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

[James Clark]

On 14/08/2024 5:41 pm, Arnaldo Carvalho de Melo wrote:
> On Wed, Aug 14, 2024 at 05:28:42PM +0100, James Clark wrote:
> > On 07/08/2024 9:54 am, Thorsten Leemhuis wrote:
> > > On 01.08.24 21:05, Ian Rogers wrote:
> > > > On Wed, Dec 6, 2023 at 4:09 AM Linux regression tracking #update
> > > > (Thorsten Leemhuis) <regressions@xxxxxxxxxxxxx> wrote:
> > > > > [TLDR: This mail in primarily relevant for Linux kernel regression
> > > > > tracking. See link in footer if these mails annoy you.]
> > > > >
> > > > > On 22.11.23 00:43, Bagas Sanjaya wrote:
> > > > > > On Tue, Nov 21, 2023 at 09:08:48PM +0900, Hector Martin 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.
> > > > >
> > > > > #regzbot fix: perf parse-events: Make legacy events lower priority than
> > > > > sysfs/JSON
> > > > > #regzbot ignore-activity
> > > >
> > > > Note, this is still broken.
> > >
> > > Hmmm, so all that became somewhat messy. Arnaldo, what's the way out of
> > > this? Or is this a "we are screwed one way or another and someone has to
> > > bite the bullet" situation?
> > >
> > > Ciao, Thorsten
> > >
> > > > The patch changed the priority in the case
> > > > that you do something like:
> > > >
> > > > $ perf stat -e 'armv8_pmuv3_0/cycles/' benchmark
> > > >
> > > > but if you do:
> > > >
> > > > $ perf stat -e 'cycles' benchmark
> > > >
> > > > then the broken behavior will happen as legacy events have priority
> > > > over sysfs/json events in that case. To fix this you need to revert:
> > > > 4f1b067359ac Revert "perf parse-events: Prefer sysfs/JSON hardware
> > > > events over legacy"
> > > >
> > > > This causes some testing issues resolved in this unmerged patch series:
> > > > https://lore.kernel.org/lkml/20240510053705.2462258-1-irogers@xxxxxxxxxx/
> > > >
> > > > There is a bug as the arm_dsu PMU advertises an event called "cycles"
> > > > and this PMU is present on Ampere systems. Reverting the commit above
> > > > will cause an issue as the commit 7b100989b4f6 ("perf evlist: Remove
> > > > __evlist__add_default") to fix ARM's BIG.little systems (opening a
> > > > cycles event on all PMUs not just 1) will cause the arm_dsu event to
> > > > be opened by perf record and fail as the event won't support sampling.
> > > >
> > > > The patch https://lore.kernel.org/lkml/20240525152927.665498-1-irogers@xxxxxxxxxx/
> > > > fixes this by only opening the cycles event on core PMUs when choosing
> > > > default events.
> > > >
> > > > Rather than take this patch the revert happened as Linus runs the
> > > > command "perf record -e cycles:pp" (ie using a specified event and not
> > > > defaults) and considers it a regression in the perf tool that on an
> > > > Ampere system to need to do "perf record -e
> > > > 'armv8_pmuv3_0/cycles/pp'". It was pointed out that not specifying -e
> > > > will choose the cycles event correctly and with better precision the
> > > > pp for systems that support it, but it was still considered a
> > > > regression in the perf tool so the revert was made to happen. There is
> > > > a lack of perf testing coverage for ARM, in particular as they choose
> > > > to do everything in a different way to x86. The patch in question was
> > > > in the linux-next tree for weeks without issues.
> > > >
> > > > ARM/Ampere could fix this by renaming the event from cycles to
> > > > cpu_cycles, or by following Intel's convention that anything uncore
> > > > uses the name clockticks rather than cycles. This could break people
> > > > who rely on an event called arm_dsu/cycles/ but I imagine such people
> > > > are rare. There has been no progress I'm aware of on renaming the
> > > > event.
> > > >
> > > > Making perf not terminate on opening an event for perf record seems
> > > > like the most likely workaround as that is at least something under
> > > > the tool maintainers control. ARM have discussed doing this on the
> > > > lists:
> > > > https://lore.kernel.org/lkml/f30f676e-a1d7-4d6b-94c1-3bdbd1448887@xxxxxxx/
> > > > but since the revert in v6.10 no patches have appeared for the v6.11
> > > > merge window. Feature work like coresight improvements and ARMv9 are
> > > > being actively pursued by ARM, but feature work won't resolve this
> > > > regression.
> >
> > I got some hardware with the DSU PMU so I'm going to have a go at trying to
> > send some fixes for this. My initial idea was to try incorporate the "not
> > terminate on opening" change as discussed in the link directly above. And
> > then do the revert of the "revert of prefer sysfs/json".
> >
> > FWIW I don't think Juno currently is broken if the kernel supports extended
> > type ID? I could have missed some output in this thread but it seems like
> > it's mostly related to Apple M hardware. I'm also a bit confused why the
> > "supports extended type" check fails there, but maybe the v6.9 commit
> > 25412c036 from Mark is missing?
> >
> > I sent a small fix the other day to make perf stat default arguments work on
> > Juno, and didn't notice anything out of the ordinary: https://lore.kernel.org/linux-perf-users/dac6ad1d-5aca-48b4-9dcb-ff7e54ca43f6@xxxxxxxxxx/T/#t
> > I agree that change is quite narrow but it does incrementally improve things
> > for the time being. It's possible that it would become redundant if I can
> > just include Ian's change to use strings for Perf stat.
> >
> > Of course I only think I have a handle on the issue right now, seems like it
> > has a lot of moving parts and something else always comes up. If I hit a
> > wall at some point I will come back here.
>
> Thanks for working on this, hopefully we'll get to a solution that keeps
> all the expectations expressed in this thread about not breaking
> existing muscle memory and that allows us to progress on this matter.
>
> - Arnaldo

Hi Arnaldo,

In one of your investigations here 
https://lore.kernel.org/lkml/Zld3dlJHjFMFG02v@x1/ comparing "cycles", 
"cpu-cycles" and "cpu_cycles" events on Arm you say only some of them 
open events on both core types. I wasn't able to reproduce that on 
perf-tools-next (27ac597c0e) or v6.9 (a38297e3fb) for perf record or 
stat. I guessed the 6.9 tag because you only mentioned it was on tip and 
it was 29th May. For me they all open exactly the same two legacy events 
with the extended type ID set.

It looks like the behavior you see would be caused by either missing 
this kernel change:

  5c81672865 ("arm_pmu: Add PERF_PMU_CAP_EXTENDED_HW_TYPE capability")
   (v6.6 release)

Or this userspace change, but unlikely as it was a fix for Apple M hardware:

  25412c036 ("perf print-events: make is_event_supported() more robust")
   (v6.9 release)

Do you remember if you were using a new kernel or only testing a new 
Perf? Or if you don't mind could you re-test? Hopefully not to derail 
the discussion but I just want to make sure I'm not missing some other 
third issue before I start hacking away.

I believe we still need to revert the revert of the JSON/legacy change. 
Because as Mark mentions there is no guarantee that a PMU's named event 
is the same as a legacy event of the same name, so we do want to prefer 
sysfs/JSON. There are some other edge cases like new Perf on an old 
kernel before we added extended type support, but I don't think I'll 
list all of them.

Having said that, I believe that currently all the sysfs and legacy 
events actually _are_ the same. So it's not a user facing issue _yet_, 
or at least on any hardware mentioned in these threads.

Thanks
James