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

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Date:   Tue, 21 Nov 2023 15:56:22 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Hector Martin <marcan@...can.st>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        James Clark <james.clark@....com>,
        linux-perf-users@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Asahi Linux <asahi@...ts.linux.dev>
Subject: Re: [REGRESSION] Perf (userspace) broken on big.LITTLE systems since
 v6.5

On Tue, Nov 21, 2023 at 07:41:17AM -0800, Ian Rogers wrote:
> Hi Marc,

Hi Ian,

> I'm unclear if you are running a newer perf tool on an older kernel or
> not. In any case I'll assume the kernel and perf tool versions match.
> In Linux 6.6 this patch was added to the ARM PMU:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/perf/arm_pmu.c?id=5c816728651ae425954542fed64d21d40cb75a9f
> 
> My guess is that the apple_icestorm_pmu requires a similar patch. 

The apple_icestorm_pmu PMU driver uses the arm_pmu framework, so it's using
that code (since v6.6).

> The perf tool is supposed to not use extended types when they aren't
> supported:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/pmus.c?h=perf-tools-next#n532

How does that is_event_supported() check actually work? I suspect that's giving
the wrong answer.

Regardless, I think the tool is doing something semantically wrong, see below.

> So I share your confusion as to why something broke.
> 
> PERF_TYPE_HARDWARE is a legacy type where there are hardcoded type and
> config values that correspond to an event. The PMU driver turns legacy
> events into the real types. On BIG.little systems if the legacy events
> are monitoring a task a different event is needed for each PMU (ie >1
> event). In your example you are monitoring 'ls', a task, and so
> different cycles events are necessary. In the high 32-bits (the
> extended type) the PMU is identified.

I think the interesting thing here is that the tool is mapping events with an
explicit PMU into legacy PERF_TYPE_HARDWARE events, which is the opposite
direction than intended. Regardless of whether PERF_TYPE_HARDWARE events can be
targetted to a specific PMU, if the user has requested to use a specific PMU we
should be using that PMU and related event namespace.

Marc's command line was:

	sudo taskset -c 0 ./perf stat -vvv \
		-e apple_icestorm_pmu/cycles/ \
		-e apple_firestorm_pmu/cycles/ \
		-e cycles \
	ls

... and so the apple_*_pmu events should target their respective PMUs, and the
plain 'cycles' event could legitimately be opened as a single
PERF_TYPE_HARDWARE event, or split into two directed PERF_TYPE_HARDWARE events
targetting the two PMUs.

However, thwe tool opens three (undirected?) PERF_TYPE_HARDWARE events:

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 1045843  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 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
------------------------------------------------------------

Mark.