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

From: Arnaldo Carvalho de Melo
Date: Wed Nov 22 2023 - 08:06:30 EST

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In reply to: Hector Martin: "Re: [REGRESSION] Perf (userspace) broken on big.LITTLE systems since v6.5"
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Em Wed, Nov 22, 2023 at 12:23:27PM +0900, Hector Martin escreveu:
> On 2023/11/22 1:38, Ian Rogers wrote:
> > On Tue, Nov 21, 2023 at 8:15 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >> On Tue, Nov 21, 2023 at 08:09:37AM -0800, Ian Rogers wrote:
> >>> On Tue, Nov 21, 2023 at 8:03 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >>>> On Tue, Nov 21, 2023 at 07:46:57AM -0800, Ian Rogers wrote:
> >>>>> On Tue, Nov 21, 2023 at 7:40 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >>>>>> 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):
> >>>>>>
> >>>>>> IIUC the tool is doing the wrong thing here and overriding explicit
> >>>>>> ${pmu}/${event}/ events with PERF_TYPE_HARDWARE events rather than events using
> >>>>>> that ${pmu}'s type and event namespace.
> >>>>>>
> >>>>>> Regardless of the *new* ABI that allows PERF_TYPE_HARDWARE events to be
> >>>>>> targetted to a specific PMU, it's semantically wrong to rewrite events like
> >>>>>> this since ${pmu}/${event}/ is not necessarily equivalent to a similarly-named
> >>>>>> PERF_COUNT_HW_${EVENT}.
> >>>>>
> >>>>> If you name a PMU and an event then the event should only be opened on
> >>>>> that PMU, 100% agree. There's a bunch of output, but when the legacy
> >>>>> cycles event is opened it appears to be because it was explicitly
> >>>>> requested.
> >>>>
> >>>> I think you've missed that the named PMU events are being erreously transformed
> >>>> into PERF_TYPE_HARDWARE events. Look at the -vvv output, e.g.
> >>>>
> >>>> 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 1045843 cpu -1 group_fd -1 flags 0x8 = 4
> >>>>
> >>>> ... which should not be PERF_TYPE_HARDWARE && PERF_COUNT_HW_CPU_CYCLES.
> >>>>
> >>>> Marc said that he bisected the issue down to commit:
> >>>>
> >>>> 5ea8f2ccffb23983 ("perf parse-events: Support hardware events as terms")
> >>>>
> >>>> ... so it looks like something is going wrong when the events are being parsed,
> >>>> e.g. losing the HW PMU information?
> >>>
> >>> Ok, I think I'm getting confused by other things. This looks like the issue.
> >>>
> >>> I think it may be working as intended, but not how you intended :-) If
> >>> a core PMU is listed and then a legacy event, the legacy event should

The point is that "cycles" when prefixed with "pmu/" shouldn't be
considered "cycles" as HW/0, in that setting it is "cycles" for that
PMU. (but we only have "cpu_cycles" for at least the a53 and a72 PMUs I
have access in a Libre Computer rockchip 3399-pc hybrid board, if we use
it, then we get what we want/had before, see below):

And there is an attempt at using the specified PMU, see the first
perf_event_open:

root@roc-rk3399-pc:~# strace -e perf_event_open perf stat -vv -e cycles,armv8_cortex_a53/cycles/,armv8_cortex_a72/cycles/ echo
Using CPUID 0x00000000410fd082
------------------------------------------------------------
perf_event_attr:
type 0 (PERF_TYPE_HARDWARE)
config 0x700000000
disabled 1
------------------------------------------------------------
sys_perf_event_open: pid 0 cpu -1 group_fd -1 flags 0x8perf_event_open({type=PERF_TYPE_HARDWARE, size=0 /* PERF_ATTR_SIZE_??? */, config=0x7<<32|PERF_COUNT_HW_CPU_CYCLES, sample_period=0, sample_type=0, read_format=0, disabled=1, precise_ip=0 /* arbitrary skid */, ...}, 0, -1, -1, PERF_FLAG_FD_CLOEXEC) = -1 ENOENT (No such file or directory)

//// HERE: it tries config=0x7<<32|PERF_COUNT_HW_CPU_CYCLES taking into
//account the PMU number 0x7

root@roc-rk3399-pc:~# cat /sys/devices/armv8_cortex_a53/type
7
root@roc-rk3399-pc:~#

But then we don't have "cycles" in that PMU:

root@roc-rk3399-pc:~# ls -la /sys/devices/armv8_cortex_a53/events/cycles
ls: cannot access '/sys/devices/armv8_cortex_a53/events/cycles': No such file or directory
root@roc-rk3399-pc:~#

Maybe:

root@roc-rk3399-pc:~# taskset -c 5,6 perf stat -v -e armv8_cortex_a53/cpu_cycles/,armv8_cortex_a72/cpu_cycles/ echo
Using CPUID 0x00000000410fd034
Control descriptor is not initialized

armv8_cortex_a53/cpu_cycles/: 0 2079000 0
armv8_cortex_a72/cpu_cycles/: 2488961 2079000 2079000

Performance counter stats for 'echo':

<not counted> armv8_cortex_a53/cpu_cycles/ (0.00%)
2488961 armv8_cortex_a72/cpu_cycles/

0.003449266 seconds time elapsed

0.003502000 seconds user
0.000000000 seconds sys

root@roc-rk3399-pc:~# taskset -c 0,1,2,3,4 perf stat -v -e armv8_cortex_a53/cpu_cycles/,armv8_cortex_a72/cpu_cycles/ echo
Using CPUID 0x00000000410fd034
Control descriptor is not initialized

armv8_cortex_a53/cpu_cycles/: 2986601 6999416 6999416
armv8_cortex_a72/cpu_cycles/: 0 6999416 0

Performance counter stats for 'echo':

2986601 armv8_cortex_a53/cpu_cycles/
<not counted> armv8_cortex_a72/cpu_cycles/ (0.00%)

0.011434508 seconds time elapsed

0.003911000 seconds user
0.007454000 seconds sys

root@roc-rk3399-pc:~#

root@roc-rk3399-pc:~# cat /sys/devices/armv8_cortex_a53/events/cpu_cycles
event=0x0011
root@roc-rk3399-pc:~# cat /sys/devices/armv8_cortex_a72/events/cpu_cycles
event=0x0011
root@roc-rk3399-pc:~#

And the syscalls seem sane:

root@roc-rk3399-pc:~# strace -e perf_event_open taskset -c 0,1,2,3,4 perf stat -v -e armv8_cortex_a53/cpu_cycles/,armv8_cortex_a72/cpu_cycles/ echo
Using CPUID 0x00000000410fd034
Control descriptor is not initialized
perf_event_open({type=0x7 /* PERF_TYPE_??? */, size=0x88 /* PERF_ATTR_SIZE_??? */, config=0x11, sample_period=0, sample_type=PERF_SAMPLE_IDENTIFIER, read_format=PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING, disabled=1, inherit=1, enable_on_exec=1, precise_ip=0 /* arbitrary skid */, exclude_guest=1, ...}, 14573, -1, -1, PERF_FLAG_FD_CLOEXEC) = 3
perf_event_open({type=0x8 /* PERF_TYPE_??? */, size=0x88 /* PERF_ATTR_SIZE_??? */, config=0x11, sample_period=0, sample_type=PERF_SAMPLE_IDENTIFIER, read_format=PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING, disabled=1, inherit=1, enable_on_exec=1, precise_ip=0 /* arbitrary skid */, exclude_guest=1, ...}, 14573, -1, -1, PERF_FLAG_FD_CLOEXEC) = 4

--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=14573, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
armv8_cortex_a53/cpu_cycles/: 3227098 4480875 4480875
armv8_cortex_a72/cpu_cycles/: 0 4480875 0

Performance counter stats for 'echo':

3227098 armv8_cortex_a53/cpu_cycles/
<not counted> armv8_cortex_a72/cpu_cycles/ (0.00%)

0.008381759 seconds time elapsed

0.004064000 seconds user
0.004121000 seconds sys

--- SIGCHLD {si_signo=SIGCHLD, si_code=SI_USER, si_pid=14572, si_uid=0} ---
+++ exited with 0 +++
root@roc-rk3399-pc:~#

As:

root@roc-rk3399-pc:~# cat /sys/devices/armv8_cortex_a53/type
7
root@roc-rk3399-pc:~# cat /sys/devices/armv8_cortex_a72/type
8
root@roc-rk3399-pc:~#

See the type=0x7 and type=0x8.

So what we need here seems to be to translate the generic term "cycles"
to "cpu_cycles" when a PMU is explicitely passed in the event name and
it doesn't have "cycles" and then just retry.

- Arnaldo

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In reply to: Hector Martin: "Re: [REGRESSION] Perf (userspace) broken on big.LITTLE systems since v6.5"
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