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Date: Wed, 29 May 2024 12:18:58 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Ben Gainey <Ben.Gainey@....com>
Cc: "alexander.shishkin@...ux.intel.com" <alexander.shishkin@...ux.intel.com>,
"peterz@...radead.org" <peterz@...radead.org>, "acme@...nel.org" <acme@...nel.org>,
"mingo@...hat.com" <mingo@...hat.com>, James Clark <James.Clark@....com>,
"adrian.hunter@...el.com" <adrian.hunter@...el.com>, "irogers@...gle.com" <irogers@...gle.com>,
"jolsa@...nel.org" <jolsa@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>, Mark Rutland <Mark.Rutland@....com>
Subject: Re: [PATCH v6 4/4] tools/perf: Allow inherit + PERF_SAMPLE_READ when
opening events
Hello,
On Wed, May 29, 2024 at 10:48 AM Ben Gainey <Ben.Gainey@....com> wrote:
>
> On Mon, 2024-05-27 at 18:47 +0100, Ben Gainey wrote:
> > On Thu, 2024-05-23 at 18:41 -0700, Namhyung Kim wrote:
> > > On Tue, May 21, 2024 at 6:30 AM Ben Gainey <ben.gainey@....com>
> > > wrote:
> > > >
> > > > The tool will now default to this new mode if the user specifies
> > > > a
> > > > sampling group when not in system-wide mode, and when --no-
> > > > inherit
> > > > is not specified.
> > > >
> > > > This change updates evsel to allow the combination of inherit
> > > > and PERF_SAMPLE_READ.
> > > >
> > > > A fallback is implemented for kernel versions where this feature
> > > > is
> > > > not
> > > > supported.
> > >
> > > But I'm afraid the test would fail on old kernels. Maybe we need
> > > to
> > > put it in the selftests.
> > >
> >
> > Sorry, not clear what you mean?
> >
> > Is the issue that the fallback on older kernels fails, or that the
> > "perf test" tests fail?
> >
> > Thanks
> > Ben
>
> Just to follow up, I've rechecked the fallback on an unmodified 6.9.1
> kernel with the following:
>
> perf record -vv -e '{cycles,instructions}:S' ls
>
> With an unpatched version of perf running on an unpatched kernel, the
> cycles event is opened as:
>
> ------------------------------------------------------------
> perf_event_attr:
> type 0 (PERF_TYPE_HARDWARE)
> size 136
> config 0 (PERF_COUNT_HW_CPU_CYCLES)
> { sample_period, sample_freq } 4000
> sample_type IP|TID|TIME|READ|ID|PERIOD
> read_format ID|GROUP|LOST
> disabled 1
> exclude_kernel 1
> exclude_hv 1
> mmap 1
> comm 1
> freq 1
> enable_on_exec 1
> task 1
> sample_id_all 1
> exclude_guest 1
> mmap2 1
> comm_exec 1
> ksymbol 1
> bpf_event 1
> ------------------------------------------------------------
>
> whereas with these patches applied to perf, on an unpatched kernel, the
> output is as follows
>
> ------------------------------------------------------------
> perf_event_attr:
> type 0 (PERF_TYPE_HARDWARE)
> size 136
> config 0 (PERF_COUNT_HW_CPU_CYCLES)
> { sample_period, sample_freq } 4000
> sample_type IP|TID|TIME|READ|ID|PERIOD
> read_format ID|GROUP|LOST
> disabled 1
> inherit 1
> exclude_kernel 1
> exclude_hv 1
> mmap 1
> comm 1
> freq 1
> enable_on_exec 1
> task 1
> sample_id_all 1
> exclude_guest 1
> mmap2 1
> comm_exec 1
> ksymbol 1
> bpf_event 1
> ------------------------------------------------------------
> sys_perf_event_open: pid 3442954 cpu 0 group_fd -1 flags 0x8
> sys_perf_event_open failed, error -22
> Using PERF_SAMPLE_READ / :S modifier is not compatible with
> inherit, falling back to no-inherit.
> ------------------------------------------------------------
> perf_event_attr:
> type 0 (PERF_TYPE_HARDWARE)
> size 136
> config 0 (PERF_COUNT_HW_CPU_CYCLES)
> { sample_period, sample_freq } 4000
> sample_type IP|TID|TIME|READ|ID|PERIOD
> read_format ID|GROUP|LOST
> disabled 1
> exclude_kernel 1
> exclude_hv 1
> mmap 1
> comm 1
> freq 1
> enable_on_exec 1
> task 1
> sample_id_all 1
> exclude_guest 1
> mmap2 1
> comm_exec 1
> ksymbol 1
> bpf_event 1
> ------------------------------------------------------------
>
> The command falls back to the same configuration as was previously
> used. The same is true for the instructions event.
>
> `perf test` fails on an unpatched kernel in "15: Setup struct
> perf_event_attr" for the test "test-record-group-sampling1" but that is
> surely expected for unpatched kernels?
>
> Is there some very-old kernel version where this would be expected to
> succeed by accident?
I don't think so but I don't want the test to fail depending on the
kernel version. Maybe we can check the allowed combination
first and skip the test if perf_event_open() fails. And then it can
verify if the kernel rejects the unsupported combinations. Not
sure if it's easy to do that in the current attr test framework.
Thanks,
Namhyung
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