Re: [PATCH v2 1/1] swiotlb: Track and report io_tlb_used high water mark in debugfs

[Christoph Hellwig <hch@infradead.org>]

On Sat, Mar 25, 2023 at 10:53:10AM -0700, Michael Kelley wrote:
> @@ -659,6 +663,14 @@ static int swiotlb_do_find_slots(struct device *dev, int area_index,
>  	area->index = wrap_area_index(mem, index + nslots);
>  	area->used += nslots;
>  	spin_unlock_irqrestore(&area->lock, flags);
> +
> +	new_used = atomic_long_add_return(nslots, &total_used);
> +	old_hiwater = atomic_long_read(&used_hiwater);
> +	do {
> +		if (new_used <= old_hiwater)
> +			break;
> +	} while (!atomic_long_try_cmpxchg(&used_hiwater, &old_hiwater, new_used));
> +
>  	return slot_index;

Hmm, so we're right in the swiotlb hot path here and add two new global
atomics?

>  static int io_tlb_used_get(void *data, u64 *val)
>  {
> -	*val = mem_used(&io_tlb_default_mem);
> +	*val = (u64)atomic_long_read(&total_used);
>  	return 0;
>  }
> +
> +static int io_tlb_hiwater_get(void *data, u64 *val)
> +{
> +	*val = (u64)atomic_long_read(&used_hiwater);

I can't see how these casts would be needed.