On Fri, 17 Apr 2020, Thomas Gleixner wrote:
> Dan Williams <[email protected]> writes:
>
> > The goal of naming it _inatomic() was specifically for the observation
> > that your driver coordinates atomic access and does not benefit from
> > the cache friendliness that non-temporal stores afford. That said
> > _inatomic() is arguably not a good choice either because that refers
> > to whether the copy is prepared to take a fault or not. What about
> > _exclusive() or _single()? Anything but _clflushopt() that conveys no
> > contextual information.
OK. I renamed it to memcpy_flushcache_single
> > Other than quibbling with the name, and one more comment below, this
> > looks ok to me.
> >
> >> Index: linux-2.6/drivers/md/dm-writecache.c
> >> ===================================================================
> >> --- linux-2.6.orig/drivers/md/dm-writecache.c 2020-04-17
> >> 14:06:35.139999000 +0200
> >> +++ linux-2.6/drivers/md/dm-writecache.c 2020-04-17
> >> 14:06:35.129999000 +0200
> >> @@ -1166,7 +1166,10 @@ static void bio_copy_block(struct dm_wri
> >> }
> >> } else {
> >> flush_dcache_page(bio_page(bio));
> >> - memcpy_flushcache(data, buf, size);
> >> + if (likely(size > 512))
> >
> > This needs some reference to how this magic number is chosen and how a
> > future developer might determine whether the value needs to be
> > adjusted.
>
> I don't think it's a good idea to make this decision in generic code as
> architectures or even CPU models might have different constraints on the
> size.
>
> So I'd rather let the architecture implementation decide and make this
>
> flush_dcache_page(bio_page(bio));
> - memcpy_flushcache(data, buf, size);
> + memcpy_flushcache_bikesheddedname(data, buf, size);
>
> and have the default fallback memcpy_flushcache() and let the
> architecture sort the size limit and the underlying technology out.
>
> So x86 can use clflushopt or implement it with movdir64b and any other
> architecture can provide their own magic soup without changing the
> callsite.
>
> Thanks,
>
> tglx
OK - so I moved the decision to memcpy_flushcache_single and I added a
comment that explains the magic number.
Mikulas
From: Mikulas Patocka <[email protected]>
Implement the function memcpy_flushcache_single which flushes cache just
like memcpy_flushcache - except that it uses cached writes and explicit
cache flushing instead of non-temporal stores.
Explicit cache flushing performs better in singlethreaded cases (i.e. the
dm-writecache target with block size greater than 512), non-temporal
stores perform better in other cases (mostly multithreaded workloads) - so
we provide these two functions and the user should select which one is
faster for his particular workload.
dm-writecache througput (on real Optane-based persistent memory):
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
Signed-off-by: Mikulas Patocka <[email protected]>
---
arch/x86/include/asm/string_64.h | 10 ++++++++
arch/x86/lib/usercopy_64.c | 46 +++++++++++++++++++++++++++++++++++++++
drivers/md/dm-writecache.c | 2 -
include/linux/string.h | 6 +++++
4 files changed, 63 insertions(+), 1 deletion(-)
Index: linux-2.6/arch/x86/include/asm/string_64.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/string_64.h 2020-04-20
15:31:46.939999000 +0200
+++ linux-2.6/arch/x86/include/asm/string_64.h 2020-04-20 15:31:46.929999000
+0200
@@ -114,6 +114,14 @@ memcpy_mcsafe(void *dst, const void *src
return 0;
}
+/*
+ * In some cases (mostly single-threaded workload), clflushopt is faster
+ * than non-temporal stores. In other situations, non-temporal stores are
+ * faster. So, we provide two functions:
+ * memcpy_flushcache using non-temporal stores
+ * memcpy_flushcache_single using clflushopt
+ * The caller should test which one is faster for the particular workload.
+ */
#ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
#define __HAVE_ARCH_MEMCPY_FLUSHCACHE 1
void __memcpy_flushcache(void *dst, const void *src, size_t cnt);
@@ -135,6 +143,8 @@ static __always_inline void memcpy_flush
}
__memcpy_flushcache(dst, src, cnt);
}
+#define __HAVE_ARCH_MEMCPY_FLUSHCACHE_CLFLUSHOPT 1
+void memcpy_flushcache_single(void *dst, const void *src, size_t cnt);
#endif
#endif /* __KERNEL__ */
Index: linux-2.6/include/linux/string.h
===================================================================
--- linux-2.6.orig/include/linux/string.h 2020-04-20 15:31:46.939999000
+0200
+++ linux-2.6/include/linux/string.h 2020-04-20 15:31:46.929999000 +0200
@@ -175,6 +175,12 @@ static inline void memcpy_flushcache(voi
memcpy(dst, src, cnt);
}
#endif
+#ifndef __HAVE_ARCH_MEMCPY_FLUSHCACHE_CLFLUSHOPT
+static inline void memcpy_flushcache_single(void *dst, const void *src, size_t
cnt)
+{
+ memcpy_flushcache(dst, src, cnt);
+}
+#endif
void *memchr_inv(const void *s, int c, size_t n);
char *strreplace(char *s, char old, char new);
Index: linux-2.6/arch/x86/lib/usercopy_64.c
===================================================================
--- linux-2.6.orig/arch/x86/lib/usercopy_64.c 2020-04-20 15:31:46.939999000
+0200
+++ linux-2.6/arch/x86/lib/usercopy_64.c 2020-04-20 15:38:13.159999000
+0200
@@ -199,6 +199,52 @@ void __memcpy_flushcache(void *_dst, con
}
EXPORT_SYMBOL_GPL(__memcpy_flushcache);
+void memcpy_flushcache_single(void *_dst, const void *_src, size_t size)
+{
+ unsigned long dest = (unsigned long) _dst;
+ unsigned long source = (unsigned long) _src;
+
+ /*
+ * dm-writecache througput (on real Optane-based persistent memory):
+ * measured with dd:
+ *
+ * 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 see that movnti performs better for 512-byte blocks, and
+ * clflushopt performs better for 1024-byte and larger blocks. So, we
+ * prefer clflushopt for sizes >= 768.
+ */
+
+ if (static_cpu_has(X86_FEATURE_CLFLUSHOPT) &&
likely(boot_cpu_data.x86_clflush_size == 64) &&
+ likely(size >= 768)) {
+ if (unlikely(!IS_ALIGNED(dest, 64))) {
+ size_t len = min_t(size_t, size, ALIGN(dest, 64) -
dest);
+
+ memcpy((void *) dest, (void *) source, len);
+ clflushopt((void *)dest);
+ dest += len;
+ source += len;
+ size -= len;
+ }
+ do {
+ memcpy((void *)dest, (void *)source, 64);
+ clflushopt((void *)dest);
+ dest += 64;
+ source += 64;
+ size -= 64;
+ } while (size >= 64)
+ if (unlikely(size != 0)) {
+ memcpy((void *)dest, (void *)source, size);
+ clflushopt((void *)dest);
+ }
+ return;
+ }
+ memcpy_flushcache((void *)dest, (void *)source, size);
+}
+EXPORT_SYMBOL_GPL(memcpy_flushcache_single);
+
void memcpy_page_flushcache(char *to, struct page *page, size_t offset,
size_t len)
{
Index: linux-2.6/drivers/md/dm-writecache.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-writecache.c 2020-04-20 15:31:46.939999000
+0200
+++ linux-2.6/drivers/md/dm-writecache.c 2020-04-20 15:32:35.549999000
+0200
@@ -1166,7 +1166,7 @@ static void bio_copy_block(struct dm_wri
}
} else {
flush_dcache_page(bio_page(bio));
- memcpy_flushcache(data, buf, size);
+ memcpy_flushcache_single(data, buf, size);
}
bvec_kunmap_irq(buf, &flags);
--
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