Re: [PATCH net v3] net: mana: Optimize irq affinity for low vcpu configs

[Paolo Abeni <pabeni@redhat.com>]

On 6/1/26 12:27 PM, Shradha Gupta wrote:
> 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.
> 
> 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
> 
> =======================================================
> Throughput Impact(in Gbps, same env)
> =======================================================
> TCP conn	with patch	w/o patch
> 20480		15.65		7.73
> 10240		15.63		8.93
> 8192		15.64		9.69
> 6144		15.64		13.16
> 4096		15.69		15.75
> 2048		15.69		15.83
> 1024		15.71		15.28
> 
> 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>

Why do you consider this patch a fix? To me is a configuration
improvement and should land on net-next.

> @@ -1717,11 +1719,24 @@ static int irq_setup(unsigned int *irqs, unsigned int len, int node,
>  	return 0;
>  }
>  
> +/* should be called with cpus_read_lock() held */

Minor nit: s/should/must/ or just drop the comment, as
`for_each_online_cpu()` usage implies that.

> +static void 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--;
> +	}

As this is another heuristic regarding irq spreading, why don't you
implement that inside irq_setup()?

> @@ -1767,13 +1784,42 @@ static int mana_gd_setup_dyn_irqs(struct pci_dev *pdev, int nvec)
>  	 * 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 = irq_setup(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();
> +		if (nvec > first_unassigned) {

An empty line is needed between the variable declaration and the code.

/P