Re: [PATCH v3] perf stat: Start tool events when enable_on_exec is used

[sashiko-bot@kernel.org]

> When perf stat runs a workload, target__enable_on_exec() is true and the
> tool events are left to the kernel to start on exec.  Tool events are
> however emulated in perf (duration_time, user_time, system_time,
> num_cpus) and have nothing in the kernel to start them, so they stay
> disabled and are read as zero.
> 
> For duration_time this makes CPUs_utilized divide by zero, and plain
> "perf stat" prints "nan CPUs":
> 
>   $ perf stat -e task-clock,duration_time -- sleep 0.1
>                0      duration_time
> 
> Start the tool events explicitly in that case.  They are started through
> evsel__tool_pmu_enable() rather than evsel__enable(), because the latter
> also enables the members of a mixed PMU group, and those cannot be used
> here: get_group_fd() passes the leader's descriptor to
> sys_perf_event_open(), which fails with EINVAL for a userspace PMU
> leader, so the members end up unsupported with core.fd released.  Only
> the tool event itself is started; the kernel starts the members of a
> group it can open.
> 
> Skip the events that failed to open, the same way the evlist and the
> counter read paths skip them, so a tool event that never opened is not
> started and then read through a NULL descriptor.
> 
> Fixes: 0b18bced5444 ("perf tool_pmu: Make tool PMU events respect enable/disable")
> Assisted-by: LLM
> 

Sashiko has reviewed this patch and found no issues.

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