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@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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"
--
dm-devel mailing list
dm-devel@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/dm-devel
