[PATCH 1/2] x86: don't unnecessarily call dma_alloc_from_contiguous()

Akinobu Mita
2014-09-28T15:52:30Z

If CONFIG_DMA_CMA is enabled, dma_generic_alloc_coherent() tries to
allocate memory region by dma_alloc_from_contiguous() before trying to
use alloc_pages().

This wastes CMA region by small DMA-coherent buffers which can be
allocated by alloc_pages(). And it also causes performance degradation,
as this is trying to drive _all_ dma mapping allocations through a
_very_ small window, reported by Peter Hurley.

This fixes it by trying to allocate by alloc_pages() first in
dma_generic_alloc_coherent() as dma_alloc_from_contiguous should be
called only for huge allocation.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reported-by: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: x86@kernel.org
Cc: iommu@lists.linux-foundation.org
---
arch/x86/kernel/pci-dma.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
index a25e202..0402266 100644
--- a/arch/x86/kernel/pci-dma.c
+++ b/arch/x86/kernel/pci-dma.c
@@ -99,20 +99,20 @@ void *dma_generic_alloc_coherent(struct device *dev, size_t size,

flag &= ~__GFP_ZERO;
again:
- page = NULL;
+ page = alloc_pages_node(dev_to_node(dev), flag | __GFP_NOWARN,
+ get_order(size));
/* CMA can be used only in the context which permits sleeping */
- if (flag & __GFP_WAIT) {
+ if (!page && (flag & __GFP_WAIT)) {
page = dma_alloc_from_contiguous(dev, count, get_order(size));
if (page && page_to_phys(page) + size > dma_mask) {
dma_release_from_contiguous(dev, page, count);
page = NULL;
}
}
- /* fallback */
- if (!page)
- page = alloc_pages_node(dev_to_node(dev), flag, get_order(size));
- if (!page)
+ if (!page) {
+ warn_alloc_failed(flag, get_order(size), NULL);
return NULL;
+ }

addr = page_to_phys(page);
if (addr + size > dma_mask) {
--
1.9.1