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 14:56:16 -0700
From:   Keith Busch <kbusch@...nel.org>
To:     Jonathan Derrick <jonathan.derrick@...ux.dev>
Cc:     korantwork@...il.com, nirmal.patel@...ux.intel.com,
        lpieralisi@...nel.org, 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 Thu, Dec 22, 2022 at 02:15:20AM -0700, Jonathan Derrick wrote:
> On 12/22/22 12:26 AM, korantwork@...il.com wrote:
> > 
> > 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.

With 12 drives and only 6 CPUs, the bypass mode is going to get more irq
context switching. Sounds like the non-bypass mode is aggregating and
spreading interrupts across the cores better, but there's probably some
cpu:drive count tipping point where performance favors the other way.

The fio jobs could also probably set their cpus_allowed differently to
get better performance in the bypass mode.