Re: [PATCH] Revert "arm64: Increase the max granular size"

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Mar 18, 2016 at 09:05:37PM +0000, Chalamarla, Tirumalesh wrote:
> On 3/16/16, 2:32 AM, "linux-arm-kernel on behalf of Ganesh Mahendran" <linux-arm-kernel-bounces@lists.infradead.org on behalf of opensource.ganesh@gmail.com> wrote:
> >Reverts commit 97303480753e ("arm64: Increase the max granular size").
> >
> >The commit 97303480753e ("arm64: Increase the max granular size") will
> >degrade system performente in some cpus.
> >
> >We test wifi network throughput with iperf on Qualcomm msm8996 CPU:
> >----------------
> >run on host:
> >  # iperf -s
> >run on device:
> >  # iperf -c <device-ip-addr> -t 100 -i 1
> >----------------
> >
> >Test result:
> >----------------
> >with commit 97303480753e ("arm64: Increase the max granular size"):
> >    172MBits/sec
> >
> >without commit 97303480753e ("arm64: Increase the max granular size"):
> >    230MBits/sec
> >----------------
> >
> >Some module like slab/net will use the L1_CACHE_SHIFT, so if we do not
> >set the parameter correctly, it may affect the system performance.
> >
> >So revert the commit.
> 
> Is there any explanation why is this so? May be there is an
> alternative to this, apart from reverting the commit.

I agree we need an explanation but in the meantime, this patch has
caused a regression on certain systems.

> Until now it seems L1_CACHE_SHIFT is the max of supported chips. But
> now we are making it 64byte, is there any reason why not 32. 

We may have to revisit this logic and consider L1_CACHE_BYTES the
_minimum_ of cache line sizes in arm64 systems supported by the kernel.
Do you have any benchmarks on Cavium boards that would show significant
degradation with 64-byte L1_CACHE_BYTES vs 128?

For non-coherent DMA, the simplest is to make ARCH_DMA_MINALIGN the
_maximum_ of the supported systems:

diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
index 5082b30bc2c0..4b5d7b27edaf 100644
--- a/arch/arm64/include/asm/cache.h
+++ b/arch/arm64/include/asm/cache.h
@@ -18,17 +18,17 @@
 
 #include <asm/cachetype.h>
 
-#define L1_CACHE_SHIFT		7
+#define L1_CACHE_SHIFT		6
 #define L1_CACHE_BYTES		(1 << L1_CACHE_SHIFT)
 
 /*
  * Memory returned by kmalloc() may be used for DMA, so we must make
- * sure that all such allocations are cache aligned. Otherwise,
- * unrelated code may cause parts of the buffer to be read into the
- * cache before the transfer is done, causing old data to be seen by
- * the CPU.
+ * sure that all such allocations are aligned to the maximum *known*
+ * cache line size on ARMv8 systems. Otherwise, unrelated code may cause
+ * parts of the buffer to be read into the cache before the transfer is
+ * done, causing old data to be seen by the CPU.
  */
-#define ARCH_DMA_MINALIGN	L1_CACHE_BYTES
+#define ARCH_DMA_MINALIGN	(128)
 
 #ifndef __ASSEMBLY__
 
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 392c67eb9fa6..30bafca1aebf 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -976,9 +976,9 @@ void __init setup_cpu_features(void)
 	if (!cwg)
 		pr_warn("No Cache Writeback Granule information, assuming cache line size %d\n",
 			cls);
-	if (L1_CACHE_BYTES < cls)
-		pr_warn("L1_CACHE_BYTES smaller than the Cache Writeback Granule (%d < %d)\n",
-			L1_CACHE_BYTES, cls);
+	if (ARCH_DMA_MINALIGN < cls)
+		pr_warn("ARCH_DMA_MINALIGN smaller than the Cache Writeback Granule (%d < %d)\n",
+			ARCH_DMA_MINALIGN, cls);
 }
 
 static bool __maybe_unused