int nvec)
 		return -ENOMEM;
 
 	/*
+	 * In this function, num_msix_usable = HWC IRQ + Queue IRQ.
+	 * nvec is only Queue IRQ (HWC already setup).
 	 * While processing the next pci irq vector, we start with index 1,
 	 * as IRQ vector at index 0 is already processed for HWC.
 	 * However, the population of irqs array starts with index 0, to be
-	 * further used in irq_setup()
+	 * further used in mana_irq_setup_numa_aware()
 	 */
 	for (i = 1; i <= nvec; i++) {
 		msi = i;
@@ -1975,18 +1992,51 @@ static int mana_gd_setup_dyn_irqs(struct pci_dev *pdev, int nvec)
 	}
 
 	/*
-	 * When calling irq_setup() for dynamically added IRQs, if number of
-	 * CPUs is more than or equal to allocated MSI-X, we need to skip the
-	 * first CPU sibling group since they are already affinitized to HWC IRQ
+	 * When calling mana_irq_setup_numa_aware() for dynamically added IRQs,
+	 * if number of CPUs is more than or equal to allocated MSI-X, we need to
+	 * skip the first CPU sibling group since they are already affinitized to
+	 * HWC IRQ
 	 */
 	cpus_read_lock();
-	if (gc->num_msix_usable <= num_online_cpus())
-		skip_first_cpu = true;
+	if (gc->num_msix_usable <= num_online_cpus()) {
+		err = mana_irq_setup_numa_aware(irqs, nvec, gc->numa_node,
+						true);
+		if (err) {
+			cpus_read_unlock();
+			goto free_irq;
+		}
+	} else {
+		/*
+		 * When num_msix_usable are more than num_online_cpus, our
+		 * queue IRQs should be equal to num of online vCPUs.
+		 * We try to make sure queue IRQs spread across all vCPUs.
+		 * In such a case NUMA or CPU core affinity does not matter.
+		 * Note: in this case the total mana IRQ should always be
+		 * num_online_cpus + 1. The first HWC IRQ is already handled
+		 * in HWC setup calls
+		 * However, if CPUs went offline since num_msix_usable was
+		 * computed, queue IRQs will be more than num_online_cpus().
+		 * In such cases remaining extra IRQs will retain their default
+		 * affinity.
+		 */
+		int first_unassigned = num_online_cpus();
 
-	err = irq_setup(irqs, nvec, gc->numa_node, skip_first_cpu);
-	if (err) {
-		cpus_read_unlock();
-		goto free_irq;
+		if (nvec > first_unassigned) {
+			char buf[32];
+
+			if (first_unassigned == nvec - 1)
+				snprintf(buf, sizeof(buf), "%d",
+					 first_unassigned);
+			else
+				snprintf(buf, sizeof(buf), "%d-%d",
+					 first_unassigned, nvec - 1);
+
+			dev_dbg(&pdev->dev,
+				"MANA IRQ indices #%s will retain the default CPU affinity\n",
+				buf);
+		}
+
+		mana_irq_setup_linear(irqs, nvec);
 	}
 
 	cpus_read_unlock();
@@ -2041,7 +2091,7 @@ static int mana_gd_setup_irqs(struct pci_dev *pdev, int nvec)
 		nvec -= 1;
 	}
 
-	err = irq_setup(irqs, nvec, gc->numa_node, false);
+	err = mana_irq_setup_numa_aware(irqs, nvec, gc->numa_node, false);
 	if (err) {
 		cpus_read_unlock();
 		goto free_irq;

base-commit: 96e7f9122aae0ed000ee321f324b812a447906d9
-- 
2.34.1