linux-kernel - [PATCH 1/2] x86: don't unnecessarily call dma_alloc_from

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Date:	Mon, 29 Sep 2014 00:52:03 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Cc:	Akinobu Mita <akinobu.mita@...il.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Don Dutile <ddutile@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Yinghai Lu <yinghai@...nel.org>, x86@...nel.org,
	iommu@...ts.linux-foundation.org
Subject: [PATCH 1/2] x86: don't unnecessarily call dma_alloc_from_contiguous()

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@...il.com>
Reported-by: Peter Hurley <peter@...leysoftware.com>
Cc: Peter Hurley <peter@...leysoftware.com>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc: David Woodhouse <dwmw2@...radead.org>
Cc: Don Dutile <ddutile@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Andi Kleen <andi@...stfloor.org>
Cc: Yinghai Lu <yinghai@...nel.org>
Cc: x86@...nel.org
Cc: iommu@...ts.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

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