[PATCH v2] memcpy_flushcache: use cache flusing for larger lengths

[Mikulas Patocka <mpatocka@redhat.com>]

This is the second version of the patch - it adds a test for 
boot_cpu_data.x86_clflush_size. There may be CPUs with different cache 
line size and we don't want to run the 64-byte aligned loop on them.

Mikulas



From: Mikulas Patocka <mpatocka@redhat.com>

memcpy_flushcache: use cache flusing for larger lengths

I tested dm-writecache performance on a machine with Optane nvdimm and it
turned out that for larger writes, cached stores + cache flushing perform
better than non-temporal stores. This is the throughput of dm-writecache
measured with this command:
dd if=/dev/zero of=/dev/mapper/wc bs=64 oflag=direct

block size	512		1024		2048		4096
movnti		496 MB/s	642 MB/s	725 MB/s	744 MB/s
clflushopt	373 MB/s	688 MB/s	1.1 GB/s	1.2 GB/s

We can see that for smaller block, movnti performs better, but for larger
blocks, clflushopt has better performance.

This patch changes the function __memcpy_flushcache accordingly, so that
with size >= 768 it performs cached stores and cache flushing. Note that
we must not use the new branch if the CPU doesn't have clflushopt - in
that case, the kernel would use inefficient "clflush" instruction that has
very bad performance.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 arch/x86/lib/usercopy_64.c |   36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

Index: linux-2.6/arch/x86/lib/usercopy_64.c
===================================================================
--- linux-2.6.orig/arch/x86/lib/usercopy_64.c	2020-03-24 15:15:36.644945091 -0400
+++ linux-2.6/arch/x86/lib/usercopy_64.c	2020-03-30 07:17:51.450290007 -0400
@@ -152,6 +152,42 @@ void __memcpy_flushcache(void *_dst, con
 			return;
 	}
 
+	if (static_cpu_has(X86_FEATURE_CLFLUSHOPT) && size >= 768 && likely(boot_cpu_data.x86_clflush_size == 64)) {
+		while (!IS_ALIGNED(dest, 64)) {
+			asm("movq    (%0), %%r8\n"
+			    "movnti  %%r8,   (%1)\n"
+			    :: "r" (source), "r" (dest)
+			    : "memory", "r8");
+			dest += 8;
+			source += 8;
+			size -= 8;
+		}
+		do {
+			asm("movq    (%0), %%r8\n"
+			    "movq   8(%0), %%r9\n"
+			    "movq  16(%0), %%r10\n"
+			    "movq  24(%0), %%r11\n"
+			    "movq    %%r8,   (%1)\n"
+			    "movq    %%r9,  8(%1)\n"
+			    "movq   %%r10, 16(%1)\n"
+			    "movq   %%r11, 24(%1)\n"
+			    "movq  32(%0), %%r8\n"
+			    "movq  40(%0), %%r9\n"
+			    "movq  48(%0), %%r10\n"
+			    "movq  56(%0), %%r11\n"
+			    "movq    %%r8, 32(%1)\n"
+			    "movq    %%r9, 40(%1)\n"
+			    "movq   %%r10, 48(%1)\n"
+			    "movq   %%r11, 56(%1)\n"
+			    :: "r" (source), "r" (dest)
+			    : "memory", "r8", "r9", "r10", "r11");
+			clflushopt((void *)dest);
+			dest += 64;
+			source += 64;
+			size -= 64;
+		} while (size >= 64);
+	}
+
 	/* 4x8 movnti loop */
 	while (size >= 32) {
 		asm("movq    (%0), %%r8\n"
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