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

From: Arnaldo Carvalho de Melo
Date: Thu Mar 05 2020 - 15:25:15 EST

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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
:-\

# 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

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Next in thread: Liang, Kan: "Re: [PATCH 02/12] perf tools: Support PERF_SAMPLE_BRANCH_HW_INDEX"
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