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

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Message-ID: <68b40228-d2e6-c63c-751b-972bfea93f41@linux.intel.com>
Date:   Thu, 9 Feb 2023 16:57:59 -0700
From:   "Patel, Nirmal" <nirmal.patel@...ux.intel.com>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Xinghui Li <korantwork@...il.com>,
        Jonathan Derrick <jonathan.derrick@...ux.dev>,
        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 2/9/2023 4:05 PM, Keith Busch wrote:
> On Tue, Feb 07, 2023 at 01:32:20PM -0700, Patel, Nirmal wrote:
>> On 2/6/2023 8:18 PM, Xinghui Li wrote:
>>> Keith Busch <kbusch@...nel.org> 于2023年2月7日周二 02:28写道：
>>>> I suspect bypass is the better choice if "num_active_cpus() > pci_msix_vec_count(vmd->dev)".
>>> For this situation, My speculation is that the PCIE nodes are
>>> over-mounted and not just because of the CPU to Drive ratio.
>>> We considered designing online nodes, because we were concerned that
>>> the IO of different chunk sizes would adapt to different MSI-X modes.
>>> I privately think that it may be logically complicated if programmatic
>>> judgments are made.
>> Also newer CPUs have more MSIx (128) which means we can still have
>> better performance without bypass. It would be better if user have
>> can chose module parameter based on their requirements. Thanks.
> So what? More vectors just pushes the threshold to when bypass becomes
> relevant, which is exactly why I suggested it. There has to be an empirical
> answer to when bypass beats muxing. Why do you want a user tunable if there's a
> verifiable and automated better choice?

Make sense about the automated choice. I am not sure what is the exact
tipping point. The commit message includes only two cases. one 1 drive
1 CPU and second 12 drives 6 CPU. Also performance gets worse from 8
drives to 12 drives.
One the previous comments also mentioned something about FIO changing
cpus_allowed; will there be an issue when VMD driver decides to bypass
the remapping during the boot up, but FIO job changes the cpu_allowed?