linux-kernel - Re: [2.6.35-rc3] select useful number of entries for DMA debugging...

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Date:	Sun, 11 Jul 2010 14:00:03 +0100
From:	Daniel J Blueman <daniel.blueman@...il.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [2.6.35-rc3] select useful number of entries for DMA debugging...

On 11 July 2010 12:54, Daniel J Blueman <daniel.blueman@...il.com> wrote:
> On 10 July 2010 16:43, Joerg Roedel <joro@...tes.org> wrote:
>> On Sat, Jul 10, 2010 at 03:52:04PM +0100, Daniel J Blueman wrote:
>>> On 9 July 2010 22:33, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>>> > On Mon, Jul 5, 2010 at 1:35 AM, Daniel J Blueman
>>> > <daniel.blueman@...il.com> wrote:
>>> >> When booting 2.6.35-rc3 on some different x86 boxes with DMA debugging
>>> >> enabled, I've consistently seen it exhaust the allocated entries during
>>> >> boot, giving 'DMA-API: debugging out of memory - disabling'.
>>> >>
>>> >> Increase number of entries to allow DMA debugging again.
>>> >
>>> > Rather than increase the default that gets allocated whenever anybody
>>> > enables the DMA debugging, I'd really prefer to see people use the
>>> > kernel command line option if they run out. After all, it's a (pretty
>>> > esoteric) debug option, and the number of required entries depends on
>>> > machine configuration. I'd rather not make the default cover a huge
>>> > number, when you could just add
>>> >
>>> >   dma_debug_entries=65536
>>> >
>>> > on the kernel boot command line instead for machines that want/need it..
>>>
>>> That said, I am seeing the DMA pool exhaust on a single socket Core i5
>>> system with Intel graphics and no other adapters - seems like a fairly
>>> common case. If eg 25% of developers will be using similar to this,
>>> maybe it's good to make DMA debugging less immediately esoteric?
>>>
>>> On the other hand, I would immediately agree if the exhaustion
>>> occurred on an atypical setup.
>>
>> How much memory do you have in this machine? We could probably make the
>> number of pre-allocated entries dependent on the memory available in the
>> machine like Ingo suggested some time ago to avoid such problems.
>
> I have 4GB. Since this change is specific to x86, I guess the only
> corner case we need to protect from this change is developers on small
> x86 embedded systems such as MIDs, so lowering the allocated size on
> <1GB systems makes sense.

How about something like this? If you think ~18800 entries on a 1GB
system is better, let me know and I'll quickly respin. The calculation
seems reasonable for other arches.

When booting 2.6.35-rc3 on some different x86 boxes with DMA debugging
enabled, I've seen consistent exhaustion of the allocated entries during
boot, giving 'DMA-API: debugging out of memory - disabling'.

Increase the default number of entries to allow DMA debugging again, but
assign a reasonable lower limit for systems with less memory (eg ~37600
entries on a 1GB system), to prevent excessive use.

Signed-off-by: Daniel J Blueman <daniel.blueman@...il.com>

diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
index a4ac764..0766fcf 100644
--- a/arch/x86/kernel/pci-dma.c
+++ b/arch/x86/kernel/pci-dma.c
@@ -52,7 +52,7 @@ struct device x86_dma_fallback_dev = {
 EXPORT_SYMBOL(x86_dma_fallback_dev);

 /* Number of entries preallocated for DMA-API debugging */
-#define PREALLOC_DMA_DEBUG_ENTRIES       32768
+#define PREALLOC_DMA_DEBUG_ENTRIES       65536

 int dma_set_mask(struct device *dev, u64 mask)
 {
diff --git a/lib/dma-debug.c b/lib/dma-debug.c
index ba8b670..2d1f965 100644
--- a/lib/dma-debug.c
+++ b/lib/dma-debug.c
@@ -736,6 +736,10 @@ void dma_debug_init(u32 num_entries)

 	if (req_entries)
 		num_entries = req_entries;
+	else
+		/* for systems with less memory, limit to a proportional amount of memory */
+		/* eg 1GB memory, 4KB pages ~ 37600 entries */
+		num_entries = min(num_entries, (unsigned int)(totalram_pages >>
(PAGE_SHIFT - 10)));

 	if (prealloc_memory(num_entries) != 0) {
 		pr_err("DMA-API: debugging out of memory error - disabled\n");
-- 
Daniel J Blueman
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