RE: [PATCH] x86/asm/64: Align start of __clear_user() loop to 16-bytes
From: Alexey Dobriyan > Sent: 18 June 2020 14:17 ... > > > diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c > > > index fff28c6f73a2..b0dfac3d3df7 100644 > > > --- a/arch/x86/lib/usercopy_64.c > > > +++ b/arch/x86/lib/usercopy_64.c > > > @@ -24,6 +24,7 @@ unsigned long __clear_user(void __user *addr, unsigned > > > long size) > > > asm volatile( > > > " testq %[size8],%[size8]\n" > > > " jz 4f\n" > > > + " .align 16\n" > > > "0: movq $0,(%[dst])\n" > > > " addq $8,%[dst]\n" > > > " decl %%ecx ; jnz 0b\n" > > > > You can do better that that loop. > > Change 'dst' to point to the end of the buffer, negate the count > > and divide by 8 and you get: > > "0: movq $0,($[dst],%%ecx,8)\n" > > " add $1,%%ecx" > > " jnz 0b\n" > > which might run at one iteration per clock especially on cpu that pair > > the add and jnz into a single uop. > > (You need to use add not inc.) > > /dev/zero should probably use REP STOSB etc just like everything else. Almost certainly it shouldn't, and neither should anything else. Potentially it could use whatever memset() is patched to. That MIGHT be 'rep stos' on some cpu variants, but in general it is slow. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
Re: [PATCH] x86/asm/64: Align start of __clear_user() loop to 16-bytes
On Thu, Jun 18, 2020 at 04:39:35PM +, David Laight wrote: > From: Alexey Dobriyan > > Sent: 18 June 2020 14:17 > ... > > > > diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c > > > > index fff28c6f73a2..b0dfac3d3df7 100644 > > > > --- a/arch/x86/lib/usercopy_64.c > > > > +++ b/arch/x86/lib/usercopy_64.c > > > > @@ -24,6 +24,7 @@ unsigned long __clear_user(void __user *addr, > > > > unsigned long size) > > > > asm volatile( > > > > " testq %[size8],%[size8]\n" > > > > " jz 4f\n" > > > > + " .align 16\n" > > > > "0: movq $0,(%[dst])\n" > > > > " addq $8,%[dst]\n" > > > > " decl %%ecx ; jnz 0b\n" > > > > > > You can do better that that loop. > > > Change 'dst' to point to the end of the buffer, negate the count > > > and divide by 8 and you get: > > > "0: movq $0,($[dst],%%ecx,8)\n" > > > " add $1,%%ecx" > > > " jnz 0b\n" > > > which might run at one iteration per clock especially on cpu that pair > > > the add and jnz into a single uop. > > > (You need to use add not inc.) > > > > /dev/zero should probably use REP STOSB etc just like everything else. > > Almost certainly it shouldn't, and neither should anything else. > Potentially it could use whatever memset() is patched to. > That MIGHT be 'rep stos' on some cpu variants, but in general > it is slow. Yes, that's what I meant: alternatives choosing REP variant. memset loops are so 21-st century.
RE: [PATCH] x86/asm/64: Align start of __clear_user() loop to 16-bytes
From: Matt Fleming
> Sent: 18 June 2020 11:20
> x86 CPUs can suffer severe performance drops if a tight loop, such as
> the ones in __clear_user(), straddles a 16-byte instruction fetch
> window, or worse, a 64-byte cacheline. This issues was discovered in the
> SUSE kernel with the following commit,
>
> 1153933703d9 ("x86/asm/64: Micro-optimize __clear_user() - Use immediate
> constants")
>
> which increased the code object size from 10 bytes to 15 bytes and
> caused the 8-byte copy loop in __clear_user() to be split across a
> 64-byte cacheline.
>
> Aligning the start of the loop to 16-bytes makes this fit neatly inside
> a single instruction fetch window again and restores the performance of
> __clear_user() which is used heavily when reading from /dev/zero.
>
> Here are some numbers from running libmicro's read_z* and pread_z*
> microbenchmarks which read from /dev/zero:
>
> Zen 1 (Naples)
>
> libmicro-file
> 5.7.0-rc6 5.7.0-rc6
> 5.7.0-rc6
> revert-1153933703d9+
> align16+
> Time mean95-pread_z100k 9.9195 ( 0.00%) 5.9856 ( 39.66%)
> 5.9938 ( 39.58%)
> Time mean95-pread_z10k1.1378 ( 0.00%) 0.7450 ( 34.52%)
> 0.7467 ( 34.38%)
> Time mean95-pread_z1k 0.2623 ( 0.00%) 0.2251 ( 14.18%)
> 0.2252 ( 14.15%)
> Time mean95-pread_zw100k 9.9974 ( 0.00%) 6.0648 ( 39.34%)
> 6.0756 ( 39.23%)
> Time mean95-read_z100k9.8940 ( 0.00%) 5.9885 ( 39.47%)
> 5.9994 ( 39.36%)
> Time mean95-read_z10k 1.1394 ( 0.00%) 0.7483 ( 34.33%)
> 0.7482 ( 34.33%)
>
> Note that this doesn't affect Haswell or Broadwell microarchitectures
> which seem to avoid the alignment issue by executing the loop straight
> out of the Loop Stream Detector (verified using perf events).
Which cpu was affected?
At least one source (www.agner.org/optimize) implies that both ivy
bridge and sandy bridge have uop caches that mean (If I've read it
correctly) the loop shouldn't be affected by the alignment).
> diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
> index fff28c6f73a2..b0dfac3d3df7 100644
> --- a/arch/x86/lib/usercopy_64.c
> +++ b/arch/x86/lib/usercopy_64.c
> @@ -24,6 +24,7 @@ unsigned long __clear_user(void __user *addr, unsigned long
> size)
> asm volatile(
> " testq %[size8],%[size8]\n"
> " jz 4f\n"
> + " .align 16\n"
> "0: movq $0,(%[dst])\n"
> " addq $8,%[dst]\n"
> " decl %%ecx ; jnz 0b\n"
You can do better that that loop.
Change 'dst' to point to the end of the buffer, negate the count
and divide by 8 and you get:
"0: movq $0,($[dst],%%ecx,8)\n"
" add $1,%%ecx"
" jnz 0b\n"
which might run at one iteration per clock especially on cpu that pair
the add and jnz into a single uop.
(You need to use add not inc.)
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT,
UK
Registration No: 1397386 (Wales)
Re: [PATCH] x86/asm/64: Align start of __clear_user() loop to 16-bytes
On Thu, Jun 18, 2020 at 10:48:05AM +, David Laight wrote:
> From: Matt Fleming
> > Sent: 18 June 2020 11:20
> > x86 CPUs can suffer severe performance drops if a tight loop, such as
> > the ones in __clear_user(), straddles a 16-byte instruction fetch
> > window, or worse, a 64-byte cacheline. This issues was discovered in the
> > SUSE kernel with the following commit,
> >
> > 1153933703d9 ("x86/asm/64: Micro-optimize __clear_user() - Use immediate
> > constants")
> >
> > which increased the code object size from 10 bytes to 15 bytes and
> > caused the 8-byte copy loop in __clear_user() to be split across a
> > 64-byte cacheline.
> >
> > Aligning the start of the loop to 16-bytes makes this fit neatly inside
> > a single instruction fetch window again and restores the performance of
> > __clear_user() which is used heavily when reading from /dev/zero.
> >
> > Here are some numbers from running libmicro's read_z* and pread_z*
> > microbenchmarks which read from /dev/zero:
> >
> > Zen 1 (Naples)
> >
> > libmicro-file
> > 5.7.0-rc6 5.7.0-rc6
> > 5.7.0-rc6
> > revert-1153933703d9+
> >align16+
> > Time mean95-pread_z100k 9.9195 ( 0.00%) 5.9856 ( 39.66%)
> > 5.9938 ( 39.58%)
> > Time mean95-pread_z10k1.1378 ( 0.00%) 0.7450 ( 34.52%)
> > 0.7467 ( 34.38%)
> > Time mean95-pread_z1k 0.2623 ( 0.00%) 0.2251 ( 14.18%)
> > 0.2252 ( 14.15%)
> > Time mean95-pread_zw100k 9.9974 ( 0.00%) 6.0648 ( 39.34%)
> > 6.0756 ( 39.23%)
> > Time mean95-read_z100k9.8940 ( 0.00%) 5.9885 ( 39.47%)
> > 5.9994 ( 39.36%)
> > Time mean95-read_z10k 1.1394 ( 0.00%) 0.7483 ( 34.33%)
> > 0.7482 ( 34.33%)
> >
> > Note that this doesn't affect Haswell or Broadwell microarchitectures
> > which seem to avoid the alignment issue by executing the loop straight
> > out of the Loop Stream Detector (verified using perf events).
>
> Which cpu was affected?
> At least one source (www.agner.org/optimize) implies that both ivy
> bridge and sandy bridge have uop caches that mean (If I've read it
> correctly) the loop shouldn't be affected by the alignment).
>
> > diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
> > index fff28c6f73a2..b0dfac3d3df7 100644
> > --- a/arch/x86/lib/usercopy_64.c
> > +++ b/arch/x86/lib/usercopy_64.c
> > @@ -24,6 +24,7 @@ unsigned long __clear_user(void __user *addr, unsigned
> > long size)
> > asm volatile(
> > " testq %[size8],%[size8]\n"
> > " jz 4f\n"
> > + " .align 16\n"
> > "0: movq $0,(%[dst])\n"
> > " addq $8,%[dst]\n"
> > " decl %%ecx ; jnz 0b\n"
>
> You can do better that that loop.
> Change 'dst' to point to the end of the buffer, negate the count
> and divide by 8 and you get:
> "0: movq $0,($[dst],%%ecx,8)\n"
> " add $1,%%ecx"
> " jnz 0b\n"
> which might run at one iteration per clock especially on cpu that pair
> the add and jnz into a single uop.
> (You need to use add not inc.)
/dev/zero should probably use REP STOSB etc just like everything else.
