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SubjectRe: [PATCH] perf/x86/intel: Fix n_metric for the canceled group
On Wed, Sep 30, 2020 at 07:29:35AM -0700, kan.liang@linux.intel.com wrote:
> From: Kan Liang <kan.liang@linux.intel.com>
>
> When a group that has TopDown members is failed to be scheduled, any
> later TopDown groups will not return valid values.
>
> Here is an example.
>
> A background perf that occupies all the GP counters and the fixed
> counter 1.
> $perf stat -e "{cycles,cycles,cycles,cycles,cycles,cycles,cycles,
> cycles,cycles}:D" -a
>
> A user monitors a TopDown group. It works well, because the fixed
> counter 3 and the PERF_METRICS are available.
> $perf stat -x, --topdown -- ./workload
> retiring,bad speculation,frontend bound,backend bound,
> 18.0,16.1,40.4,25.5,
>
> Then the user tries to monitor a group that has TopDown members.
> Because of the cycles event, the group is failed to be scheduled.
> $perf stat -x, -e '{slots,topdown-retiring,topdown-be-bound,
> topdown-fe-bound,topdown-bad-spec,cycles}'
> -- ./workload
> <not counted>,,slots,0,0.00,,
> <not counted>,,topdown-retiring,0,0.00,,
> <not counted>,,topdown-be-bound,0,0.00,,
> <not counted>,,topdown-fe-bound,0,0.00,,
> <not counted>,,topdown-bad-spec,0,0.00,,
> <not counted>,,cycles,0,0.00,,
>
> The user tries to monitor a TopDown group again. It doesn't work anymore.
> $perf stat -x, --topdown -- ./workload
>
> ,,,,,
>
> In a txn, cancel_txn() is to truncate the event_list for a canceled
> group and update the number of events added in this transaction.
> However, the number of TopDown events added in this transaction is not
> updated. The kernel will probably fail to add new Topdown events.
>
> Check if the canceled group has Topdown events. If so, subtract the
> TopDown events from n_metric accordingly.
>
> Fixes: 7b2c05a15d29 ("perf/x86/intel: Generic support for hardware TopDown metrics")
> Reported-by: Andi Kleen <ak@linux.intel.com>
> Reviewed-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
> ---
> arch/x86/events/core.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 0f3d01562ded..4cb3ccbe2d62 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2017,6 +2017,7 @@ static void x86_pmu_cancel_txn(struct pmu *pmu)
> {
> unsigned int txn_flags;
> struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> + int i;
>
> WARN_ON_ONCE(!cpuc->txn_flags); /* no txn in flight */
>
> @@ -2031,6 +2032,15 @@ static void x86_pmu_cancel_txn(struct pmu *pmu)
> */
> __this_cpu_sub(cpu_hw_events.n_added, __this_cpu_read(cpu_hw_events.n_txn));
> __this_cpu_sub(cpu_hw_events.n_events, __this_cpu_read(cpu_hw_events.n_txn));
> +
> + /* Subtract Topdown events in the canceled group from n_metric */
> + if (x86_pmu.intel_cap.perf_metrics && cpuc->n_metric) {
> + for (i = 0; i < cpuc->n_txn; i++) {
> + if (is_metric_event(cpuc->event_list[i + cpuc->n_events]))
> + __this_cpu_dec(cpu_hw_events.n_metric);
> + }
> + WARN_ON_ONCE(__this_cpu_read(cpu_hw_events.n_metric) < 0);
> + }
> perf_pmu_enable(pmu);
> }


Urgh, I'd much rather we add n_txn_metric. But also, while looking at
this, don't we have the same problem with n_pair ?

Something like this perhaps...

---
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 757e49755e7c..9b7792c0b6fb 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -1066,6 +1066,7 @@ static int add_nr_metric_event(struct cpu_hw_events *cpuc,
if (cpuc->n_metric == INTEL_TD_METRIC_NUM)
return -EINVAL;
cpuc->n_metric++;
+ cpuc->n_txn_metric++;
}

return 0;
@@ -1089,8 +1090,10 @@ static int collect_event(struct cpu_hw_events *cpuc, struct perf_event *event,
return -EINVAL;

cpuc->event_list[n] = event;
- if (is_counter_pair(&event->hw))
+ if (is_counter_pair(&event->hw)) {
cpuc->n_pair++;
+ cpuc->n_txn_pair++;
+ }

return 0;
}
@@ -2062,6 +2065,8 @@ static void x86_pmu_start_txn(struct pmu *pmu, unsigned int txn_flags)

perf_pmu_disable(pmu);
__this_cpu_write(cpu_hw_events.n_txn, 0);
+ __this_cpu_write(cpu_hw_events.n_txn_metric, 0);
+ __this_cpu_write(cpu_hw_events.n_txn_pair, 0);
}

/*
@@ -2087,6 +2092,8 @@ static void x86_pmu_cancel_txn(struct pmu *pmu)
*/
__this_cpu_sub(cpu_hw_events.n_added, __this_cpu_read(cpu_hw_events.n_txn));
__this_cpu_sub(cpu_hw_events.n_events, __this_cpu_read(cpu_hw_events.n_txn));
+ __this_cpu_sub(cpu_hw_events.n_metric, __this_cpu_read(cpu_hw_events.n_txn_metric));
+ __this_cpu_sub(cpu_hw_events.n_pair, __this_cpu_read(cpu_hw_events.n_txn_pair));
perf_pmu_enable(pmu);
}

diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index 345442410a4d..6348105b6d30 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -235,6 +235,8 @@ struct cpu_hw_events {
they've never been enabled yet */
int n_txn; /* the # last events in the below arrays;
added in the current transaction */
+ int n_txn_metric;
+ int n_txn_pair;
int assign[X86_PMC_IDX_MAX]; /* event to counter assignment */
u64 tags[X86_PMC_IDX_MAX];

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