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Subject[PATCH v3 RESEND] x86: optimize memcpy_flushcache


On Fri, 22 Jun 2018, Ingo Molnar wrote:

>
> * Mikulas Patocka <mpatocka@redhat.com> wrote:
>
> > On Thu, 21 Jun 2018, Ingo Molnar wrote:
> >
> > >
> > > * Mike Snitzer <snitzer@redhat.com> wrote:
> > >
> > > > From: Mikulas Patocka <mpatocka@redhat.com>
> > > > Subject: [PATCH v2] x86: optimize memcpy_flushcache
> > > >
> > > > In the context of constant short length stores to persistent memory,
> > > > memcpy_flushcache suffers from a 2% performance degradation compared to
> > > > explicitly using the "movnti" instruction.
> > > >
> > > > Optimize 4, 8, and 16 byte memcpy_flushcache calls to explicitly use the
> > > > movnti instruction with inline assembler.
> > >
> > > Linus requested asm optimizations to include actual benchmarks, so it would be
> > > nice to describe how this was tested, on what hardware, and what the before/after
> > > numbers are.
> > >
> > > Thanks,
> > >
> > > Ingo
> >
> > It was tested on 4-core skylake machine with persistent memory being
> > emulated using the memmap kernel option. The dm-writecache target used the
> > emulated persistent memory as a cache and sata SSD as a backing device.
> > The patch results in 2% improved throughput when writing data using dd.
> >
> > I don't have access to the machine anymore.
>
> I think this information is enough, but do we know how well memmap emulation
> represents true persistent memory speed and cache management characteristics?
> It might be representative - but I don't know for sure, nor probably most
> readers of the changelog.
>
> So could you please put all this into an updated changelog, and also add a short
> description that outlines exactly which codepaths end up using this method in a
> typical persistent memory setup? All filesystem ops - or only reads, etc?
>
> Thanks,
>
> Ingo

Here I resend it:


From: Mikulas Patocka <mpatocka@redhat.com>
Subject: [PATCH] x86: optimize memcpy_flushcache

I use memcpy_flushcache in my persistent memory driver for metadata
updates, there are many 8-byte and 16-byte updates and it turns out that
the overhead of memcpy_flushcache causes 2% performance degradation
compared to "movnti" instruction explicitly coded using inline assembler.

The tests were done on a Skylake processor with persistent memory emulated
using the "memmap" kernel parameter. dd was used to copy data to the
dm-writecache target.

This patch recognizes memcpy_flushcache calls with constant short length
and turns them into inline assembler - so that I don't have to use inline
assembler in the driver.

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

---
arch/x86/include/asm/string_64.h | 20 +++++++++++++++++++-
arch/x86/lib/usercopy_64.c | 4 ++--
2 files changed, 21 insertions(+), 3 deletions(-)

Index: linux-2.6/arch/x86/include/asm/string_64.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/string_64.h
+++ linux-2.6/arch/x86/include/asm/string_64.h
@@ -149,7 +149,25 @@ memcpy_mcsafe(void *dst, const void *src

#ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
#define __HAVE_ARCH_MEMCPY_FLUSHCACHE 1
-void memcpy_flushcache(void *dst, const void *src, size_t cnt);
+void __memcpy_flushcache(void *dst, const void *src, size_t cnt);
+static __always_inline void memcpy_flushcache(void *dst, const void *src, size_t cnt)
+{
+ if (__builtin_constant_p(cnt)) {
+ switch (cnt) {
+ case 4:
+ asm ("movntil %1, %0" : "=m"(*(u32 *)dst) : "r"(*(u32 *)src));
+ return;
+ case 8:
+ asm ("movntiq %1, %0" : "=m"(*(u64 *)dst) : "r"(*(u64 *)src));
+ return;
+ case 16:
+ asm ("movntiq %1, %0" : "=m"(*(u64 *)dst) : "r"(*(u64 *)src));
+ asm ("movntiq %1, %0" : "=m"(*(u64 *)(dst + 8)) : "r"(*(u64 *)(src + 8)));
+ return;
+ }
+ }
+ __memcpy_flushcache(dst, src, cnt);
+}
#endif

#endif /* __KERNEL__ */
Index: linux-2.6/arch/x86/lib/usercopy_64.c
===================================================================
--- linux-2.6.orig/arch/x86/lib/usercopy_64.c
+++ linux-2.6/arch/x86/lib/usercopy_64.c
@@ -153,7 +153,7 @@ long __copy_user_flushcache(void *dst, c
return rc;
}

-void memcpy_flushcache(void *_dst, const void *_src, size_t size)
+void __memcpy_flushcache(void *_dst, const void *_src, size_t size)
{
unsigned long dest = (unsigned long) _dst;
unsigned long source = (unsigned long) _src;
@@ -216,7 +216,7 @@ void memcpy_flushcache(void *_dst, const
clean_cache_range((void *) dest, size);
}
}
-EXPORT_SYMBOL_GPL(memcpy_flushcache);
+EXPORT_SYMBOL_GPL(__memcpy_flushcache);

void memcpy_page_flushcache(char *to, struct page *page, size_t offset,
size_t len)
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