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

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Message-ID: <Y+V8O9kDH5ZXWdBF@kbusch-mbp.dhcp.thefacebook.com>
Date:   Thu, 9 Feb 2023 16:05:31 -0700
From:   Keith Busch <kbusch@...nel.org>
To:     "Patel, Nirmal" <nirmal.patel@...ux.intel.com>
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 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?