linux_dsm_epyc7002/arch
Kim Phillips 0f4cd769c4 perf/x86/amd/ibs: Fix sample bias for dispatched micro-ops
When counting dispatched micro-ops with cnt_ctl=1, in order to prevent
sample bias, IBS hardware preloads the least significant 7 bits of
current count (IbsOpCurCnt) with random values, such that, after the
interrupt is handled and counting resumes, the next sample taken
will be slightly perturbed.

The current count bitfield is in the IBS execution control h/w register,
alongside the maximum count field.

Currently, the IBS driver writes that register with the maximum count,
leaving zeroes to fill the current count field, thereby overwriting
the random bits the hardware preloaded for itself.

Fix the driver to actually retain and carry those random bits from the
read of the IBS control register, through to its write, instead of
overwriting the lower current count bits with zeroes.

Tested with:

perf record -c 100001 -e ibs_op/cnt_ctl=1/pp -a -C 0 taskset -c 0 <workload>

'perf annotate' output before:

 15.70  65:   addsd     %xmm0,%xmm1
 17.30        add       $0x1,%rax
 15.88        cmp       %rdx,%rax
              je        82
 17.32  72:   test      $0x1,%al
              jne       7c
  7.52        movapd    %xmm1,%xmm0
  5.90        jmp       65
  8.23  7c:   sqrtsd    %xmm1,%xmm0
 12.15        jmp       65

'perf annotate' output after:

 16.63  65:   addsd     %xmm0,%xmm1
 16.82        add       $0x1,%rax
 16.81        cmp       %rdx,%rax
              je        82
 16.69  72:   test      $0x1,%al
              jne       7c
  8.30        movapd    %xmm1,%xmm0
  8.13        jmp       65
  8.24  7c:   sqrtsd    %xmm1,%xmm0
  8.39        jmp       65

Tested on Family 15h and 17h machines.

Machines prior to family 10h Rev. C don't have the RDWROPCNT capability,
and have the IbsOpCurCnt bitfield reserved, so this patch shouldn't
affect their operation.

It is unknown why commit db98c5faf8 ("perf/x86: Implement 64-bit
counter support for IBS") ignored the lower 4 bits of the IbsOpCurCnt
field; the number of preloaded random bits has always been 7, AFAICT.

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: "Arnaldo Carvalho de Melo" <acme@kernel.org>
Cc: <x86@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Borislav Petkov" <bp@alien8.de>
Cc: Stephane Eranian <eranian@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: "Namhyung Kim" <namhyung@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lkml.kernel.org/r/20190826195730.30614-1-kim.phillips@amd.com
2019-08-30 14:27:47 +02:00
..
alpha Merge branch 'akpm' (patches from Andrew) 2019-07-17 08:58:04 -07:00
arc Merge branch 'akpm' (patches from Andrew) 2019-07-17 08:58:04 -07:00
arm dma-mapping fixes for 5.3-rc 2019-08-24 20:00:11 -07:00
arm64 arm64 fixes: 2019-08-16 10:51:47 -07:00
c6x Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gerg/m68knommu 2019-07-10 21:42:03 -07:00
csky treewide: add "WITH Linux-syscall-note" to SPDX tag of uapi headers 2019-07-25 11:05:10 +02:00
h8300 h8300 update for 5.3 2019-07-17 09:36:38 -07:00
hexagon hexagon: switch to generic version of pte allocation 2019-07-21 09:53:00 -07:00
ia64 Merge branch 'work.mount0' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2019-07-19 10:42:02 -07:00
m68k arch: mark syscall number 435 reserved for clone3 2019-07-15 00:39:33 +02:00
microblaze clone3-v5.3 2019-07-11 10:09:44 -07:00
mips Wimplicit-fallthrough patches for 5.3-rc6 2019-08-22 11:26:10 -07:00
nds32 treewide: add "WITH Linux-syscall-note" to SPDX tag of uapi headers 2019-07-25 11:05:10 +02:00
nios2 nios2 update for v5.3-rc1 2019-07-12 15:38:05 -07:00
openrisc dma-mapping updates for Linux 5.3 2019-07-12 15:13:55 -07:00
parisc parisc: fix compilation errrors 2019-08-24 19:48:42 -07:00
powerpc dma-mapping fixes for 5.3-rc 2019-08-14 10:31:11 -07:00
riscv riscv: Make __fstate_clean() work correctly. 2019-08-14 13:20:46 -07:00
s390 Bugfixes (arm and x86) and cleanups. 2019-08-09 15:46:29 -07:00
sh sh: kernel: hw_breakpoint: Fix missing break in switch statement 2019-08-11 16:15:16 -05:00
sparc treewide: add "WITH Linux-syscall-note" to SPDX tag of uapi headers 2019-07-25 11:05:10 +02:00
um um: fix time travel mode 2019-08-23 00:39:53 +02:00
unicore32 Kconfig updates for v5.3 2019-07-12 16:06:27 -07:00
x86 perf/x86/amd/ibs: Fix sample bias for dispatched micro-ops 2019-08-30 14:27:47 +02:00
xtensa xtensa: add missing isync to the cpu_reset TLB code 2019-08-12 15:05:48 -07:00
.gitignore
Kconfig Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-07-20 10:33:44 -07:00