[PATCH] x86: introduce memcpy_flushcache_clflushopt
From: Mikulas Patocka
Date: Fri Apr 17 2020 - 08:47:35 EST
On Thu, 16 Apr 2020, Dan Williams wrote:
> On Thu, Apr 16, 2020 at 1:24 AM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> >
> >
> >
> > On Thu, 9 Apr 2020, Mikulas Patocka wrote:
> >
> > > With dm-writecache on emulated pmem (with the memmap argument), we get
> > >
> > > With the original kernel:
> > > 8508 - 11378
> > > real 0m4.960s
> > > user 0m0.638s
> > > sys 0m4.312s
> > >
> > > With dm-writecache hacked to use cached writes + clflushopt:
> > > 8505 - 11378
> > > real 0m4.151s
> > > user 0m0.560s
> > > sys 0m3.582s
> >
> > I did some multithreaded tests:
> > http://people.redhat.com/~mpatocka/testcases/pmem/microbenchmarks/pmem-multithreaded.txt
> >
> > And it turns out that for singlethreaded access, write+clwb performs
> > better, while for multithreaded access, non-temporal stores perform
> > better.
> >
> > 1 sequential write-nt 8 bytes 1.3 GB/s
> > 2 sequential write-nt 8 bytes 2.5 GB/s
> > 3 sequential write-nt 8 bytes 2.8 GB/s
> > 4 sequential write-nt 8 bytes 2.8 GB/s
> > 5 sequential write-nt 8 bytes 2.5 GB/s
> >
> > 1 sequential write 8 bytes + clwb 1.6 GB/s
> > 2 sequential write 8 bytes + clwb 2.4 GB/s
> > 3 sequential write 8 bytes + clwb 1.7 GB/s
> > 4 sequential write 8 bytes + clwb 1.2 GB/s
> > 5 sequential write 8 bytes + clwb 0.8 GB/s
> >
> > For one thread, we can see that write-nt 8 bytes has 1.3 GB/s and write
> > 8+clwb has 1.6 GB/s, but for multiple threads, write-nt has better
> > throughput.
> >
> > The dm-writecache target is singlethreaded (all the copying is done while
> > holding the writecache lock), so it benefits from clwb.
> >
> > Should memcpy_flushcache be changed to write+clwb? Or are there some
> > multithreaded users of memcpy_flushcache that would be hurt by this
> > change?
>
> Maybe this is asking for a specific memcpy_flushcache_inatomic()
> implementation for your use case, but leave nt-writes for the general
> case?
Yes - I have created this patch that adds a new function
memcpy_flushcache_clflushopt and makes dm-writecache use it.
Mikulas
From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Implement the function memcpy_flushcache_clflushopt 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 some 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 <mpatocka@xxxxxxxxxx>
---
arch/x86/include/asm/string_64.h | 10 ++++++++++
arch/x86/lib/usercopy_64.c | 32 ++++++++++++++++++++++++++++++++
drivers/md/dm-writecache.c | 5 ++++-
include/linux/string.h | 6 ++++++
4 files changed, 52 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 2020-04-17 14:06:35.139999000 +0200
+++ linux-2.6/arch/x86/include/asm/string_64.h 2020-04-17 14:06:35.129999000 +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_clflushopt 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_clflushopt(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 2020-04-17 14:06:35.139999000 +0200
+++ linux-2.6/include/linux/string.h 2020-04-17 14:06:35.129999000 +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_clflushopt(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 2020-04-17 14:06:35.139999000 +0200
+++ linux-2.6/arch/x86/lib/usercopy_64.c 2020-04-17 14:25:18.569999000 +0200
@@ -199,6 +199,38 @@ void __memcpy_flushcache(void *_dst, con
}
EXPORT_SYMBOL_GPL(__memcpy_flushcache);
+void memcpy_flushcache_clflushopt(void *_dst, const void *_src, size_t size)
+{
+ unsigned long dest = (unsigned long) _dst;
+ unsigned long source = (unsigned long) _src;
+
+ if (static_cpu_has(X86_FEATURE_CLFLUSHOPT) && likely(boot_cpu_data.x86_clflush_size == 64)) {
+ 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;
+ }
+ while (size >= 64) {
+ memcpy((void *)dest, (void *)source, 64);
+ clflushopt((void *)dest);
+ dest += 64;
+ source += 64;
+ 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_clflushopt);
+
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 2020-04-17 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))
+ memcpy_flushcache_clflushopt(data, buf, size);
+ else
+ memcpy_flushcache(data, buf, size);
}
bvec_kunmap_irq(buf, &flags);