linux-kernel - Re: [PATCH 1/1] swiotlb: Track and report io_tlb

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Date:   Wed, 22 Mar 2023 15:32:30 +0100
From:   Petr Tesarik <petrtesarik@...weicloud.com>
To:     Michael Kelley <mikelley@...rosoft.com>, hch@....de,
        m.szyprowski@...sung.com, robin.murphy@....com,
        decui@...rosoft.com, tiala@...rosoft.com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] swiotlb: Track and report io_tlb_used high water mark
 in debugfs

Hi,

On 2/27/2023 5:41 PM, Michael Kelley wrote:
> swiotlb currently reports the total number of slabs and the instantaneous
> in-use slabs in debugfs. But with increased usage of swiotlb for all I/O
> in Confidential Computing (coco) VMs, it has become difficult to know
> how much memory to allocate for swiotlb bounce buffers, either via the
> automatic algorithm in the kernel or by specifying a value on the
> kernel boot line. The current automatic algorithm generously allocates
> swiotlb bounce buffer memory, and may be wasting significant memory in
> many use cases.

I like the idea. Tracking and exposing the high watermark is definitely a step in the right direction. I would also appreciate an indicator of fragmentation, but that can be implemented later.

However, you apparently have a scenario where swiotlb is used intensely. Are you able to measure swiotlb performance? If yes, can you suggest a suitable method? I'm asking for my work on making swiotlb more flexible (able to grow and shrink).

Petr T