netdev - Re: AMD IOMMU problem after NIC uses multi-page allocation

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Message-ID: <ZCU9KZMlGMWb2ezZ@8bytes.org>
Date:   Thu, 30 Mar 2023 09:41:29 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Jakub Kicinski <kuba@...nel.org>,
        Vasant Hegde <vasant.hegde@....com>,
        Robin Murphy <robin.murphy@....com>
Cc:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        iommu@...ts.linux.dev,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Saeed Mahameed <saeed@...nel.org>
Subject: Re: AMD IOMMU problem after NIC uses multi-page allocation

Also adding Vasant and Robin.

Vasant, Robin, any idea?

On Wed, Mar 29, 2023 at 06:14:07PM -0700, Jakub Kicinski wrote:
> Hi Joerg, Suravee,
> 
> I see an odd NIC behavior with AMD IOMMU in lazy mode (on 5.19).
> 
> The NIC allocates a buffer for Rx packets which is MTU rounded up 
> to page size. If I run it with 1500B MTU or 9000 MTU everything is
> fine, slight but manageable perf hit.
> 
> But if I flip the MTU to 9k, run some traffic and then go back to 1.5k 
> - 70%+ of CPU cycles are spent in alloc_iova (and children).
> 
> Does this ring any bells?