Re: [REGRESSION] Perf (userspace) broken on big.LITTLE systems since v6.5

[Mark Rutland]

On Tue, Nov 21, 2023 at 03:24:25PM +0000, Marc Zyngier wrote:
> On Tue, 21 Nov 2023 13:40:31 +0000,
> Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > 
> > [Adding key people on Cc]
> > 
> > On Tue, 21 Nov 2023 12:08:48 +0000,
> > Hector Martin <marcan@xxxxxxxxx> wrote:
> > > 
> > > Perf broke on all Apple ARM64 systems (tested almost everything), and
> > > according to maz also on Juno (so, probably all big.LITTLE) since v6.5.
> > 
> > I can confirm that at least on 6.7-rc2, perf is pretty busted on any
> > asymmetric ARM platform. It isn't clear what criteria is used to pick
> > the PMU, but nothing works anymore.
> > 
> > The saving grace in my case is that Debian still ships a 6.1 perftool
> > package, but that's obviously not going to last.
> > 
> > I'm happy to test potential fixes.
> 
> At Mark's request, I've dumped a couple of perf (as of -rc2) runs with
> -vvv.  And it is quite entertaining (this is taskset to an 'icestorm'
> CPU):

Looking at this with fresh(er) eyes, I think there's a userspace bug here,
regardless of whether one believes it's correct to convert a named-pmu event to
a PERF_TYPE_HARDWARE event directed at that PMU.

It looks like the userspace tool is dropping the extended type ID after an
initial probe, and requests events with plain PERF_TYPE_HARDWARE (without an
extended type ID), which explains why we seem to get events from one PMU only.

More detail below...

Marc, if you have time, could you run the same commands (on the same kernel)
with a perf tool build from v6.4?

> <quote>
> maz@valley-girl:~/hot-poop/arm-platforms/tools/perf$ sudo taskset -c 0 ./perf stat -vvv -e apple_icestorm_pmu/cycles/ -e
>  apple_firestorm_pmu/cycles/ -e cycles ls
> Using CPUID 0x00000000612f0280
> Attempt to add: apple_icestorm_pmu/cycles=0/
> ..after resolving event: apple_icestorm_pmu/cycles=0/
> Opening: unknown-hardware:HG
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   config                           0xb00000000
>   disabled                         1
> ------------------------------------------------------------

Here config[31:0] is 0 (PERF_COUNT_HW_CPU_CYCLES), and config[63:32] is 0xb,
which is presumably the PMU ID for the apple_icestorm_pmu.

The attr doesn't contain exclude_guest=1, so this will be rejected by the PMU
driver due to its mode exclusion requirements.

> sys_perf_event_open: pid 0  cpu -1  group_fd -1  flags 0x8
> sys_perf_event_open failed, error -95

... which is what we see here (this is EOPNOTSUPP, which __hw_perf_event_init()
in drivers/perf/arm_pmu.c returns when the mode requested mode exclusion
options aren't supported).

So far, so good...

> Attempt to add: apple_firestorm_pmu/cycles=0/
> ..after resolving event: apple_firestorm_pmu/cycles=0/
> Control descriptor is not initialized
> Opening: apple_icestorm_pmu/cycles/
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   size                             136
>   config                           0 (PERF_COUNT_HW_CPU_CYCLES)
>   sample_type                      IDENTIFIER
>   read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
>   disabled                         1
>   inherit                          1
>   enable_on_exec                   1
>   exclude_guest                    1
> ------------------------------------------------------------

... but here, the extended type ID has been dropped, and this event is no
longer directed towards the apple_firestorm_pmu PMU, so the kernel can direct
this to *any* CPU PMU...

> sys_perf_event_open: pid 1045843  cpu -1  group_fd -1  flags 0x8 = 3

... and *some* PMU accepts it.

> Opening: apple_firestorm_pmu/cycles/
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   size                             136
>   config                           0 (PERF_COUNT_HW_CPU_CYCLES)
>   sample_type                      IDENTIFIER
>   read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
>   disabled                         1
>   inherit                          1
>   enable_on_exec                   1
>   exclude_guest                    1
> ------------------------------------------------------------

Likewise here, no extended type ID...

> sys_perf_event_open: pid 1045843  cpu -1  group_fd -1  flags 0x8 = 4
> Opening: cycles
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   size                             136
>   config                           0 (PERF_COUNT_HW_CPU_CYCLES)
>   sample_type                      IDENTIFIER
>   read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
>   disabled                         1
>   inherit                          1
>   enable_on_exec                   1
>   exclude_guest                    1
> ------------------------------------------------------------

Likewise here, no extended type ID...

> sys_perf_event_open: pid 1045843  cpu -1  group_fd -1  flags 0x8 = 5
> arch			builtin-diff.o      builtin-mem.o	 common-cmds.h    perf-completion.sh
> bench			builtin-evlist.c    builtin-probe.c	 CREDITS	  perf.h
> Build			builtin-evlist.o    builtin-probe.o	 design.txt	  perf-in.o
> builtin-annotate.c	builtin-ftrace.c    builtin-record.c	 dlfilters	  perf-iostat
> builtin-annotate.o	builtin-ftrace.o    builtin-record.o	 Documentation    perf-iostat.sh
> builtin-bench.c		builtin.h	    builtin-report.c	 FEATURE-DUMP	  perf.o
> builtin-bench.o		builtin-help.c      builtin-report.o	 include	  perf-read-vdso.c
> builtin-buildid-cache.c  builtin-help.o      builtin-sched.c	 jvmti		  perf-sys.h
> builtin-buildid-cache.o  builtin-inject.c    builtin-script.c	 libapi	  PERF-VERSION-FILE
> builtin-buildid-list.c	builtin-inject.o    builtin-script.o	 libperf	  perf-with-kcore
> builtin-buildid-list.o	builtin-kallsyms.c  builtin-stat.c	 libsubcmd	  pmu-events
> builtin-c2c.c		builtin-kallsyms.o  builtin-stat.o	 libsymbol	  python
> builtin-c2c.o		builtin-kmem.c      builtin-timechart.c  Makefile	  python_ext_build
> builtin-config.c	builtin-kvm.c	    builtin-top.c	 Makefile.config  scripts
> builtin-config.o	builtin-kvm.o	    builtin-top.o	 Makefile.perf    tests
> builtin-daemon.c	builtin-kwork.c     builtin-trace.c	 MANIFEST	  trace
> builtin-daemon.o	builtin-list.c      builtin-version.c	 perf		  ui
> builtin-data.c		builtin-list.o      builtin-version.o	 perf-archive	  util
> builtin-data.o		builtin-lock.c      check-headers.sh	 perf-archive.sh
> builtin-diff.c		builtin-mem.c	    command-list.txt	 perf.c
> apple_icestorm_pmu/cycles/: -1: 0 873709 0
> apple_firestorm_pmu/cycles/: -1: 0 873709 0
> cycles: -1: 0 873709 0
> apple_icestorm_pmu/cycles/: 0 873709 0
> apple_firestorm_pmu/cycles/: 0 873709 0
> cycles: 0 873709 0
> 
>  Performance counter stats for 'ls':
> 
>      <not counted>      apple_icestorm_pmu/cycles/                                              (0.00%)
>      <not counted>      apple_firestorm_pmu/cycles/                                             (0.00%)
>      <not counted>      cycles                                                                  (0.00%)
> 
>        0.000002250 seconds time elapsed
> 
>        0.000000000 seconds user
>        0.000000000 seconds sys

So it looks like the tool has expanded the requested
'apple_icestorm_pmu/cycles/' event into three cycles events, each opened
without an extended type ID.

AFAICT, the kernel has done exactly what it has always done for
PERF_TYPE_HARDWARE/PERF_COUNT_HW_CPU_CYCLES events: pick the first PMU which
said it can handle them.

> If I run the same thing on another CPU cluster (firestorm), I get
> this:
> 
> <quote>
> maz@valley-girl:~/hot-poop/arm-platforms/tools/perf$ sudo taskset -c 2 ./perf stat -vvv -e apple_icestorm_pmu/cycles/ -e
>  apple_firestorm_pmu/cycles/ -e cycles ls
> Using CPUID 0x00000000612f0280
> Attempt to add: apple_icestorm_pmu/cycles=0/
> ..after resolving event: apple_icestorm_pmu/cycles=0/
> Opening: unknown-hardware:HG
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   config                           0xb00000000
>   disabled                         1
> ------------------------------------------------------------
> sys_perf_event_open: pid 0  cpu -1  group_fd -1  flags 0x8
> sys_perf_event_open failed, error -95

Again, we see one request with an extended type ID, which fails due to mode exclusion requirements...

> Attempt to add: apple_firestorm_pmu/cycles=0/
> ..after resolving event: apple_firestorm_pmu/cycles=0/
> Control descriptor is not initialized
> Opening: apple_icestorm_pmu/cycles/
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   size                             136
>   config                           0 (PERF_COUNT_HW_CPU_CYCLES)
>   sample_type                      IDENTIFIER
>   read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
>   disabled                         1
>   inherit                          1
>   enable_on_exec                   1
>   exclude_guest                    1
> ------------------------------------------------------------
> sys_perf_event_open: pid 1045925  cpu -1  group_fd -1  flags 0x8 = 3
> Opening: apple_firestorm_pmu/cycles/
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   size                             136
>   config                           0 (PERF_COUNT_HW_CPU_CYCLES)
>   sample_type                      IDENTIFIER
>   read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
>   disabled                         1
>   inherit                          1
>   enable_on_exec                   1
>   exclude_guest                    1
> ------------------------------------------------------------
> sys_perf_event_open: pid 1045925  cpu -1  group_fd -1  flags 0x8 = 4
> Opening: cycles
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   size                             136
>   config                           0 (PERF_COUNT_HW_CPU_CYCLES)
>   sample_type                      IDENTIFIER
>   read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
>   disabled                         1
>   inherit                          1
>   enable_on_exec                   1
>   exclude_guest                    1
> ------------------------------------------------------------

... but all subsequent requests do not have an extended type ID, and the kernel
directs these to whichever PMU accepts the event first...

> sys_perf_event_open: pid 1045925  cpu -1  group_fd -1  flags 0x8 = 5
> arch			builtin-diff.o      builtin-mem.o	 common-cmds.h    perf-completion.sh
> bench			builtin-evlist.c    builtin-probe.c	 CREDITS	  perf.h
> Build			builtin-evlist.o    builtin-probe.o	 design.txt	  perf-in.o
> builtin-annotate.c	builtin-ftrace.c    builtin-record.c	 dlfilters	  perf-iostat
> builtin-annotate.o	builtin-ftrace.o    builtin-record.o	 Documentation    perf-iostat.sh
> builtin-bench.c		builtin.h	    builtin-report.c	 FEATURE-DUMP	  perf.o
> builtin-bench.o		builtin-help.c      builtin-report.o	 include	  perf-read-vdso.c
> builtin-buildid-cache.c  builtin-help.o      builtin-sched.c	 jvmti		  perf-sys.h
> builtin-buildid-cache.o  builtin-inject.c    builtin-script.c	 libapi	  PERF-VERSION-FILE
> builtin-buildid-list.c	builtin-inject.o    builtin-script.o	 libperf	  perf-with-kcore
> builtin-buildid-list.o	builtin-kallsyms.c  builtin-stat.c	 libsubcmd	  pmu-events
> builtin-c2c.c		builtin-kallsyms.o  builtin-stat.o	 libsymbol	  python
> builtin-c2c.o		builtin-kmem.c      builtin-timechart.c  Makefile	  python_ext_build
> builtin-config.c	builtin-kvm.c	    builtin-top.c	 Makefile.config  scripts
> builtin-config.o	builtin-kvm.o	    builtin-top.o	 Makefile.perf    tests
> builtin-daemon.c	builtin-kwork.c     builtin-trace.c	 MANIFEST	  trace
> builtin-daemon.o	builtin-list.c      builtin-version.c	 perf		  ui
> builtin-data.c		builtin-list.o      builtin-version.o	 perf-archive	  util
> builtin-data.o		builtin-lock.c      check-headers.sh	 perf-archive.sh
> builtin-diff.c		builtin-mem.c	    command-list.txt	 perf.c
> apple_icestorm_pmu/cycles/: -1: 1035101 469125 469125
> apple_firestorm_pmu/cycles/: -1: 1035035 469125 469125
> cycles: -1: 1034653 469125 469125
> apple_icestorm_pmu/cycles/: 1035101 469125 469125
> apple_firestorm_pmu/cycles/: 1035035 469125 469125
> cycles: 1034653 469125 469125
> 
>  Performance counter stats for 'ls':
> 
>          1,035,101      apple_icestorm_pmu/cycles/                                            
>          1,035,035      apple_firestorm_pmu/cycles/                                           
>          1,034,653      cycles                                                                
> 
>        0.000001333 seconds time elapsed
> 
>        0.000000000 seconds user
>        0.000000000 seconds sys
> </quote>

... and in this case the workload was run on a CPU affine ot that arbitrary
PMU, hence we managed to count.

So AFAICT, this is a userspace bug, maybe related to the way we probe for
supported PMU features?

Thanks,
Mark.