linux-kernel - Re: [PATCH] PCI: vmd: Do not disable MSI-X remapping in VMD 28C0 controller

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Date:   Thu, 22 Dec 2022 02:15:20 -0700
From:   Jonathan Derrick <jonathan.derrick@...ux.dev>
To:     korantwork@...il.com, nirmal.patel@...ux.intel.com,
        lpieralisi@...nel.org
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xinghui Li <korantli@...cent.com>
Subject: Re: [PATCH] PCI: vmd: Do not disable MSI-X remapping in VMD 28C0
 controller



On 12/22/22 12:26 AM, korantwork@...il.com wrote:
> From: Xinghui Li <korantli@...cent.com>
> 
> Commit ee81ee84f873("PCI: vmd: Disable MSI-X remapping when possible")
> disable the vmd MSI-X remapping for optimizing pci performance.However,
> this feature severely negatively optimized performance in multi-disk
> situations.
> 
> In FIO 4K random test, we test 1 disk in the 1 CPU
> 
> when disable MSI-X remapping:
> read: IOPS=1183k, BW=4622MiB/s (4847MB/s)(1354GiB/300001msec)
> READ: bw=4622MiB/s (4847MB/s), 4622MiB/s-4622MiB/s (4847MB/s-4847MB/s),
> io=1354GiB (1454GB), run=300001-300001msec
> 
> When not disable MSI-X remapping:
> read: IOPS=1171k, BW=4572MiB/s (4795MB/s)(1340GiB/300001msec)
> READ: bw=4572MiB/s (4795MB/s), 4572MiB/s-4572MiB/s (4795MB/s-4795MB/s),
> io=1340GiB (1438GB), run=300001-300001msec
> 
> However, the bypass mode could increase the interrupts costs in CPU.
> We test 12 disks in the 6 CPU,
Well the bypass mode was made to improve performance where you have >4 
drives so this is pretty surprising. With bypass mode disabled, VMD will 
intercept and forward interrupts, increasing costs.

I think Nirmal would want to to understand if there's some other factor 
going on here.

> 
> When disable MSI-X remapping:
> read: IOPS=562k, BW=2197MiB/s (2304MB/s)(644GiB/300001msec)
> READ: bw=2197MiB/s (2304MB/s), 2197MiB/s-2197MiB/s (2304MB/s-2304MB/s),
> io=644GiB (691GB), run=300001-300001msec
> 
> When not disable MSI-X remapping:
> read: IOPS=1144k, BW=4470MiB/s (4687MB/s)(1310GiB/300005msec)
> READ: bw=4470MiB/s (4687MB/s), 4470MiB/s-4470MiB/s (4687MB/s-4687MB/s),
> io=1310GiB (1406GB), run=300005-300005msec
> 
> Signed-off-by: Xinghui Li <korantli@...cent.com>
> ---
>   drivers/pci/controller/vmd.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
> index e06e9f4fc50f..9f6e9324d67d 100644
> --- a/drivers/pci/controller/vmd.c
> +++ b/drivers/pci/controller/vmd.c
> @@ -998,8 +998,7 @@ static const struct pci_device_id vmd_ids[] = {
>   		.driver_data = VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP,},
>   	{PCI_DEVICE(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_VMD_28C0),
>   		.driver_data = VMD_FEAT_HAS_MEMBAR_SHADOW |
> -				VMD_FEAT_HAS_BUS_RESTRICTIONS |
> -				VMD_FEAT_CAN_BYPASS_MSI_REMAP,},
> +				VMD_FEAT_HAS_BUS_RESTRICTIONS,},
>   	{PCI_DEVICE(PCI_VENDOR_ID_INTEL, 0x467f),
>   		.driver_data = VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP |
>   				VMD_FEAT_HAS_BUS_RESTRICTIONS |