linux-kernel - Re: [PATCH] swiotlb: add kernel error message to track iobtlb overflow buffer usage

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Message-Id: <20120904162804.baad7cc0.akpm@linux-foundation.org>
Date:	Tue, 4 Sep 2012 16:28:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	shuah.khan@...com
Cc:	fujita.tomonori@....ntt.co.jp, paul.gortmaker@...driver.com,
	bhelgaas@...gle.com, amwang@...hat.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-kernel@...r.kernel.org, shuahkhan@...il.com
Subject: Re: [PATCH] swiotlb: add kernel error message to track iobtlb
 overflow buffer usage

On Fri, 31 Aug 2012 18:55:59 -0600
Shuah Khan <shuah.khan@...com> wrote:

> Add kernel error message to track iotlb overflow buffer triggers to understand
> how often the overflow buffer gets used.
> 
> Signed-off-by: Shuah Khan <shuah.khan@...com>
> ---
>  lib/swiotlb.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/lib/swiotlb.c b/lib/swiotlb.c
> index 45bc1f8..597d7b9 100644
> --- a/lib/swiotlb.c
> +++ b/lib/swiotlb.c
> @@ -682,6 +682,7 @@ dma_addr_t swiotlb_map_page(struct device *dev, struct page *page,
>  	if (!map) {
>  		swiotlb_full(dev, size, dir, 1);
>  		map = io_tlb_overflow_buffer;
> +		dev_err(dev, "SWIOTLB is full. Mapping overflow buffer.\n");
>  	}
>  
>  	dev_addr = swiotlb_virt_to_bus(dev, map);

hm.  I would view this as a developer-only debugging thing which
shouldn't be present in production code.  Plus there's a potential that
some poor person will have his logs flooded with this message.

A more typical way of handling this would be to increment a stats
counter, accessible in procfs or debugfs.

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