Re: [PATCH 02/12] perf tools: Support PERF_SAMPLE_BRANCH_HW_INDEX

From: Arnaldo Carvalho de Melo
Date: Thu Mar 05 2020 - 18:17:50 EST

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In reply to: Liang, Kan: "Re: [PATCH 02/12] perf tools: Support PERF_SAMPLE_BRANCH_HW_INDEX"
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Em Thu, Mar 05, 2020 at 04:02:10PM -0500, Liang, Kan escreveu:
>
>
> On 3/5/2020 3:25 PM, Arnaldo Carvalho de Melo wrote:
> > Em Fri, Feb 28, 2020 at 08:30:01AM -0800, kan.liang@xxxxxxxxxxxxxxx escreveu:
> > > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > >
> > > A new branch sample type PERF_SAMPLE_BRANCH_HW_INDEX has been introduced
> > > in latest kernel.
> > >
> > > Enable HW_INDEX by default in LBR call stack mode.
> > > If kernel doesn't support the sample type, switching it off.
> > >
> > > Add HW_INDEX in attr_fprintf as well. User can check whether the branch
> > > sample type is set via debug information or header.
> >
> > So while this works with a kernel where PERF_SAMPLE_BRANCH_HW_INDEX is
> > present and we get, from the committer notes I was putting together
> > while testing/applying this cset:
> >
> > First collect some samples with LBR callchains, system wide, for a few
> > seconds:
> >
> > # perf record --call-graph lbr -a sleep 5
> > [ perf record: Woken up 1 times to write data ]
> > [ perf record: Captured and wrote 0.625 MB perf.data (224 samples) ]
> > #
> >
> > Now lets use 'perf evlist -v' to look at the branch_sample_type:
> >
> > # perf evlist -v
> > cycles: size: 120, { sample_period, sample_freq }: 4000, sample_type: IP|TID|TIME|CALLCHAIN|CPU|PERIOD|BRANCH_STACK, read_format: ID, disabled: 1, inherit: 1, mmap: 1, comm: 1, freq: 1, task: 1, precise_ip: 3, sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1, ksymbol: 1, bpf_event: 1, branch_sample_type: USER|CALL_STACK|NO_FLAGS|NO_CYCLES|HW_INDEX
> > #
> >
> > So the machine has the kernel feature, and it was correctly added to
> > perf_event_attr.branch_sample_type, for the default 'cycles' event.
> >
> > Cool, and look at that 'attr.precise_ip: 3' part, the kernel is OK with
> > having that together with attr.branch_sample_type with HW_INDEX set.
> >
> > The problem happens when I go test this in an older kernel, where the
> > kernel doesn't know about HW_INDEX, we get it disabled but then
> > precise_ip is set to zero in its detection , even if at the end we get
> > it to 3, as expected, which got me a bit confused, I'll investigate this
> > a bit more to try and avoid these extra probes for the max precise level
> > that fails in older kernels due to branch_sample_type having HW_INDEX
> > :-\
>
> It looks like this is an expected behavior for the event with maximum
> precise config for current perf tool.
>
> The related commits are as below:
> commit ID: 4e8a5c155137 ("perf evsel: Fix max perf_event_attr.precise_ip
> detection")
> commit ID: cd136189370c ("perf evsel: Do not rely on errno values for
> precise_ip fallback")
>
> Before handling any standard fallback (not just HW_INDEX), perf tool will
> try all the precise_ip value first.

Right, and since it uses HW_INDEX and the kernel doesn't support that,
that precise_ip max value detection will fail, it will go back to
whatever it was and try again later, then the fallback will remove the
HW_INDEX and it will work.

What I said is that if we don't set branch_sample_type when detecting
the max precise_ip, then that detection will work and we will not
needlessly go on testing with precise_ip = 3, 2, 1, 0.

My doubt was more about if HW_INDEX (or any other branch_sample_type)
had to be tested in conjunction with precise_ip, which I think isn't
the case from what I saw in my tests.

- Arnaldo

> Thanks,
> Kan
>
> >
> >
> > # perf record -vv --call-graph lbr -a sleep 5
> > <SNIP>
> > ------------------------------------------------------------
> > perf_event_attr:
> > size 120
> > { sample_period, sample_freq } 4000
> > sample_type IP|TID|TIME|CALLCHAIN|CPU|PERIOD|BRANCH_STACK
> > read_format ID
> > disabled 1
> > inherit 1
> > mmap 1
> > comm 1
> > freq 1
> > task 1
> > precise_ip 3
> > sample_id_all 1
> > exclude_guest 1
> > mmap2 1
> > comm_exec 1
> > ksymbol 1
> > bpf_event 1
> > branch_sample_type USER|CALL_STACK|NO_FLAGS|NO_CYCLES|HW_INDEX
> > ------------------------------------------------------------
> > sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8
> > sys_perf_event_open failed, error -95
> > decreasing precise_ip by one (2)
> > ------------------------------------------------------------
> > perf_event_attr:
> > size 120
> > { sample_period, sample_freq } 4000
> > sample_type IP|TID|TIME|CALLCHAIN|CPU|PERIOD|BRANCH_STACK
> > read_format ID
> > disabled 1
> > inherit 1
> > mmap 1
> > comm 1
> > freq 1
> > task 1
> > precise_ip 2
> > sample_id_all 1
> > exclude_guest 1
> > mmap2 1
> > comm_exec 1
> > ksymbol 1
> > bpf_event 1
> > branch_sample_type USER|CALL_STACK|NO_FLAGS|NO_CYCLES|HW_INDEX
> > ------------------------------------------------------------
> > sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8
> > sys_perf_event_open failed, error -95
> > decreasing precise_ip by one (1)
> > ------------------------------------------------------------
> > perf_event_attr:
> > size 120
> > { sample_period, sample_freq } 4000
> > sample_type IP|TID|TIME|CALLCHAIN|CPU|PERIOD|BRANCH_STACK
> > read_format ID
> > disabled 1
> > inherit 1
> > mmap 1
> > comm 1
> > freq 1
> > task 1
> > precise_ip 1
> > sample_id_all 1
> > exclude_guest 1
> > mmap2 1
> > comm_exec 1
> > ksymbol 1
> > bpf_event 1
> > branch_sample_type USER|CALL_STACK|NO_FLAGS|NO_CYCLES|HW_INDEX
> > ------------------------------------------------------------
> > sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8
> > sys_perf_event_open failed, error -95
> > decreasing precise_ip by one (0)
> > ------------------------------------------------------------
> > perf_event_attr:
> > size 120
> > { sample_period, sample_freq } 4000
> > sample_type IP|TID|TIME|CALLCHAIN|CPU|PERIOD|BRANCH_STACK
> > read_format ID
> > disabled 1
> > inherit 1
> > mmap 1
> > comm 1
> > freq 1
> > task 1
> > sample_id_all 1
> > exclude_guest 1
> > mmap2 1
> > comm_exec 1
> > ksymbol 1
> > bpf_event 1
> > branch_sample_type USER|CALL_STACK|NO_FLAGS|NO_CYCLES|HW_INDEX
> > ------------------------------------------------------------
> > sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8
> > sys_perf_event_open failed, error -22
> > switching off branch HW index support
> > ------------------------------------------------------------
> > perf_event_attr:
> > size 120
> > { sample_period, sample_freq } 4000
> > sample_type IP|TID|TIME|CALLCHAIN|CPU|PERIOD|BRANCH_STACK
> > read_format ID
> > disabled 1
> > inherit 1
> > mmap 1
> > comm 1
> > freq 1
> > task 1
> > precise_ip 3
> > sample_id_all 1
> > exclude_guest 1
> > mmap2 1
> > comm_exec 1
> > ksymbol 1
> > bpf_event 1
> > branch_sample_type USER|CALL_STACK|NO_FLAGS|NO_CYCLES
> > ------------------------------------------------------------
> >
> >
> > > Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > > ---
> > > tools/perf/util/evsel.c | 15 ++++++++++++---
> > > tools/perf/util/evsel.h | 1 +
> > > tools/perf/util/perf_event_attr_fprintf.c | 1 +
> > > 3 files changed, 14 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > > index 05883a45de5b..816d930d774e 100644
> > > --- a/tools/perf/util/evsel.c
> > > +++ b/tools/perf/util/evsel.c
> > > @@ -712,7 +712,8 @@ static void __perf_evsel__config_callchain(struct evsel *evsel,
> > > attr->branch_sample_type = PERF_SAMPLE_BRANCH_USER |
> > > PERF_SAMPLE_BRANCH_CALL_STACK |
> > > PERF_SAMPLE_BRANCH_NO_CYCLES |
> > > - PERF_SAMPLE_BRANCH_NO_FLAGS;
> > > + PERF_SAMPLE_BRANCH_NO_FLAGS |
> > > + PERF_SAMPLE_BRANCH_HW_INDEX;
> > > }
> > > } else
> > > pr_warning("Cannot use LBR callstack with branch stack. "
> > > @@ -763,7 +764,8 @@ perf_evsel__reset_callgraph(struct evsel *evsel,
> > > if (param->record_mode == CALLCHAIN_LBR) {
> > > perf_evsel__reset_sample_bit(evsel, BRANCH_STACK);
> > > attr->branch_sample_type &= ~(PERF_SAMPLE_BRANCH_USER |
> > > - PERF_SAMPLE_BRANCH_CALL_STACK);
> > > + PERF_SAMPLE_BRANCH_CALL_STACK |
> > > + PERF_SAMPLE_BRANCH_HW_INDEX);
> > > }
> > > if (param->record_mode == CALLCHAIN_DWARF) {
> > > perf_evsel__reset_sample_bit(evsel, REGS_USER);
> > > @@ -1673,6 +1675,8 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,
> > > evsel->core.attr.ksymbol = 0;
> > > if (perf_missing_features.bpf)
> > > evsel->core.attr.bpf_event = 0;
> > > + if (perf_missing_features.branch_hw_idx)
> > > + evsel->core.attr.branch_sample_type &= ~PERF_SAMPLE_BRANCH_HW_INDEX;
> > > retry_sample_id:
> > > if (perf_missing_features.sample_id_all)
> > > evsel->core.attr.sample_id_all = 0;
> > > @@ -1784,7 +1788,12 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,
> > > * Must probe features in the order they were added to the
> > > * perf_event_attr interface.
> > > */
> > > - if (!perf_missing_features.aux_output && evsel->core.attr.aux_output) {
> > > + if (!perf_missing_features.branch_hw_idx &&
> > > + (evsel->core.attr.branch_sample_type & PERF_SAMPLE_BRANCH_HW_INDEX)) {
> > > + perf_missing_features.branch_hw_idx = true;
> > > + pr_debug2("switching off branch HW index support\n");
> > > + goto fallback_missing_features;
> > > + } else if (!perf_missing_features.aux_output && evsel->core.attr.aux_output) {
> > > perf_missing_features.aux_output = true;
> > > pr_debug2_peo("Kernel has no attr.aux_output support, bailing out\n");
> > > goto out_close;
> > > diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
> > > index 99a0cb60c556..33804740e2ca 100644
> > > --- a/tools/perf/util/evsel.h
> > > +++ b/tools/perf/util/evsel.h
> > > @@ -119,6 +119,7 @@ struct perf_missing_features {
> > > bool ksymbol;
> > > bool bpf;
> > > bool aux_output;
> > > + bool branch_hw_idx;
> > > };
> > > extern struct perf_missing_features perf_missing_features;
> > > diff --git a/tools/perf/util/perf_event_attr_fprintf.c b/tools/perf/util/perf_event_attr_fprintf.c
> > > index 651203126c71..355d3458d4e6 100644
> > > --- a/tools/perf/util/perf_event_attr_fprintf.c
> > > +++ b/tools/perf/util/perf_event_attr_fprintf.c
> > > @@ -50,6 +50,7 @@ static void __p_branch_sample_type(char *buf, size_t size, u64 value)
> > > bit_name(ABORT_TX), bit_name(IN_TX), bit_name(NO_TX),
> > > bit_name(COND), bit_name(CALL_STACK), bit_name(IND_JUMP),
> > > bit_name(CALL), bit_name(NO_FLAGS), bit_name(NO_CYCLES),
> > > + bit_name(HW_INDEX),
> > > { .name = NULL, }
> > > };
> > > #undef bit_name
> > > --
> > > 2.17.1
> > >
> >

--

- Arnaldo

Next message: Rafael J. Wysocki: "[PATCH] ACPI: EC: Eliminate EC_FLAGS_QUERY_HANDSHAKE"
Previous message: Rafael J. Wysocki: "[PATCH 1/2] ACPI: EC: Simplify acpi_ec_ecdt_start() and acpi_ec_init()"
In reply to: Liang, Kan: "Re: [PATCH 02/12] perf tools: Support PERF_SAMPLE_BRANCH_HW_INDEX"
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