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Date:   Fri, 7 Apr 2023 12:55:35 +0200
From:   Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To:     Michael Kelley <mikelley@microsoft.com>
Cc:     hch@lst.de, m.szyprowski@samsung.com, robin.murphy@arm.com,
        decui@microsoft.com, tiala@microsoft.com, iommu@lists.linux.dev,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/1] swiotlb: Track and report io_tlb_used high water
 mark in debugfs
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On Fri, 31 Mar 2023 21:45:00 -0700
Michael Kelley <mikelley@microsoft.com> 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.
> 
> To support better understanding of swiotlb usage, add tracking of the
> the high water mark usage of the default swiotlb bounce buffer memory
> pool. Report the high water mark in debugfs along with the other swiotlb
> metrics.  Allow the high water mark to be reset to zero at runtime by
> writing to it.
> 
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> ---
> Changes in v3:
> * Do high water mark accounting only when CONFIG_DEBUG_FS=y. As
>   as a result, add back the mem_used() function for the "swiotlb
>   buffer is full" error message. [Christoph -- I didn't hear back
>   whether this approach addresses your concern about one additional
>   atomic operation when slots are allocated and again when freed. I've
>   gone ahead with this new version, and we can obviously have further
>   discussion.]
> 
> * Remove unnecessary u64 casts. [Christoph Hellwig]
> 
> * Track slot usage and the high water mark only for io_tlb_default_mem.
>   Previous versions incorrectly included per-device pools. [Petr Tesarik]
> 
> Changes in v2:
> * Only reset the high water mark to zero when the specified new value
>   is zero, to prevent confusion about the ability to reset to some
>   other value [Dexuan Cui]
> 
>  kernel/dma/swiotlb.c | 41 ++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index d3d6be0..6587a3d 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -76,6 +76,9 @@ struct io_tlb_slot {
>  static unsigned long default_nslabs = IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT;
>  static unsigned long default_nareas;
>  
> +static atomic_long_t total_used = ATOMIC_LONG_INIT(0);
> +static atomic_long_t used_hiwater = ATOMIC_LONG_INIT(0);
> +
>  /**
>   * struct io_tlb_area - IO TLB memory area descriptor
>   *
> @@ -594,6 +597,7 @@ static int swiotlb_do_find_slots(struct device *dev, int area_index,
>  	unsigned long flags;
>  	unsigned int slot_base;
>  	unsigned int slot_index;
> +	unsigned long old_hiwater, new_used;
>  
>  	BUG_ON(!nslots);
>  	BUG_ON(area_index >= mem->nareas);
> @@ -663,6 +667,17 @@ static int swiotlb_do_find_slots(struct device *dev, int area_index,
>  		area->index = 0;
>  	area->used += nslots;
>  	spin_unlock_irqrestore(&area->lock, flags);
> +
> +	if (IS_ENABLED(CONFIG_DEBUG_FS) && mem == &io_tlb_default_mem) {

Yes, this works fine now, but why are total_used and used_hiwater
global variables? If you make them fields in struct io_tlb_mem
(possibly guarded with #ifdef CONFIG_DEBUG_FS), you can get rid of the
check. Of course, in instances other than io_tlb_default_mem these
fields would not be exported to userspace through debugfs, but if really
needed, I can at least find them in a crash dump (or read them through
/proc/kcore).

Petr T